Android声音焦点----从音乐回到Luncher调节音量显示的是Music的音量

声音的类型有：定义在AudioSystem.java文件中

/* The default audio stream */
public static final int STREAM_DEFAULT = -1;
/* The audio stream for phone calls */
public static final int STREAM_VOICE_CALL = 0;
/* The audio stream for system sounds */
public static final int STREAM_SYSTEM = 1;
/* The audio stream for the phone ring and message alerts */
public static final int STREAM_RING = 2;
/* The audio stream for music playback */
public static final int STREAM_MUSIC = 3;
/* The audio stream for alarms */
public static final int STREAM_ALARM = 4;
/* The audio stream for notifications */
public static final int STREAM_NOTIFICATION = 5;
/* @hide The audio stream for phone calls when connected on bluetooth */
public static final int STREAM_BLUETOOTH_SCO = 6;
/* @hide The audio stream for enforced system sounds in certain countries (e.g camera in Japan) */
public static final int STREAM_SYSTEM_ENFORCED = 7;
/* @hide The audio stream for DTMF tones */
public static final int STREAM_DTMF = 8;
/* @hide The audio stream for text to speech (TTS) */
public static final int STREAM_TTS = 9;

正常情况下在音、视频界面调节音量调节的Stream是STREAM_MUSIC，回到Luncher调节的是STREAM_VOICE_CALL，但这个切换有点延迟StreamOverride.sDelayMs，导致退出音、视频界面后马上调节，改变的还是STREAM_MUSIC的大小，如图所示：

可以修改/frameworks/base/media/java/android/media/AudioService.java文件

 /**
     * For code clarity for getActiveStreamType(int)
     * @param delay_ms max time since last STREAM_MUSIC activity to consider
     * @return true if STREAM_MUSIC is active in streams handled by AudioFlinger now or
     *     in the last "delay_ms" ms.
     */
    private boolean isAfMusicActiveRecently(int delay_ms) {
        return AudioSystem.isStreamActive(AudioSystem.STREAM_MUSIC, delay_ms)
                || AudioSystem.isStreamActiveRemotely(AudioSystem.STREAM_MUSIC, delay_ms);
    }

    private int getActiveStreamType(int suggestedStreamType) {
        switch (mPlatformType) {
        case PLATFORM_VOICE:
            if (isInCommunication()) {
                if (AudioSystem.getForceUse(AudioSystem.FOR_COMMUNICATION)
                        == AudioSystem.FORCE_BT_SCO) {
                    // Log.v(TAG, "getActiveStreamType: Forcing STREAM_BLUETOOTH_SCO...");
                    return AudioSystem.STREAM_BLUETOOTH_SCO;
                } else {
                    // Log.v(TAG, "getActiveStreamType: Forcing STREAM_VOICE_CALL...");
                    return AudioSystem.STREAM_VOICE_CALL;
                }
            } else if (suggestedStreamType == AudioManager.USE_DEFAULT_STREAM_TYPE) {
                if (isAfMusicActiveRecently(StreamOverride.sDelayMs)) {
                    if (DEBUG_VOL)
                        Log.v(TAG, "getActiveStreamType: Forcing STREAM_MUSIC stream active");
                    return AudioSystem.STREAM_MUSIC;
                    } else {
                        if (DEBUG_VOL)
                            Log.v(TAG, "getActiveStreamType: Forcing STREAM_RING b/c default");
                        return AudioSystem.STREAM_RING;
                }
            } else if (isAfMusicActiveRecently(0)) {
                if (DEBUG_VOL)
                    Log.v(TAG, "getActiveStreamType: Forcing STREAM_MUSIC stream active");
                return AudioSystem.STREAM_MUSIC;
            }
            break;
        case PLATFORM_TELEVISION:
            if (suggestedStreamType == AudioManager.USE_DEFAULT_STREAM_TYPE) {
                    // TV always defaults to STREAM_MUSIC
                    return AudioSystem.STREAM_MUSIC;
            }
            break;
        default:
            if (isInCommunication()) {
                if (AudioSystem.getForceUse(AudioSystem.FOR_COMMUNICATION)
                        == AudioSystem.FORCE_BT_SCO) {
                    if (DEBUG_VOL) Log.v(TAG, "getActiveStreamType: Forcing STREAM_BLUETOOTH_SCO");
                    return AudioSystem.STREAM_BLUETOOTH_SCO;
                } else {
                    if (DEBUG_VOL)  Log.v(TAG, "getActiveStreamType: Forcing STREAM_VOICE_CALL");
                    return AudioSystem.STREAM_VOICE_CALL;
                }
            } else if (AudioSystem.isStreamActive(AudioSystem.STREAM_NOTIFICATION,
                    StreamOverride.sDelayMs) ||
                    AudioSystem.isStreamActive(AudioSystem.STREAM_RING,
                            StreamOverride.sDelayMs)) {
                if (DEBUG_VOL) Log.v(TAG, "getActiveStreamType: Forcing STREAM_NOTIFICATION");
                return AudioSystem.STREAM_NOTIFICATION;
            } else if (suggestedStreamType == AudioManager.USE_DEFAULT_STREAM_TYPE) {
                if (isAfMusicActiveRecently(StreamOverride.sDelayMs)) {
                    if (DEBUG_VOL) Log.v(TAG, "getActiveStreamType: forcing STREAM_MUSIC");
                    return AudioSystem.STREAM_MUSIC;
                } else {
                    if (DEBUG_VOL) Log.v(TAG,
                            "getActiveStreamType: using STREAM_NOTIFICATION as default");
                    return AudioSystem.STREAM_NOTIFICATION;
                }
            }
            break;
        }
        if (DEBUG_VOL) Log.v(TAG, "getActiveStreamType: Returning suggested type "
                + suggestedStreamType);
        return suggestedStreamType;
    }

把isAfMusicActiveRecently(StreamOverride.sDelayMs) 改为： isAfMusicActiveRecently(0) 或一个小的值。