/*
 * Copyright (C) 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 com.android.internal.listeners;

import android.annotation.Nullable;

import java.util.Objects;
import java.util.concurrent.Executor;
import java.util.function.Consumer;

/**
 * A listener transport object which can run listener operations on an executor.
 *
 * @param <TListener> listener type
 */
public interface ListenerTransport<TListener> {

    /**
     * Should return a valid listener until {@link #unregister()} is invoked, and must return
     * null after that. Recommended (but not required) that this is implemented via a volatile
     * variable.
     */
    @Nullable TListener getListener();

    /**
     * Must be implemented so that {@link #getListener()} returns null after this is invoked.
     */
    void unregister();

    /**
     * Executes the given operation for the listener.
     */
    default void execute(Executor executor, Consumer<TListener> operation) {
        Objects.requireNonNull(operation);

        if (getListener() == null) {
            return;
        }

        executor.execute(() -> {
            TListener listener = getListener();
            if (listener == null) {
                return;
            }

            operation.accept(listener);
        });
    }
}
