How and where to use Transformations.switchMap?

Question

In recent Android Architecture Components library released by Google, we have two static functions in the Transformations class. While the map function is straight forward and easily understandable, I am finding it hard to properly understand the switchMap function.

The official documentation of switchMap can be found here.

Can somebody explain how and where to use the switchMap function with a practical example?

Answer 1

In the map() function

LiveData userLiveData = ...;
LiveData userName = Transformations.map(userLiveData, user -> {
     return user.firstName + " " + user.lastName; // Returns String
});

everytime the value of userLiveData changes, userName will be updated too. Notice that we are returning a String.

In the switchMap() function:

MutableLiveData userIdLiveData = ...;
LiveData userLiveData = Transformations.switchMap(userIdLiveData, id ->
    repository.getUserById(id)); // Returns LiveData

void setUserId(String userId) {
     this.userIdLiveData.setValue(userId);
}

everytime the value of userIdLiveData changes, repository.getUserById(id) will be called, just like the map function. But repository.getUserById(id) returns a LiveData. So everytime that the value of the LiveData returned by repository.getUserById(id) changes, the value of userLiveData will change too. So the value of userLiveData will depend on changes of userIdLiveData and changes of the value of repository.getUserById(id).

Practical example of switchMap(): imagine you have a user profile with a follow button and a next profile button which sets another profile info. Next profile button will call setUserId() with another id so userLiveData will change and UI will change. Follow button will call the DAO to add one follower more to that user, so the user will have 301 followers instead of 300. userLiveData will have this update that comes from the repository, which comes from the DAO.