6. Data
Data is central to most applications, and understanding how to manage data and transform it into information the user can interact with is critical. Windows 8 applications can interact with data in a variety of ways. You can save local data, retrieve syndicated content from the Web, and parse local resources that are stored in JSON format. You can query XML documents, use WinRT controls to direct the user to select files from the file system, and manipulate collections of data using a structured query language.
In this chapter, you learn about the different types of data that are available to your Windows 8 application and techniques for manipulating, loading, storing, encrypting, signing, and querying data. You’ll find that the WinRT provides several ready-to-use APIs that make working with data a breeze. This chapter explores these APIs and how to best integrate them into your application.
Application Settings
You were exposed to application settings in Chapter 5, Application Lifecycle. Common cases for using application settings include
• Simple settings that are accessed through the Settings charm and can be synchronized between machines (Roaming)
• Local data storage persisted between application sessions (Local)
• Local persistent cache to enable occasionally disconnected scenarios (Local)
• Temporary cached data used as a workspace or to improve performance of the application (Temporary)
The settings use a simple dictionary to store values and require the values you store to be basic WinRT types. It is possible to store more complex types. In Chapter 5, you learned how to manually serialize and de-serialize an item by writing to a file in local storage. You serialize complex types using a serialization helper. An example of this exists in the SuspensionManager class that is included in the project templates. You can search for the file SuspensionManager.cs on your system to browse the source code.
The SuspensionManager class uses the DataContractSerializer to serialize complex types in a dictionary:
DataContractSerializer serializer =
new DataContractSerializer(typeof(Dictionary<string, object>),
knownTypes_);
serializer.WriteObject(sessionData, sessionState_);
The serializer (in this case, the DataContractSerializer class) automatically inspects the properties on the target class and composes XML to represent the class. The XML is written to a file in the folder allocated for the current application. Similar to the various containers for application settings (local, roaming, and temporary), there is a local folder specific to the user and application that you can use to create directories and read and write files. Accessing the folder is as simple as
StorageFile file =
await ApplicationData.Current.LocalFolder.
CreateFileAsync(filename,
CreationCollisionOption.ReplaceExisting);
You can access a roaming or temporary folder as well. The Create CompletionOption is a feature that allows you generate filenames that don’t conflict with existing data. The options (passed in as an enum to the file method) include:
• FailIfExists—The operation will throw an exception if a file with that name already exists.
• GenerateUniqueName—The operation will append a sequence to the end of the filename to ensure it is a unique, new file.
• OpenIfExists—If the file already exists, instead of creating a new file, the operation will simply open the existing file for writing.
• ReplaceExisting—Any existing file will be overwritten. The example will always overwrite the file with the XML for the dictionary.
After the dictionary has been written, the serialization helper is used to de-serialize the data when the application resumes after a termination:
DataContractSerializer serializer =
new DataContractSerializer(typeof(Dictionary<string, object>),
knownTypes_);
sessionState_ = (Dictionary<string, object>)serializer
.ReadObject(inStream.AsStreamForRead());
The local storage can be used for more than just saving state. As demonstrated in Chapter 5, you may also use it to store data. It can also be used to store assets like text files and images. A common design is to use local storage to save cloud-based data that is unlikely to change as a local cache. This will allow your application to operate even when the user is not connected to the Internet and in some cases may improve the performance of the application when the network is experiencing high latency. In the next section, you learn more about how to access and save data using the Windows Runtime.
Accessing and Saving Data
Take a moment to download the Wintellog project for Chapter 6, Data, from the book website at http://windows8applications.codeplex.com/.
You may need to remove TFS bindings before you run the project. This is a sample project that demonstrates several techniques for accessing and saving data. The application takes blog feeds from various Wintellect employees and caches them locally on your Windows 8 device. Each time you launch the application, it scans for new items and pulls those down. These blogs cover cutting-edge content ranging from the latest information about Windows 8 to topics like Azure, SQL Server, and more. You may recognize some of the blog authors including Jeff Prosise, Jeffrey Richter, and John Robbins.
You learned in Chapter 5 about the various storage locations and how you can use either settings or the file system itself. The application currently uses settings to track the first time it runs. That process takes several minutes as it reads a feed with blog entries and parses the web pages for display. An extended splash screen is used due to the longer startup time. You can see the check to see if the application has been initialized in the ExtendedSplashScreen_Loaded method in SplashPage.xaml.cs:
ProgressText.Text = ApplicationData.Current.LocalSettings.Values
.ContainsKey("Initialized") && (bool)ApplicationData.Current.
LocalSettings.Values["Initialized"]
? "Loading blogs..." :
"Initializing for first use: this may take several minutes...";
After the process is completed, the flag is set to true. This allows the application to display a warning about the startup time the first time it runs. Subsequent launches will load the majority of data from a local cache to improve the speed of the application:
ApplicationData.Current.LocalSettings.Values["Initialized"]
= true;
There are several classes involved with loading and saving the data. Take a look at the StorageUtility class. This class is used to simplify the process of saving items to local storage and restoring them when the application is launched. In SaveItem, you can see the process to create a folder and a file and handling potential collisions as described in Chapter 5 (extra code has been removed for clarity):
var folder = await ApplicationData.Current.LocalFolder
.CreateFolderAsync(folderName,
CreationCollisionOption.OpenIfExists);
var file = await folder.CreateFileAsync(item.Id.GetHashCode().ToString(),
CreationCollisionOption.ReplaceExisting);
Notice that the method itself is marked with an async keyword, and the file system operations are preceded by await. You learn about these keywords in the next section. Unlike the example in Chapter 5 that manually wrote the properties to storage, the StorageUtility class takes a generic type to make it easier to save any type that can be serialized. The code uses the same engine that handles complex types transmitted via web services (you will learn more about web services later in this chapter). This code uses the DataContractJsonSerializer to take the snapshot of the instance that is saved:
var stream = await file.OpenAsync(FileAccessMode.ReadWrite);
using (var outStream = stream.GetOutputStreamAt(0))
{
var serializer = new DataContractJsonSerializer(typeof(T));
serializer.WriteObject(outStream.AsStreamForWrite(), item);
await outStream.FlushAsync();
}
The file is created through the previous call and used to retrieve a stream. The instance of the DataContractJsonSerializer is passed the type of the class to be serialized. The serialized object is written to the stream attached to the file and then flushed to store this to disk. The entire operation is wrapped in a try ... catch block to handle any potential file system errors that may occur. This is common for cache code because if the local operation fails, the data can always be retrieved again from the cloud.
To see how the serialization works and where the files are stored, run the application and allow it to initialize and pass you to the initial grouped item list. Hold down the Windows Key and press R to get the run dialog. In the dialog, type the following:
%userprofile%AppDataLocalPackages
Press the Enter key, and it will open the folder.
This is where the application-specific data for your login will be stored. You can either try to match the folder name to the package identifier or type Groups into the search box to locate the folder used by the Wintellog application. When you open the folder, you’ll see several folders with numbers for the name and a single folder called Groups, similar to what is shown in Figure 6.1.
Figure 6.1. The local cache for the Wintellog application
To simplify the generation of filenames, the application currently just uses the hash code for the unique identifier of the group or item to establish a filename. A hash code is simply a value that makes it easier to compare complex objects. You can read more about hash codes online at http://msdn.microsoft.com/en-us/library/system.object.gethashcode.aspx.
Hash codes are not guaranteed to be unique, but in the case of strings, it is highly unlikely that the combination of a group and a post would cause a collision. The Groups folder contains a list of files for each group. Navigate to that folder and open one of the items in Notepad. You’ll see the JSON serialized value for a BlogGroup instance.
The JSON is stored in a compact format on disk. The following example shows the JSON value for my blog, formatted to make it easier to read:
{
"Id" : "http://www.wintellect.com/CS/blogs/jlikness/default.aspx",
"PageUri" :
"http://www.wintellect.com/CS/blogs/jlikness/default.aspx",
"Title" : "Jeremy Likness' Blog",
"RssUri" : "http://www.wintellect.com/CS/blogs/jlikness/rss.aspx"
}
The syntax is straightforward. The braces enclose the object being defined and contain a list of keys (the name of the property) and values (what the property is set to). If you inspect any of the serialized posts (those are contained in a folder with the same name as the group hash code), you will notice the ImageUriList property uses a bracket to specify an array:
"ImageUriList" : [
"http://www.wintellect.com/.../Screen_thumb_42317207.png",
"http://www.wintellect.com/.../someotherimage.png" ]
You may have already looked at the BlogGroup class and noticed that not all of the properties are being stored. For example, the item counts are always computed when the items are loaded for the group, so they do not need to be serialized. This particular approach requires that you mark the class as a DataContract and then explicitly tag the properties you wish to serialize. The BlogGroup class is tagged like this:
[DataContract]
public class BlogGroup : BaseItem
Any properties to be serialized are tagged using the DataMember attribute:
[DataMember]
public Uri RssUri { get; set; }
If you have written web services using Windows Communication Foundation (WCF) in the past, you will be familiar with this format for tagging classes. You may not have realized it could be used for direct serialization without going through the web service stack. The default DataContractSerializer outputs XML, so remember to specify the DataContractJsonSerializer if you want to use JSON.
It is common to put code that initializes a class in the constructor for that class. When you use the serialization engines provided by the system, the constructor is not called. This actually makes sense because the implication is that the class was already created and is serialized in a state that reflects the initialization. If you do need code to run when the class is deserialized, you can specify a member for the engine to call by tagging it with the OnDeserialized attribute. In the Wintellog example, you can see an instance of this in the BlogItem class. This ensures an event is registered regardless of whether the class was created using the new keyword or was deserialized.
The process to restore is similar. You still reference the file but this time open it for read access. The same serialization engine is used to create an instance of the type from the serialized data:
var folder = await ApplicationData.Current.LocalFolder
.GetFolderAsync(folderName);
var file = await folder.GetFileAsync(hashCode);
var inStream = await file.OpenSequentialReadAsync();
var serializer = new DataContractJsonSerializer(typeof(T));
var retVal = (T)serializer.ReadObject(inStream.AsStreamForRead());
You can see when you start the application that the process of loading web sites, saving the data, and restoring items from the cache takes time. In the Windows Runtime, any process that takes more than a few milliseconds is defined as asynchronous. This is different from a synchronous call. To understand the difference, it is important to be familiar with the concept of threading.
The Need for Speed and Threading
In a nutshell, threading provides a way to execute different processes at the same time (concurrently). One job of the processor in your device is to schedule these threads. If you only have one processor, multiple threads take turns to run. If you have multiple processors, threads can run on different processors at the same time.
When the user launches an application, the system creates a main application thread that is responsible for performing most of the work, including responding to user input and drawing graphics on the screen. The fact that it manages the user interface has led to a convention of calling this thread the “UI thread.” By default, your code will execute on the UI thread unless you do something to spin off a separate thread.
The problem with making synchronous calls from the UI thread is that all processing must wait for your code to complete. If your code takes several seconds, this means the routines that check for touch events or update graphics will not run during that period. In other words, your application will freeze and become unresponsive.
The Windows Runtime team purposefully designed the framework to avoid this scenario by introducing asynchronous calls for any methods that might potentially take longer than 50 milliseconds to execute. Instead of running synchronously, these methods will spin off a separate thread to perform work and leave the UI thread free. At some point when their work is complete, they return their results. When the new await keyword is used, the results are marshaled automatically to the calling thread, which in many cases is the UI thread. A common mistake is to try to update the display without returning to the UI thread; this will generate an exception called a cross-thread access violation because only the UI thread is allowed to manage those resources.
Managing asynchronous calls in traditional C# was not only difficult, but resulted in code that was hard to read and maintain. Listing 6.1 provides an example using a traditional event-based model. Breakfast, lunch, and dinner happen asynchronously, but one meal must be completed before the next can begin. In the event-based model, an event handler is registered with the meal so the meal can flag when it is done. A method is called to kick off the process, which by convention ends with the text Async.
Listing 6.1. Asynchronous Meals Using the Event Model
public void EatMeals()
{
var breakfast = new Breakfast();
breakfast.MealCompleted += breakfast_MealCompleted;
breakfast.BeginBreakfastAsync();
}
void breakfast_MealCompleted(object sender, EventArgs e)
{
var lunch = new Lunch();
lunch.MealCompleted += lunch_MealCompleted;
lunch.BeginLunchAsync();
}
void lunch_MealCompleted(object sender, EventArgs e)
{
var dinner = new Dinner();
dinner.MealCompleted += dinner_MealCompleted;
dinner.BeginDinnerAsync();
}
void dinner_MealCompleted(object sender, EventArgs e)
{
// done;
}
This example is already complex. Every step requires a proper registration (subscription) to the completion event and then passes control to an entirely separate method when the task is done. The fact that the process continues in a separate method means that access to any local method variables is lost and any information must be passed through the subsequent calls. This is how many applications become overly complex and difficult to maintain.
The Task Parallel Library (TPL) was introduced in .NET 4.0 to simplify the process of managing parallel, concurrent, and asynchronous code. Using the TPL, you can create meals as individual tasks and execute them like this:
var breakfast = new Breakfast();
var lunch = new Lunch();
var dinner = new Dinner();
var t1 = Task.Run(() => breakfast.BeginBreakfast())
.ContinueWith(breakfastResult => lunch.
BeginLunch(breakfastResult))
.ContinueWith(lunchResult => dinner.BeginDinner(lunchResult));
This helped simplify the process quite a bit, but the code is still not easy to read and understand or maintain. The Windows Runtime has a considerable amount of APIs that use the asynchronous model. To make developing applications that use asynchronous method calls even easier, Visual Studio 2012 provides support for two new keywords called async and await.
Understanding async and await
The async and await keywords provide a simplified approach to asynchronous programming. A method that is going to perform work asynchronously and should not block the calling thread is marked with the async keyword. Within that method, you can call other asynchronous methods to launch long running tasks. Methods marked with the async keyword can have one of three return values.
All async operations in the Windows Runtime return one of four interfaces. The interface that is implemented depends on whether or not the operation returns a result to the caller and whether or not it supports tracking progress. Table 6.1 lists the available interfaces.
Table 6.1. Interfaces Available for async Operations
In C#, there are several ways you can both wrap calls to asynchronous methods as well as define them. Methods that call asynchronous operations are tagged with the async keyword. Methods with the async keyword that return void are most often event handlers. Event handlers require a void return type. For example, when you want to run an asynchronous task from a button tap, the signature of the event handler looks like this:
private void button1_Click(object sender, RoutedEventArgs e)
{
// do stuff
}
To wait for asynchronous calls to finish without blocking the UI thread, you must add the async keyword so the signature looks like this:
private async void button1_Click(object sender, RoutedEventArgs e)
{
// do stuff
await DoSomethingAsynchronously();
}
Failure to add the async modifier to a method that uses await will result in a compiler error. Aside from the special case of event handlers, you might want to create a long-running task that must complete before other code can run but does not return any values. For those methods, you return a Task. This type exists in the System.Threading.Tasks namespace. For example:
public async Task LongRunningNoReturnValue()
{
await TakesALongTime();
return;
}
Notice that the compiler does the work for you. In your method, you simply return without sending a value. The compiler will recognize the method as a long-running Task and create the Task “behind the scenes” for you. The final return type is a Task that is closed with a specific return type. Listing 6.2 demonstrates how to take a simple method that computes a factorial and wrap it in an asynchronous call. The DoFactorialExample method asynchronously computes the factorial for the number 5 and then puts the result into the Text property as a string.
Listing 6.2. Creating an Asynchronous Method That Returns a Result
public long Factorial(int factor)
{
long factorial = 1;
for (int i = 1; i <= factor; i++)
{
factorial *= i;
}
return factorial;
}
public async Task<long> FactorialAsync(int factor)
{
return await Task.Run(() => Factorial(factor));
}
public async void DoFactorialExample()
{
var result = await FactorialAsync(5);
Result = result.ToString();
}
Note how easy it was to take an existing synchronous method (Factorial) and provide it as an asynchronous method (FactorialAsync) and then call it to get the result with the await keyword (DoFactorialExample). The Task.Run call is what creates the new thread. The flow between threads is illustrated in Figure 6.2. Note the UI thread is left free to continue processing while the factorial computes, and the result is updated and can be displayed to the user.
Figure 6.2. Asynchronous flow between threads
The examples here use the Task Parallel Library (TPL) because it existed in previous versions of the .NET Framework. It is also possible to create asynchronous processes using Windows Runtime methods like ThreadPool.RunAsync. You can learn more about asynchronous programming in the Windows Runtime in the development center at http://msdn.microsoft.com/en-us/library/windows/apps/hh464924.aspx. For a quickstart on using the await operator, visit http://msdn.microsoft.com/en-us/library/windows/apps/hh452713.aspx.
Lambda Expressions
The parameter that was passed to the Task.Run method is called a lambda expression. A lambda expression is simply an anonymous function. It starts with the signature of the function (if the Run method took parameters, those would be specified inside the parenthesis) and ends with the body of the function. I like to refer to the special arrow => as the gosinta for “goes into.” Take the expression from the earlier code snippet that is passed into Task.Run:
()=>Factorial(factor)
This can be read as “nothing goes into a call to Factorial with parameter factor.” You can use lambda expressions to provide methods “on the fly.” In the previous examples showing lunch, breakfast, and dinner, special methods were defined to handle the completion events. A lambda expression could also be used like this:
breakfast.MealCompleted += (sender, eventArgs)
=>
{
// do something
};
In this case, the lambda reads as “the sender and eventArgs goes into a set of statements that do something.” The parameters triggered by the event are available in the body of the lambda expression, as are local variables defined in the surrounding methods. Lambda expressions are used as a short-hand convention for passing in delegates.
There are a few caveats to be aware of when using lambda expressions. Unless you assign a lambda expression to a variable, it is no longer available to reference from code, so you cannot unregister an event handler that is defined with a lambda expression. Lambda expressions that refer to variables within the method capture those variables so they can live longer than the method scope (this is because the lambda expression may be referenced after the method is complete), so you must be aware of the side effects for this. You can learn more about lambda expressions online at http://msdn.microsoft.com/en-us/library/bb397687(v=vs.110).aspx.
IO Helpers
The PathIO and FileIO classes provide special helper methods for reading and writing storage files. The PathIO class allows you to perform file operations by passing the absolute path to the file. Creating a text file and writing data can be accomplished in a single line of code:
await PathIO.WriteTextAsync("ms-appdata:///local/tmp.txt", "Text.");
The ms-appdata prefix is a special URI that will point to local storage for the application. You can also access local resources that are embedded in your application using the ms-appx prefix. In the sample application, an initial list of blogs to load is stored in JSON format under Assets/Blogs.js. The code to access the list is in the BlogDataSource class (under the DataModel folder)—the file is accessed and loaded with a single line of code:
var content = await PathIO
.ReadTextAsync("ms-appx:///Assets/Blogs.js");
The FileIO class performs similar operations. Instead of taking a path and automatically opening the file, it accepts a parameter of type IStorageFile. Use the FileIO helpers when you already have a reference to the file or need to perform some type of processing that can’t be done by simply referencing the path.
Table 6.2 provides the list of available methods you can use. All of the methods take an absolute file path for the PathIO class and an IStorageFile object (obtained using the storage API) for the FileIO class:
Table 6.2. File Helper Methods from the PathIO and FileIO Classes
Take advantage of these helpers where it makes sense. They will help simplify your code tremendously.
Embedded Resources
There are several ways you can embed data within your application and read it back. A common reason to embed data is to provide seed values for a local database or cache, configuration items, and special files such as license agreements. You can embed any type of resource, including images and text files. The applications you have worked with already include image resources.
To specify how a resource is embedded, right-click the resource name in the Solution Explorer and select Properties or select the item and press Alt + Enter. Figure 6.3 shows the result of highlighting the file Blogs.js in the Assets folder and selecting the Properties dialog. Note the Build Action and Copy to Output Directory properties.
Figure 6.3. Properties for a resource
When you set the action to Content, the resource is copied into a folder that is relative to the package for your application. In addition to the storage containers you learned about in Chapter 5, every package has an install location that contains the local assets you have specified the Content build action for. This will include resources such as images.
You can find the location where the package is installed using the Package class:
var package = Windows.ApplicationModel.Package.Current;
var installedLocation = package.InstalledLocation;
var loc = String.Format("Installed Location: {0}",
installedLocation.Path);
An easier way to access these files is to use the ms-appx prefix. Open the BlogDataSource.cs file. The Blogs.js file is loaded in the LoadLiveGroups method. It is loaded by using the special package prefix, like this:
var content = await PathIO.ReadTextAsync(
"ms-appx:///Assets/Blogs.js");
It is also possible to embed resources directly into the executable for your application. These resources are not visible in the file system but can still be accessed through code. To embed a resource, set the Build Action to Embedded Resource. Accessing the resource is a little more complex.
To read the contents of an embedded resource, you must access the current assembly. An assembly is a building block for applications. One way to get the assembly is to inspect the information about a class you have defined:
var assembly = typeof(BlogDataSource).GetTypeInfo().Assembly;
The assembly is what the resource is embedded within. Once you have a reference to the assembly, you can grab a stream to the resource using the GetManifestResourceStream method. There is a trick to how you reference the resource, however. The resource will be named as part of the namespace for your assembly. Therefore, a resource at the root of a project with the default namespace Wintellog will be given the path:
Wintellog.ResourceName
The reference to the ReadMe.txt file in the Common folder is therefore Wintellog.Common.ReadMe.txt. This file is not ordinarily embedded in the project; the properties have been updated to illustrate this example. After you have retrieved the stream for the resource, you can use a stream reader to read it back. When the assembly reference is obtained, you can return the contents like this:
var stream = assembly.GetManifestResourceStream(txtFile);
var reader = new StreamReader(stream);
var result = await reader.ReadToEndAsync();
return result;
You will typically use embedded resources only when you wish to obfuscate the data by hiding it in the assembly. Note this will not completely hide the data because anyone with the right tools will be able to inspect the assembly to examine its contents, including embedded resources. Embedding assets using the Content build action not only makes it easier to inspect the assets from your application, but also has the added advantage of allowing you to enumerate the file system using the installed location of the current package when there are multiple assets to manage.
Collections
Collections are the primary structures you will use to manipulate data within your application. These classes implement common interfaces that provide consistent methods for querying and managing the data in the collection. Collections are often bound to UI controls. In the Wintellog example, a collection of blogs provides the grouped few and is bound to the GridView control. A collection of posts within the blogs feed the detail view within a group.
The Windows Runtime has a set of commonly used collection types. These types are mapped automatically to .NET Framework types by the CLR. In code, you won’t reference the Windows Runtime types directly. Instead, you manipulate the .NET equivalent, and the CLR handles conversion automatically. Table 6.3 lists the Windows Runtime type and the .NET equivalent along with a brief description and example classes that implement the interface.
Table 6.3. Collection Types in the Windows Runtime and .NET
One important list that is not mapped to the Windows Runtime is the ObservableCollection<T>. This is a special list because it works with the data-binding system you learned about in Chapter 3, Extensible Application Markup Language (XAML). The ObservableCollection<T> implements the INotifyCollectionChanged interface, which is designed to notify listeners when the list changes—for example, when items are added or removed or the entire list is refreshed.
For performance, the data-binding system does not constantly examine the lists you bind to UI controls. Instead, the initially bound list is used to generate the controls on the display. When you manipulate the list, the data-binding system receives a notification through the CollectionChanged event and can use the list of added and removed items to refresh the controls being displayed. Without the interface, the only way to have a list refresh the UI is to raise a PropertyChanged event for the property that exposes the list. This is inefficient because it results in the entire list being refreshed rather than only the items that changed.
Language Integrated Query (LINQ)
One major advantage of using collections is the ability to write queries against them using Language Integrated Query (LINQ). This feature extends the language syntax of C# to provide patterns for querying and updating data. LINQ itself works with providers for different types of data storage, such as a database backend (SQL) or an XML document. The LINQ to Objects provider supports classes that implement the IEnumerable interface and therefore can be used with most collections.
LINQ to Objects is implemented as a set of extension methods to the existing IEnumerable interface. These extension methods are declared in the System.Linq namespace. Extension methods enable you to add methods to existing types without having to create a new type. They are a special type of static method that use a special this modifier for the first parameter. You can learn more about extension methods online at http://msdn.microsoft.com/en-us/library/bb383977(v=vs.110).aspx.
There are three fundamental steps involved with a LINQ query. The first step is to provide the data source or collection you will query against. The second step is to provide the query, and the final step is to execute the query. It’s important to understand that creating a query does not actually invoke any action against the data source. The query only executes when you need it and then only processes results as you obtain them. This is referred to as deferred execution.
LINQ supports a variety of query operations. It also supports multiple syntaxes for querying data. The BlogDataSource class in the Wintellog project has a method called LinqExamples. This method is never called, but you can use it to see the various types of LINQ queries and syntaxes. The first syntax is referred to as LINQ query syntax and resembles the T-SQL syntax you may be used to working with in databases. The second syntax is method-oriented and is referred to as method syntax. The method syntax is constructed using lambda expressions.
The following series of examples shows both syntaxes, starting with the query syntax.
Queries
You can use simple queries to parse collections and return the properties of interest. The following examples produce a list of strings that represent the titles from the blog groups:
var query = from g in GroupList select g.Title;
var query2 = GroupList.Select(g => g.Title);
Filters
Filters allow you to restrict the data returned by a query. You can filter using common functions that compare and manipulate properties. In the following examples, the list is filtered to only those groups with a title that starts with the letter “A.”
var filter = from g in GroupList
where g.Title.StartsWith("A")
select g;
var filter2 = GroupList.Where(g => g.Title.StartsWith("A"));
Sorting
You can sort in both ascending and descending order and across multiple properties if needed. The following queries will sort the blogs by title:
var order = from g in GroupList
orderby g.Title
select g;
var order2 = GroupList.OrderBy(g => g.Title);
Grouping
A powerful feature of LINQ is the ability to group similar results. This is especially useful in Windows 8 applications for providing the list for controls that support groups. The following queries will create groups based on the first letter of the blog title:
var group = from g in GroupList
group g by g.Title.Substring(0, 1);
var group2 = GroupList.GroupBy(g =>
g.Title.Substring(0, 1));
Joins and Projections
You can join multiple sources together and project to new types that contain only the properties that are important to you. The following query syntax will join the items from one blog to another based on the date posted and then project the results to a new class with source and target properties for the title:
var items = from i in GroupList[0].Items
join i2 in GroupList[1].Items
on i.PostDate equals i2.PostDate
select new
{ SourceTitle = i.Title, TargetTitle = i2.Title };
Here is the same query using lambda expressions:
var items2 = GroupList[0].Items.Join(
GroupList[1].Items,
g1 => g1.PostDate,
g2 => g2.PostDate,
(g1, g2) => new { SourceTitle = g1.Title,
TargetTitle = g2.Title });
This section only touched the surface of what is possible with LINQ expressions. You can learn more about LINQ by reading the articles and tutorials available online at http://msdn.microsoft.com/en-us/library/bb383799(v=vs.110).aspx.
Web Content
The Windows Runtime makes it easy to download and process web content. To access web pages, you will use the HttpClient. The class is similar to the WebClient class that Silverlight developers may be familiar with. This class is used to send and receive basic requests over the HTTP protocol. It can be used to send any type of standard HTTP request including GET, PUT, POST, and DELETE. The client returns an instance of HttpResponseMessage with the status code and headers of the response. The Content property contains the actual contents of the web page that was retrieved if the operation was successful.
The BlogDataSource class contains a helper method that provides an instance of HttpClient. The method sets a buffer size to allow for large pages to be loaded and provides a user agent for the request to use. User agents are most often used to identify the browser making the web request. In the case of programmatic access, you can pass an agent that provides information about the application and expected compatibility. Passing an agent that is compatible with mobile devices may result in the web server returning a page that is optimized for mobile browsing.
The Windows Runtime makes it easy to fetch a page asynchronously and process the results. The following two lines of code fetch the client and retrieve the page:
var client = GetClient();
var page = await client.GetStringAsync(item.PageUri);
Images are not always embedded within the RSS feed, so the code retrieves the target page for the entry and then parses it for images. This is done using regular expressions. The syntax for a regular expression provides a concise way to match patterns in strings of text. This makes it ideal for parsing tokens like HTML tags out of the source document.
The first expression parses all image tags from the source for the web page:
public const string IMAGE_TAG = @"<(img)[^>]*>";
private static readonly Regex Tags = new Regex(IMAGE_TAG,
RegexOptions.IgnoreCase | RegexOptions.Multiline);
var matches = Tags.Matches(content);
Each tag is then parsed to pull the location of the image from the src attribute. This is used to construct an instance of an Uri that is added to the ImageUriList property of the blog post. This property is implemented as an ObservableCollection to provide notification when new images are added. A random image is displayed for each post. The image is hosted on the Internet, but Windows 8 will use a cached copy of the image when the user is offline if it has been downloaded previously.
Syndicated Content
Syndicated content is information that is available to other sites through special feeds. These feeds are most often presented in an XML format using either RSS (stands for RDF Site Summary, although it is commonly referred to as Real Simple Syndication—RDF is an abbreviation of Resource Description Framework) and Atom. Both formats have evolved as standard XML-based ways for blogs, websites, and other content providers to expose data in a consistent way so that other programs can download and consume the data.
The RSS specification is available online at http://www.rssboard.org/rss-specification.
The Atom publishing protocol is available online at http://atompub.org/.
Both formats provide a way to specify a feed, which is a set of related entries (topics, articles, or posts). Each entry may have a post date, information about the author, a set of links that reference the original source, and rich content such as images and videos. To parse the data in the past, you would either have to read the specifications and write your own special XML parser or find a third-party parser that would do it for you.
The Windows Runtime provides the SyndicationClient class to make it easy for you to interact with feeds. This class exists in the Windows.Web.Syndication namespace. The class can be used to asynchronously retrieve feed information and can be provided credentials to connect with sources that require authentication. When passed a URL, it is capable of parsing feeds in Atom (0.3 and 1.0) and RSS (0.91, 0.92, 1.0, and 2.0) format and presenting them using a common object model.
The sample program retrieves the feed just using four lines of code. Two lines are not required and are used to take advantage of the browser cache and provide a custom user agent to the host website when requesting the data. A helper method named GetSyndicationClient returns the client with some default properties set in the BlogDataSource class:
private static SyndicationClient GetSyndicationClient()
{
var client = new SyndicationClient
{ BypassCacheOnRetrieve = false };
client.SetRequestHeader("user-agent", USER_AGENT);
return client;
}
Using the client is as simple as calling a method to retrieve the feed by passing the location of the feed:
var client = GetSyndicationClient();
var feed = await client.RetrieveFeedAsync(group.RssUri);
The result of the operation, if successful, is a SyndicationFeed object. The instance contains information about the location of the feed, categories or tags hosted by the feed, contributors to the feed, links associated with the feed, and of course the items that are posted to the feed. Each SyndicationItem in the feed hosts the location of the item, categories or tags specific to that item, the title and content of the item, and an optional summary.
You can follow the code in the example to see how easy it is to parse the feed and retrieve the necessary data. There is no need for you to specify the format of the feed because the class will figure this out automatically from the feed itself. Syndication is a powerful way to expose content and consume it in Windows 8 applications.
Streams, Buffers, and Byte Arrays
The traditional method for reading data from a file, website, or other source in .NET is to use a stream. A stream enables you to transfer data into a data structure that you can read and manipulate. Streams may also provide the ability to transfer the contents of a data structure back to the stream to write it. Some streams support seeking, finding a position within a stream the same way you might skip ahead to a different scene on a DVD movie.
Streams are commonly written to byte arrays. The byte array is the preferred way to reference binary data in .NET. It can be used to manipulate data like the contents of a file or the pixels that make up the bitmap for an image. Many of the stream classes in .NET support converting a byte array to a stream or reading streams into a byte array. You can also convert other types into a byte array using the BitConverter class. The following example converts a 64-bit integer to an array of 8 bytes (8 bytes x 8 bits = 64 bits) and then back again:
var bigNumber = 4523452345234523455L;
var bytes = BitConverter.GetBytes(bigNumber);
var copyOfBigNumber = BitConverter.ToInt64(bytes, 0);
Debug.Assert(bigNumber == copyOfBigNumber);
The Windows Runtime introduces the concept of an IBuffer that behaves like a cross between a byte array and a stream. The interface itself only provides two members: a Capacity property (the maximum number of bytes that the buffer can hold) and a Length property (the number of bytes currently being used by the buffer). Many operations in the Windows Runtime either consume or produce an instance of IBuffer.
It is easy to convert between streams, byte arrays, and buffers. The methods to copy a stream into a byte array or send a byte array into a stream already exist as part of the .NET Framework. The WindowsRuntimeBufferExtensions class provides additional facilities for converting between buffers and byte arrays. It exists in the System.Runtime.InteropServices.WindowsRuntime namespace. It provides another set of extension methods including AsBuffer (cast a Byte[] instance to an IBuffer), AsStream (cast an IBuffer instance to a Stream), and ToArray (cast an IBuffer instance to a Byte[] instance).
Compressing Data
Storing large amounts of data can take up a large amount of disk space. Data compression encodes information in a way that reduces its overall size. There are two general types of compression. Lossless compression preserves the full fidelity of the original data set. Lossy compression can provide better performance and a higher compression ratio, but it may not preserve all of the original information. It is often used in image, video, and audio compression where an exact data match is not required.
The Windows 8 Runtime exposes the Compressor and Decompressor classes for compression. The Compression project provides an active example of compressing and decompressing a data stream. The project contains a text file that is almost 100 kilobytes in size and loads that text and displays it with a dialog showing the total bytes. You can then click a button to compress the text and click another button to decompress it back.
The compression task performs several steps. A local file is opened for output to store the result of the compressed text. There are various ways to encode text, so it first uses the Encoding class to convert the text to a UTF8 encoded byte array:
var storage = await ApplicationData.Current.LocalFolder
.CreateFileAsync("compressed.zip",
CreationCollisionOption.ReplaceExisting);
var bytes = Encoding.UTF8.GetBytes(_text);
You learned earlier in this chapter how to locate the folder for a specific user and application. You can examine the folder for the sample application to view the compressed file after you click the button to compress the text. The file is saved with a zip extension to illustrate that it was compressed, but it doesn’t contain a true archive, so you will be unable to decompress the file from Windows Explorer.
The next lines of code open the file for writing, create an instance of the Compressor, and write the bytes. The code then completes the compression operation and flushes all associated streams:
using (var stream = await storage.OpenStreamForWriteAsync())
{
var compressor = new Compressor(stream.AsOutputStream());
await compressor.WriteAsync(bytes.AsBuffer());
await compressor.FinishAsync();
}
When the compression operation is complete, the bytes are read back from disk to show the compressed size. You’ll find the default algorithm cuts the text file down to almost half of its original size. The decompression operation uses the Decompressor class to perform the reverse operation and retrieve the decompressed bytes in a buffer (it then saves these to disk so you can examine the result).
var decompressor = new Decompressor(stream.AsInputStream());
var bytes = new Byte[100000];
var buffer = bytes.AsBuffer();
var buf = await decompressor.ReadAsync(buffer, 999999,
InputStreamOptions.None);
When you create the classes for compression, you can pass a parameter to determine the compression algorithm that is used. Table 6.4 lists the possible values.
Table 6.4. Compression Algorithms
The Windows Runtime makes compression simple and straightforward. Use compression when you have large amounts of data to store and are concerned about the amount of disk space your application requires. Remember that compression will slow down the save operation, so be sure to experiment to find the algorithm that provides the best compression ratio and performance for the type of data you are storing. Remember that you must pass the same algorithm to the decompression routine that you used to compress the data.
Encrypting and Signing Data
Many applications store sensitive data that should be encrypted to keep it safe from prying eyes. This may be information about the user or internal data for the application itself. Other information may need to be signed. Signing generates a specialized hash of data that provides a unique signature. If the original data is tampered with, the signature of the data will change. You can verify the signature against the original to determine if the data was modified in any way.
Encryption and signing is handled in the Windows Runtime by the CryptographicEngine class. This class provides services to encrypt, decrypt, sign, and verify the signature of digital content. The EncryptionSigning project contains some simple examples of performing these operations. The main code is located in the MainPage.xaml.cs file.
Encryption and decryption operations require a special key. Think of a key as a password for the encryption and decryption operations. There are two types of keys you can use. The most straightforward is called a symmetric key, which uses the same password or “secret” to both encrypt and decrypt the information.
To produce a key, you use the SymmetricKeyAlgorithmProvider class. You initialize the class by calling OpenAlgorithm with the name of the algorithm you wish to use. You then call CreateSymmetricKey to generate the key for the encryption operation. This same key must be used to decrypt the data later on. You can read the list of valid algorithms in the MSDN documentation at http://msdn.microsoft.com/en-us/library/windows/apps/windows.security.cryptography.core.symmetrickeyalgorithmprovider.openalgorithm.aspx.
In the example application, the RC4 stream cipher is used to encrypt and decrypt the data. The user is prompted for one of two passwords, and then the passwords are repeated 100 times to fill a buffer. You can use any source data that can be converted to an array of bytes for the key. A helper utility is included in the code to help convert a string to an instance of IBuffer:
var buffer = CryptographicBuffer
.ConvertStringToBinary(str.Trim(),
BinaryStringEncoding.Utf8);
return buffer;
The CryptographicBuffer class provides a set of helper utilities for encryption, decryption, and signing operations. It supports comparing two instances of a buffer, converting between strings and binary arrays using various encodings, decoding and encoding using Base64, and generating a buffer of random data. In this example, it is used to encode the string using UTF8 to a buffer that is returned.
Using the helper method, the code produces the key like this:
var result = await GetPassword();
var provider = SymmetricKeyAlgorithmProvider.OpenAlgorithm("RC4");
var key = provider.CreateSymmetricKey(AsBuffer(result));
When the key is generated, it is a simple step to encrypt the source text with the key. The result is encoded using Base64 so that it can be updated to the TextBlock in the right column for display:
var encrypted = CryptographicEngine.Encrypt(key,
AsBuffer(BigTextBox.Text), null);
_encrypted = encrypted.ToArray();
BigTextBlock.Text = CryptographicBuffer
.EncodeToBase64String(encrypted);
When you encrypt the text, the decrypt button is enabled. The user is given the option to select a password again for the decryption. If the user chooses a password that is different from the one used in the encryption operation, the decrypt process will fail or produce illegible output. The decryption process produces a key the same way the encryption process does and then simply calls the Decrypt method on the CryptographicEngine class:
var decrypted = CryptographicEngine.Decrypt(key,
_encrypted.AsBuffer(), null);
BigTextBox.Text = AsText(decrypted).Trim();
It is also possible to encrypt and decrypt using an asymmetric key. The AsymmetricKeyAlgorithmProvider is used to generate asymmetric keys. Asymmetric encryption uses two different keys, a “public” key and a “private” key to perform encryption and decryption. This allows you to encrypt the data with your private secret but provide a public key for decryption. It allows third parties to decrypt the data while keeping your secrets safe.
You can learn more about asymmetric keys and see sample code online at http://msdn.microsoft.com/en-us/library/windows/apps/windows.security.cryptography.core.asymmetrickeyalgorithmprovider.openalgorithm.aspx.
The key used to sign data can be generated using the MacAlgorithmProvider class. This class represents a Message Authentication Code (MAC). You can create the key using any one of the popular algorithms including Message-Digest Algorithm (MD5), Secure Hash Algorithm (SHA), and Cipher-based MAC (CMAC). The key is generated much the same way as the encryption key. In the example project, a default password is used to generate the key for the signature:
var provider = MacAlgorithmProvider.OpenAlgorithm("HMAC_SHA256");
var key = provider.CreateKey(
AsBuffer(MakeBigPassword(PASSWORD1)));
The signing process generates a buffer the same way the encryption process does. The difference is that you can use the buffer output by encryption to decrypt the message and produce the original. The signature is one-way—you cannot recreate the message from the signature. It’s only function is to compare against an existing message to determine whether or not it has been tampered with.
The signature is generated with a call to the Sign method on the CryptographicEngine class:
_signature = CryptographicEngine.Sign(key,
AsBuffer(BigTextBox.Text)).ToArray();
The signature is verified with a call to VerifySignature that will return true if the text has not been altered since the signature was generated:
var result = CryptographicEngine.VerifySignature(key,
AsBuffer(BigTextBox.Text),
_signature.AsBuffer());
To see how this works, launch the sample application and tap the Sign button. Now tap the Verify button to see that the text has not been altered. Now add a space or other character to the text in the left column and tap Verify again. This time you should receive a message that the text has been tampered with.
Windows 8 provides a set of powerful algorithms for encrypting, decrypting, and signing data. The process is made extremely simple through the use of the CryptographicEngine, CryptographicBuffer, and key provider classes. Use encryption to secure data both internal to your application and for transport over the Internet and use signatures to verify that data has not been tampered with in-transit.
Web Services
A web service is a method for communication between devices over the Internet. The most common protocol for communication is the Simple Object Access Protocol (SOAP) that was designed in 1998. If you’ve worked with SOAP, you know there is nothing simple about it, and a new protocol known as Representational State Transfer (REST) is quickly gaining popularity.
Web services are important for communications in applications. Many enterprise systems expose web services for consumption by client software like this Windows 8 application. One advantage of using SOAP is that it provides a discovery mechanism through the Web Services Description Language (WSDL) that allows the client application to determine the signature and structure of the API. You can learn more about the WSDL specification online at http://www.w3.org/TR/wsdl.
Open the WeatherService project to see an example of using web services. The example uses a free web service to obtain weather information. Connecting to the service was as simple as right-clicking the References node of the Solution Explorer, choosing Add Service Reference, and entering the URL for the service. The result is shown in Figure 6.4.
Figure 6.4. Adding a SOAP-based web service
When the service is added, a client proxy is generated automatically. The proxy handles all of the work necessary to make a request to the API and return the data and presents these as asynchronous implementations of the server interfaces. In the example application, the user is prompted to enter a zip code. When the button is clicked, the zip code is validated and, if there are no errors, passed to the web service. The following line of code is all that is needed to create a proxy for connecting to the web service and to call it with the zip code (note the convention of using Async at the end of the method name):
var client = new WeatherWebService.WeatherSoapClient();
var result = await client.GetCityForecastByZIPAsync(
zip.ToString());
If the result does not indicate a successful call, a dialog is shown that indicates there was a problem. Otherwise, the results are bound to the grid and shown through data-binding. This is all it takes to data-bind the results from the web service call:
ResultsGrid.DataContext = result;
The XAML is set to show the city and state:
<StackPanel Orientation="Horizontal">
<TextBlock Text="{Binding City}"/>
<TextBlock Text=","/>
<TextBlock Text="{Binding State}"/>
</StackPanel>
Listing 6.3 shows the full XAML for the individual forecast items. The “description” field is purposefully misspelled because this is how it came across in the web service as of the time of this writing.
Listing 6.3. Data-Binding the Results from a Weather Service
<ListView Grid.Row="1" ItemsSource="{Binding ForecastResult}">
<ListView.ItemTemplate>
<DataTemplate>
<StackPanel Orientation="Horizontal">
<TextBlock Text="{Binding Date, Converter={StaticResource ConvertDate}}"
Width="200"/>
<TextBlock Text="{Binding Description}" Width="150" Margin="5 0 0 0"/>
<Image Source="{Binding Description, Converter={StaticResource ConvertImage}}"/>
<TextBlock Text="{Binding Temperatures.MorningLow}" Margin="5 0 0 0" Width="50"/>
<TextBlock Text="{Binding Temperatures.DaytimeHigh}" Margin="5 0 0 0" Width="50"/>
</StackPanel>
</DataTemplate>
</ListView.ItemTemplate>
</ListView>
The weather service documentation provided a set of icons that correspond to the description. The ImageConverter class takes the description and translates it to a file name so it can return the image:
var filename =
string.Format("ms-appx:///Assets/{0}.gif",
((string) value).Replace(" ", string.Empty).ToLower());
return new BitmapImage(new Uri(filename, UriKind.Absolute));
Figure 6.5 displays the result of my request for the weather forecast of my hometown (Woodstock, Georgia) via its zip code.
Figure 6.5. The weather forecast for Woodstock, Georgia
OData Support
The Open Data Procotol (OData) is a web protocol used for querying and updating data. It is a REST-based API built on top of Atom that uses JSON or XML for transporting information. You can read more about OData online at http://www.odata.org/.
Windows 8 applications have native support for OData clients once you download and install the client from:
http://go.microsoft.com/fwlink/?LinkId=253653
To access OData services, you simply add a service reference the same way you would for a SOAP-based web service. A popular OData service to use for demonstrations is the Netflix movie catalog. You can browse the service directly by typing http://odata.netflix.com/catalog/ into your browser.
In most browsers, you should see an XML document that contains various tags for collections you may browse. For example, the collection referred to as Titles indicates you can browse all titles using the URL, http://odata.netflix.com/catalog/Titles.
The Netflix project shows a simple demonstration of using this OData feed. The main URL was added as a service reference the same way the weather service was added in the previous example. The first step in using the service is to create a proxy to access it. This is done by taking the generated class from adding the service and passing in the service URL:
var netflix =
new NetflixCatalog(
new Uri(
"http://odata.netflix.com/Catalog/",
UriKind.Absolute));
Next, set up a collection for holding the results of an OData query. This is done using the special DataServiceCollection class:
private DataServiceCollection<Title> _collection;
...
_collection = new DataServiceCollection<Title>(netflix);
TitleGrid.ItemsSource = _collection;
Finally, specify a query to filter the data. This query is passed to the proxy and will load the results into the collection. In this example, the query will grab the first 100 titles that start with the letter “Y” in order of highest rated first:
var query = (from t in netflix.Titles
where t.Name.StartsWith("Y")
orderby t.Rating descending
select t).Take(100);
_collection.LoadAsync(query);
Finally, as data comes in, you have the option to page in additional sets of data. This is done by checking the collection for a continuation. If one exists, you can request that the service load the next set. This allows you to page in data rather than pull down an extremely large set all at once:
if (_collection.Continuation != null)
{
_collection.LoadNextPartialSetAsync();
}
Run the included sample application. You should see the titles and images start to appear asynchronously in a grid that you can scroll through. As in the previous example, the results of the web service are bound directly to the grid:
<Image Stretch="Uniform" Width="150" Height="150">
<Image.Source>
<BitmapImage UriSource="{Binding BoxArt.LargeUrl}"/>
</Image.Source>
</Image>
<TextBlock Text="{Binding Name}" Grid.Row="1"/>
The Windows 8 development environment makes it easy and straightforward to connect to web services and pull data in from external sources. Many existing applications expose web services in the form of SOAP, REST, and OData feeds. The built-in support to access and process these feeds makes it possible to build Windows 8 applications that support your existing functionality when it is exposed via web services.
Summary
This chapter explored a variety of ways you can deal with data in your Windows 8 applications. You learned how to save and retrieve data from file storage, access it over the Web, and syndicate it through RSS and Atom feeds. You learned about the built-in tools that make it easy to encrypt and sign data. Finally, you saw how easy it is to connect to existing SOAP and OData web services by generating proxies and retrieving data asynchronously from external APIs.
In the next chapter, you will learn how to keep your application alive even when it is not running through the use of tiles and notifications. Tiles provide information to the user at a glance on their Start screens and can be refreshed even when the application is not running. Notifications can be generated from within the application or by an external source to inform the user when important events happen and provide a contextual link back into the application.