📚 课程目录第1章构建音频焦点请求与释放框架

1. 构建音频焦点请求与释放框架

任务说明

在Android音视频应用中，多个播放器可能同时运行（如音乐播放器与导航语音），系统通过音频焦点（Audio Focus）机制协调音频流的使用权。本案例实现一个可复用的音频焦点管理框架，支持请求、释放、监听焦点变化并自动处理短暂丢失与永久丢失两种典型场景。

核心实现

// AudioFocusManager.java
package com.example.audiofocus;

import android.content.Context;
import android.media.AudioAttributes;
import android.media.AudioFocusRequest;
import android.media.AudioManager;
import android.os.Build;
import android.os.Handler;
import android.os.Looper;
import android.util.Log;

public class AudioFocusManager {
    private static final String TAG = "AudioFocusManager";
    private final AudioManager audioManager;
    private final FocusListener focusListener;
    private AudioFocusRequest focusRequest;
    private boolean hasFocus = false;
    private final Handler mainHandler = new Handler(Looper.getMainLooper());

    // 焦点变化回调接口
    public interface FocusListener {
        void onFocusGained();          // 获得焦点（可开始播放）
        void onFocusLostTransient();   // 短暂丢失（如来电，可暂停）
        void onFocusLostPermanent();   // 永久丢失（如其他应用抢占，应停止）
        void onFocusDucked();          // 闪避（降低音量而非暂停）
    }

    public AudioFocusManager(Context context, FocusListener listener) {
        this.audioManager = (AudioManager) context.getSystemService(Context.AUDIO_SERVICE);
        this.focusListener = listener;
        buildFocusRequest();
    }

    private void buildFocusRequest() {
        AudioAttributes attrs = new AudioAttributes.Builder()
                .setUsage(AudioAttributes.USAGE_MEDIA)
                .setContentType(AudioAttributes.CONTENT_TYPE_MUSIC)
                .build();

        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
            focusRequest = new AudioFocusRequest.Builder(AudioManager.AUDIOFOCUS_GAIN)
                    .setAudioAttributes(attrs)
                    .setAcceptsDelayedFocusGain(true)
                    .setWillPauseWhenDucked(false)
                    .setOnAudioFocusChangeListener(changeListener, mainHandler)
                    .build();
        }
    }

    private final AudioManager.OnAudioFocusChangeListener changeListener =
            new AudioManager.OnAudioFocusChangeListener() {
                @Override
                public void onAudioFocusChange(int focusChange) {
                    switch (focusChange) {
                        case AudioManager.AUDIOFOCUS_GAIN:
                            hasFocus = true;
                            focusListener.onFocusGained();
                            break;
                        case AudioManager.AUDIOFOCUS_LOSS_TRANSIENT:
                            hasFocus = false;
                            focusListener.onFocusLostTransient();
                            break;
                        case AudioManager.AUDIOFOCUS_LOSS:
                            hasFocus = false;
                            focusListener.onFocusLostPermanent();
                            break;
                        case AudioManager.AUDIOFOCUS_LOSS_TRANSIENT_CAN_DUCK:
                            focusListener.onFocusDucked();
                            break;
                    }
                }
            };

    /** 请求音频焦点 */
    public boolean requestFocus() {
        if (hasFocus) return true;
        int result;
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
            result = audioManager.requestAudioFocus(focusRequest);
        } else {
            // 低版本兼容（API 26以下）
            result = audioManager.requestAudioFocus(changeListener,
                    AudioManager.STREAM_MUSIC, AudioManager.AUDIOFOCUS_GAIN);
        }
        hasFocus = (result == AudioManager.AUDIOFOCUS_REQUEST_GRANTED);
        Log.d(TAG, "requestFocus result: " + result);
        return hasFocus;
    }

    /** 释放音频焦点 */
    public void abandonFocus() {
        if (!hasFocus) return;
        int result;
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
            result = audioManager.abandonAudioFocusRequest(focusRequest);
        } else {
            result = audioManager.abandonAudioFocus(changeListener);
        }
        hasFocus = false;
        Log.d(TAG, "abandonFocus result: " + result);
    }

    public boolean isHasFocus() {
        return hasFocus;
    }
}

使用示例

// MainActivity.java 关键片段
AudioFocusManager focusManager = new AudioFocusManager(this, new AudioFocusManager.FocusListener() {
    @Override
    public void onFocusGained() {
        // 开始或恢复播放
        mediaPlayer.start();
    }

    @Override
    public void onFocusLostTransient() {
        // 暂停播放，等待重新获得焦点
        mediaPlayer.pause();
    }

    @Override
    public void onFocusLostPermanent() {
        // 停止播放并释放资源
        mediaPlayer.stop();
        mediaPlayer.release();
    }

    @Override
    public void onFocusDucked() {
        // 降低音量至20%
        mediaPlayer.setVolume(0.2f, 0.2f);
    }
});

// 用户点击播放按钮时
focusManager.requestFocus();
// 用户点击停止或退出时
focusManager.abandonFocus();

验证方式

在真机上安装应用，准备两个音频播放源（如系统音乐播放器与本应用）。操作步骤：

启动本应用并点击播放，确认日志输出requestFocus result: 1（1表示AUDIOFOCUS_REQUEST_GRANTED），声音正常播放；
启动系统音乐播放器并播放，本应用应收到onFocusLostPermanent回调，播放停止；
关闭系统音乐播放器，再次点击本应用播放，应重新获得焦点并恢复播放；
在应用播放期间来电（或使用adb模拟短暂焦点丢失），应收到onFocusLostTransient回调并暂停，挂断后自动恢复。

判定标准：以上4个场景中回调触发顺序与日志输出符合预期，且无崩溃或ANR，即验证通过。