When using an app, there is nothing worse than it being slow and unresponsive. Computer users have come to expect every piece of software to respond immediately to every interaction. Even the most feature-rich app will be ruined if it is unpleasant to use because it doesn't manage the device resources effectively. Also, with the growing popularity of mobile computers and devices, it is more important than ever to write software that uses battery power efficiently. One of the aspects of writing software that has the largest impact on both responsiveness and battery power is memory management.
In this chapter, we will discuss techniques specific to Swift that allow us to manage memory in order to ensure that our code remains responsive and minimizes its effect on battery life and other apps. We will do so by covering the following topics:
- Computer data storage
- Value types versus reference types
- Automatic reference counting
- Strong reference cycles
- Lost objects
- Structures versus classes
Before we start looking at the code, we need to understand in some detail how data is represented in a computer. The common cliché is that all data in a computer is in 1s and 0s. This is true, but not so important when talking about memory management. Instead, we are concerned about where the data is stored. All computers, whether a desktop, laptop, tablet, or phone, store data in two places.
The first place we normally think of is the file system. It is stored on a dedicated piece of hardware; this is called a hard disk drive in many computers, but more recently, some computers have started to use solid-state drives. The other thing we hear about when buying computers is the amount of "memory" it has. Computer memory comes in "sticks" which hold less information than normal drives. All data, even if primarily stored on the Internet somewhere, must be loaded into the computer's memory so that we can interact with it.
Let's take a look at what that means for us as programmers.
The file system is designed for long-term storage of data. It is far slower to access than memory, but it is much more cost effective for storing a lot of data. As the name implies, the file system is simply a hierarchical tree of files, which we as users can interact with directly using the Finder on a Mac. This file system still exists on iPhones and iPads but it is hidden from us. However, software can still read and write the file system, thus allowing us to store data permanently, even after turning the device off.
Memory is a little more complex than the file system. It is designed to store the necessary data, temporarily for the software running currently. Unlike with a file system, all memory is lost as soon as you turn off your device. The analogy is similar to how we humans have short-term and long-term memory. While we are having a conversation or thinking about something, we have a certain subset of the information we are actively thinking about and the rest is in our long-term memory. In order to actively think about something, we have to recall it from our long-term memory into our short-term memory.
Memory is quick to access, but it is much more expensive. When computers start to act abnormally slow, it is commonly because it is very close to using up all of its memory. This is because the operating system will automatically start using the file system as a backup when memory is low. Information that is meant for short-term storage is automatically written to the file system instead, making it much slower to access again.
This is similar to how we humans have a problem processing too much information at once. If we try to add two 20-digit numbers in our head, it is going to take us a long time or simply be impossible. Instead, we often write out the partial solution on paper, as we go along. In this case, the paper is acting as our file system. It would be faster if we could just remember everything instead of taking the time to write it down and read it back, but we simply can't process that much information at one time.
This is important to consider when programming because we want to reduce the amount of memory that we use at any given time. Using a lot of memory doesn't only negatively affect our own software; it can negatively affect the entire computer's performance. Also, when the operating system has to resort to using the file system, the extra processing and extra access to a second piece of hardware causes more power usage.
Now that we understand our goal, we can start discussing how we manage memory better in Swift.