Package icyllis.modernui.lifecycle

Class Transformations

java.lang.Object
icyllis.modernui.lifecycle.Transformations
public final class Transformations extends Object
Transformation methods for LiveData.

These methods permit functional composition and delegation of LiveData instances. The transformations are calculated lazily, and will run only when the returned LiveData is observed. Lifecycle behavior is propagated from the input source LiveData to the returned one.

  • Method Details

    • map

      @UiThread @Nonnull public static <X, Y> LiveData<Y> map(@Nonnull LiveData<X> source, @Nonnull Function<? super X,? extends Y> mapFunction)
      Returns a LiveData mapped from the input source LiveData by applying mapFunction to each value set on source.

      This method is analogous to

      invalid reference 
      io.reactivex.rxjava3.core.Observable#map
      .

      transform will be executed on the UI thread.

      Here is an example mapping a simple User struct in a LiveData to a LiveData containing their full name as a String. 

      
 LiveData<User> userLiveData = ...;
 LiveData<String> userFullNameLiveData =
     Transformations.map(
         userLiveData,
         user -> user.firstName + user.lastName);
      Type Parameters:
      X - the generic type parameter of source
      Y - the generic type parameter of the returned LiveData
      Parameters:
      source - the LiveData to map from
      mapFunction - a function to apply to each value set on source in order to set it on the output LiveData
      Returns:
      a LiveData mapped from source to type <Y> by applying mapFunction to each value set.

    • switchMap

      @UiThread @Nonnull public static <X, Y> LiveData<Y> switchMap(@Nonnull LiveData<X> source, @Nonnull Function<? super X,? extends LiveData<? extends Y>> switchMapFunction)
      Returns a LiveData mapped from the input source LiveData by applying switchMapFunction to each value set on source.

      The returned LiveData delegates to the most recent LiveData created by calling switchMapFunction with the most recent value set to source, without changing the reference. In this way, switchMapFunction can change the 'backing' LiveData transparently to any observer registered to the LiveData returned by switchMap().

      Note that when the backing LiveData is switched, no further values from the older LiveData will be set to the output LiveData. In this way, the method is analogous to

      invalid reference 
      io.reactivex.rxjava3.core.Observable#switchMap
      .

      switchMapFunction will be executed on the UI thread.

      Here is an example class that holds a typed-in name of a user String (such as from an EditText) in a MutableLiveData and returns a LiveData containing a List of User objects for users that have that name. It populates that LiveData by re-querying a repository-pattern object each time the typed name changes.

      This ViewModel would permit the observing UI to update "live" as the user ID text changes. 

      
 class UserViewModel extends ViewModel {
     MutableLiveData<String> nameQueryLiveData = ...

     LiveData<List<String>> getUsersWithNameLiveData() {
         return Transformations.switchMap(
             nameQueryLiveData, myDataSource::getUsersWithNameLiveData);
     }

     void setNameQuery(String name) {
         this.nameQueryLiveData.setValue(name);
     }
 }
      Type Parameters:
      X - the generic type parameter of source
      Y - the generic type parameter of the returned LiveData
      Parameters:
      source - the LiveData to map from
      switchMapFunction - a function to apply to each value set on source to create a new delegate LiveData for the returned one
      Returns:
      a LiveData mapped from source to type <Y> by delegating to the LiveData returned by applying switchMapFunction to each value set

    • distinctUntilChanged

      @UiThread @Nonnull public static <X> LiveData<X> distinctUntilChanged(@Nonnull LiveData<X> source)
      Creates a new LiveData object that does not emit a value until the source LiveData value has been changed. The value is considered changed if Object.equals(Object) yields false.
      Type Parameters:
      X - the generic type parameter of source
      Parameters:
      source - the input LiveData
      Returns:
      a new LiveData of type X