Android音量设置流程（android8.0）_android setstreamvolume

最近一直在解安卓audio部分的bug，其中最多的就是音量设置方面的问题，在这里把自己学习到的一些东西记录一下，以供参考。

安卓java部分的音量设置首先调用到AudioManager.java中
在这里有两种方法可以设置音量setStreamVolume和adjustStreamVolume
setStreamVolume：传入index直接设置音量值
adjustStreamVolume：传入direction，根据direction和获取到的步长设置音量。

由于我们用到的是setStreamVolume方法，所以通过setStreamVolume向下看，其实如果再去看adjustStreamVolume其实也是殊途同归。

1.setStreamVolume(AudioManager.java)

    public void setStreamVolume(int streamType, int index, int flags) {
        final IAudioService service = getService();
        try {
            service.setStreamVolume(streamType, index, flags, getContext().getOpPackageName());
        } catch (RemoteException e) {
            throw e.rethrowFromSystemServer();
        }
    }
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在此处调用了AudioService的setStreamVolume方法。此处传入的streamType和index分别为设置的流类型和音量值。

关于streamType：安卓在声音方面定义了多种流的类型以控制不同的声音（比如区分音乐和铃声以及系统提示音等）

    /** Used to identify the volume of audio streams for phone calls */
    public static final int STREAM_VOICE_CALL = AudioSystem.STREAM_VOICE_CALL;
    /** Used to identify the volume of audio streams for system sounds */
    public static final int STREAM_SYSTEM = AudioSystem.STREAM_SYSTEM;
    /** Used to identify the volume of audio streams for the phone ring */
    public static final int STREAM_RING = AudioSystem.STREAM_RING;
    /** Used to identify the volume of audio streams for music playback */
    public static final int STREAM_MUSIC = AudioSystem.STREAM_MUSIC;
    /** Used to identify the volume of audio streams for alarms */
    public static final int STREAM_ALARM = AudioSystem.STREAM_ALARM;
    /** Used to identify the volume of audio streams for notifications */
    public static final int STREAM_NOTIFICATION = AudioSystem.STREAM_NOTIFICATION;
    /** @hide Used to identify the volume of audio streams for phone calls when connected
     *        to bluetooth */
    public static final int STREAM_BLUETOOTH_SCO = AudioSystem.STREAM_BLUETOOTH_SCO;
    /** @hide Used to identify the volume of audio streams for enforced system sounds
     *        in certain countries (e.g camera in Japan) */
    public static final int STREAM_SYSTEM_ENFORCED = AudioSystem.STREAM_SYSTEM_ENFORCED;
    /** Used to identify the volume of audio streams for DTMF Tones */
    public static final int STREAM_DTMF = AudioSystem.STREAM_DTMF;
    /** @hide Used to identify the volume of audio streams exclusively transmitted through the
     *        speaker (TTS) of the device */
    public static final int STREAM_TTS = AudioSystem.STREAM_TTS;
    /** Used to identify the volume of audio streams for accessibility prompts */
    public static final int STREAM_ACCESSIBILITY = AudioSystem.STREAM_ACCESSIBILITY;
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我们在AudioManager.java中可以看到源码对各种流类型进行了解释，并且调用AudioSystem中的变量进行了赋值。

   /** Used to identify the default audio stream volume */
    public static final int STREAM_DEFAULT = -1;
    /** Used to identify the volume of audio streams for phone calls */
    public static final int STREAM_VOICE_CALL = 0;
    /** Used to identify the volume of audio streams for system sounds */
    public static final int STREAM_SYSTEM = 1;
    /** Used to identify the volume of audio streams for the phone ring and message alerts */
    public static final int STREAM_RING = 2;
    /** Used to identify the volume of audio streams for music playback */
    public static final int STREAM_MUSIC = 3;
    /** Used to identify the volume of audio streams for alarms */
    public static final int STREAM_ALARM = 4;
    /** Used to identify the volume of audio streams for notifications */
    public static final int STREAM_NOTIFICATION = 5;
    /** Used to identify the volume of audio streams for phone calls when connected on bluetooth */
    public static final int STREAM_BLUETOOTH_SCO = 6;
    /** Used to identify the volume of audio streams for enforced system sounds in certain
     * countries (e.g camera in Japan) */
    public static final int STREAM_SYSTEM_ENFORCED = 7;
    /** Used to identify the volume of audio streams for DTMF tones */
    public static final int STREAM_DTMF = 8;
    /** Used to identify the volume of audio streams exclusively transmitted through the
     *  speaker (TTS) of the device */
    public static final int STREAM_TTS = 9;
    /** Used to identify the volume of audio streams for accessibility prompts */
    public static final int STREAM_ACCESSIBILITY = 10;
    /**
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2.AudioService.java

    /** @see AudioManager#setStreamVolume(int, int, int) */
    public void setStreamVolume(int streamType, int index, int flags, String callingPackage) {
       //验证流类型是否合法
        if ((streamType == AudioManager.STREAM_ACCESSIBILITY) && !canChangeAccessibilityVolume()) {
            Log.w(TAG, "Trying to call setStreamVolume() for a11y without"
                    + " CHANGE_ACCESSIBILITY_VOLUME  callingPackage=" + callingPackage);
            return;
        }
        setStreamVolume(streamType, index, flags, callingPackage, callingPackage,
                Binder.getCallingUid());
    }
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此处再调用AudioService中的setStreamVolume的重载函数，该函数多传入了一个Binder.getCallingUid()，此处用到了binder的方法获取调用端的UID来判断调用者的身份，有兴趣的可以去了解binder相关的内容，此处影响不大不多做解释。

 private void setStreamVolume(int streamType, int index, int flags, String callingPackage,
            String caller, int uid) {
        <
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