Jetpack's livedata extension mediatorlivedata

LiveData The use of has already been mentioned ：

Android Jetpack Of components LiveData Use - Source code

however Android There are also some extension classes in the Library , such as MediatorLiveData Need to introduce .

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Suppose there is a need ： stay ExitText While typing text in , Number of display text . Yes, of course EditTextChangeListener, Or just one LiveData Monitor input String You can also get length, But here are just examples .

class MainViewModel : ViewModel() {
    val message: MutableLiveData<String> = MutableLiveData()
    val count: MediatorLiveData<Int> = MediatorLiveData()

    init {
        count.value = 0

        count.addSource(message) {
            val cnt = message.value?.length ?: 0
            count.postValue(cnt)
        }
    }

    fun postMessage(message: String) {
        this.message.postValue(message)
    }
}

MediatorLiveDarta The role of , As the name suggests, it can act as an intermediary to monitor others LiveData. It can be here EditText Through postMessage to update message,count adopt addSource monitor message Update the input length after the change of .

  use Java Look at the version ：

for example ： from getNameFromServer() The value taken is "alan", and MediatorLiveData After transformation, it becomes "alan gong".

public class MyViewModel extends ViewModel {
    private MutableLiveData<String> liveEvent;
    private MediatorLiveData<String> testLiveData;
 
    public MyViewModel() {
        testLiveData = new MediatorLiveData<>();
        testLiveData.addSource(liveEvent, new Observer<String>() {
            @Override
            public void onChanged(String s) {
                testLiveData.postValue(s + " gong");
            }
        });
 
        getNameFromServer();
    }
 
    public MediatorLiveData<String> getTestLiveData() {
        return testLiveData;
    }
 
    private void getNameFromServer() {
        if (liveEvent == null) {
            liveEvent = new MutableLiveData<>();
        }
        liveEvent.setValue("alan");
    }
}
 
public class MainActivity extends AppCompatActivity {
 
    private ActivityMainBinding binding;
 
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
 
        binding = ActivityMainBinding.inflate(LayoutInflater.from(this));
        setContentView(binding.getRoot());
 
        ViewModelProvider provider = new ViewModelProvider(this, new ViewModelProvider.NewInstanceFactory());
        MyViewModel myViewModel = provider.get(MyViewModel.class);
        binding.setViewModel(myViewModel);
 
        myViewModel.getTestLiveData().observe(this, new Observer<String>() {
            @Override
            public void onChanged(String s) {
                textView.setText(s);
            }
        });
    }
}

As for the principle , I believe I can use LiveData I think it's nothing ,studio Just click this class to enter the source code and have a look .