ForwardingLiveData.java
/*
* Copyright 2020 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package androidx.camera.view;
import androidx.annotation.NonNull;
import androidx.annotation.RequiresApi;
import androidx.lifecycle.LiveData;
import androidx.lifecycle.MediatorLiveData;
/**
* A {@link LiveData} whose value is based on a source {@link LiveData} of the same type.
*
* <p> Used by {@link CameraController} to get a camera state {@link LiveData} before camera is
* bound, and keep using the same {@link LiveData} after camera is switched.
*
* <p> Setting a new source will remove the previous source.
*/
@RequiresApi(21) // TODO(b/200306659): Remove and replace with annotation on package-info.java
final class ForwardingLiveData<T> extends MediatorLiveData<T> {
private LiveData<T> mLiveDataSource;
void setSource(@NonNull LiveData<T> liveDataSource) {
if (mLiveDataSource != null) {
super.removeSource(mLiveDataSource);
}
mLiveDataSource = liveDataSource;
super.addSource(liveDataSource, this::setValue);
}
@Override
public T getValue() {
// If MediatorLiveData has no active observers, it will not receive updates
// when the source is updated, in which case the value of this class and its source
// will be out-of-sync.
// We need to retrieve the source value for the caller.
return mLiveDataSource == null ? null : mLiveDataSource.getValue();
}
}