session-android/src/org/thoughtcrime/securesms/util/SingleLiveEvent.java

/*
 *  Copyright 2017 Google Inc.
 *
 *  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 org.thoughtcrime.securesms.util;

import android.arch.lifecycle.LifecycleOwner;
import android.arch.lifecycle.MutableLiveData;
import android.arch.lifecycle.Observer;
import android.support.annotation.MainThread;
import android.support.annotation.NonNull;
import android.support.annotation.Nullable;
import android.util.Log;

import java.util.concurrent.atomic.AtomicBoolean;

/**
 * A lifecycle-aware observable that sends only new updates after subscription, used for events like
 * navigation and Snackbar messages.
 * <p>
 * This avoids a common problem with events: on configuration change (like rotation) an update
 * can be emitted if the observer is active. This LiveData only calls the observable if there's an
 * explicit call to setValue() or call().
 * <p>
 * Note that only one observer is going to be notified of changes.
 */
public class SingleLiveEvent<T> extends MutableLiveData<T> {

  private static final String TAG = SingleLiveEvent.class.getSimpleName();

  private final AtomicBoolean mPending = new AtomicBoolean(false);

  @MainThread
  public void observe(@NonNull LifecycleOwner owner, @NonNull final Observer<T> observer) {
    if (hasActiveObservers()) {
      Log.w(TAG, "Multiple observers registered but only one will be notified of changes.");
    }

    // Observe the internal MutableLiveData
    super.observe(owner, t -> {
      if (mPending.compareAndSet(true, false)) {
        observer.onChanged(t);
      }
    });
  }

  @MainThread
  public void setValue(@Nullable T t) {
    mPending.set(true);
    super.setValue(t);
  }

  /**
   * Used for cases where T is Void, to make calls cleaner.
   */
  @MainThread
  public void call() {
    setValue(null);
  }
}