自动恢复 android 播放背景音乐

Question

我正在开发一个在后台将音乐作为音频服务运行的音乐播放器。

假设音乐正在后台通过我的应用播放，用户从其他应用（例如：Youtube）播放另一个媒体文件，然后退出该应用。这会暂停通过我的应用程序播放的音乐，现在用户必须手动恢复音乐。

如何在外部媒体文件关闭后自动恢复音乐文件？

Answer 1

我认为您正在寻找控制 Audio Focus 的方法。这讨论了当另一个应用程序/您的应用程序控制音频焦点时如何实现更改，即增益调整（降低音量）。

mAudioManager = (AudioManager) Context.getSystemService(Context.AUDIO_SERVICE);
mPlaybackAttributes = new AudioAttributes.Builder()
        .setUsage(AudioAttributes.USAGE_GAME)
        .setContentType(AudioAttributes.CONTENT_TYPE_MUSIC)
        .build();
mFocusRequest = new AudioFocusRequest.Builder(AudioManager.AUDIOFOCUS_GAIN)
        .setAudioAttributes(mPlaybackAttributes)
        .setAcceptsDelayedFocusGain(true)
        .setOnAudioFocusChangeListener(mMyFocusListener, mMyHandler)
        .build();
mMediaPlayer = new MediaPlayer();
final Object mFocusLock = new Object();

boolean mPlaybackDelayed = false;
boolean mPlaybackNowAuthorized = false;

// ...
int res = mAudioManager.requestAudioFocus(mFocusRequest);
synchronized(mFocusLock) {
    if (res == AudioManager.AUDIOFOCUS_REQUEST_FAILED) {
        mPlaybackNowAuthorized = false;
    } else if (res == AudioManager.AUDIOFOCUS_REQUEST_GRANTED) {
        mPlaybackNowAuthorized = true;
        playbackNow();
    } else if (res == AudioManager.AUDIOFOCUS_REQUEST_DELAYED) {
       mPlaybackDelayed = true;
       mPlaybackNowAuthorized = false;
    }
}

// ...
@Override
public void onAudioFocusChange(int focusChange) {
    switch (focusChange) {
        case AudioManager.AUDIOFOCUS_GAIN:
            if (mPlaybackDelayed || mResumeOnFocusGain) {
                synchronized(mFocusLock) {
                    mPlaybackDelayed = false;
                    mResumeOnFocusGain = false;
                }
                playbackNow();
            }
            break;
        case AudioManager.AUDIOFOCUS_LOSS:
            synchronized(mFocusLock) {
                mResumeOnFocusGain = false;
                mPlaybackDelayed = false;
            }
            pausePlayback();
            break;
        case AudioManager.AUDIOFOCUS_LOSS_TRANSIENT:
            synchronized(mFocusLock) {
                mResumeOnFocusGain = true;
                mPlaybackDelayed = false;
            }
            pausePlayback();
            break;
        case AudioManager.AUDIOFOCUS_LOSS_TRANSIENT_CAN_DUCK:
            // ... pausing or ducking depends on your app
            break;
        }
    }
}

不过请注意如documentation所述：

Permanent loss of focus

If the audio focus loss is permanent (AUDIOFOCUS_LOSS), another app is playing audio. Your app should pause playback immediately, as it won't ever receive an AUDIOFOCUS_GAIN callback. To restart playback, the user must take an explicit action, like pressing the play transport control in a notification or app UI.

这是假设接收到永久音频焦点丢失，但是我可以看到一些解决方法，例如轮询音频焦点，但最好不要这样做。如前所述，他们明确表示：

the user must take an explicit action

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