To close this chapter off with a bang, we will implement a mechanism in which each Avatar implementation takes a turn in trying to get the value. If the first implementation returns the ErrNoAvatarURL error, we will try the next and so on until we find a useable value.

In avatar.go, underneath the Avatar type, add the following type definition:

type TryAvatars []Avatar

The TryAvatars type is simply a slice of Avatar objects; therefore, we will add the following GetAvatarURL method:

func (a TryAvatars) GetAvatarURL(u ChatUser) (string, error) {
  for _, avatar := range a {
    if url, err := avatar.GetAvatarURL(u); err == nil {
      return url, nil
    }
  }
  return "", ErrNoAvatarURL
}

This means that TryAvatars is now a valid Avatar implementation and can be used in place of any specific implementation. In the preceding method, we iterated over the slice of Avatar objects in an order, calling GetAvatarURL for each one. If no error is returned, we return the URL; otherwise, we carry on looking. Finally, if we are unable to find a value, we just return ErrNoAvatarURL as per the interface design.

Update the avatars global variable in main.go to use our new implementation:

var avatars Avatar = TryAvatars{
  UseFileSystemAvatar,
  UseAuthAvatar,
  UseGravatar}

Here we created a new instance of our TryAvatars slice type while putting the other Avatar implementations inside it. The order matters since it iterates over the objects in the order in which they appear in the slice. So, first our code will check to see whether the user has uploaded a picture; if they haven't, the code will check whether the authentication service has a picture for us to use. If both the approaches fail, a Gravatar URL will be generated, which in the worst case (for example, if the user hasn't added a Gravatar picture), will render a default placeholder image.

To see our new functionality in action, perform the following steps: