camera/camera-core/src/main/java/androidx/camera/core/impl/utils/executor/MainThreadExecutor.java

/*
 * Copyright 2019 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.core.impl.utils.executor;

import android.os.Handler;
import android.os.Looper;

import java.util.concurrent.Executor;
import java.util.concurrent.ScheduledExecutorService;

/**
 * Helper class for retrieving an {@link ScheduledExecutorService} which will post to the main
 * thread.
 *
 * <p>Since {@link ScheduledExecutorService} implements {@link Executor}, this can also be used
 * as a simple Executor.
 */
final class MainThreadExecutor {
    private static volatile ScheduledExecutorService sInstance;

    private MainThreadExecutor() {
    }

    static ScheduledExecutorService getInstance() {
        if (sInstance != null) {
            return sInstance;
        }
        synchronized (MainThreadExecutor.class) {
            if (sInstance == null) {
                sInstance = new HandlerScheduledExecutorService(
                        new Handler(Looper.getMainLooper()));
            }
        }

        return sInstance;
    }
}