来电振铃时按音量键静音

所以的键盘输入事件都是由PhoneWindowManager这个类来处理，最终是由interceptKeyBeforeQueueing方法处理

public int interceptKeyBeforeQueueing(KeyEvent event, int policyFlags, boolean isScreenOn) {
    .....
    switch (keyCode) {
            case KeyEvent.KEYCODE_VOLUME_DOWN:
            case KeyEvent.KEYCODE_VOLUME_UP:
            case KeyEvent.KEYCODE_VOLUME_MUTE: {
if (telephonyService != null) {
                        try {
                            if (telephonyService.isRinging()) {
                                // If an incoming call is ringing, either VOLUME key means
                                // "silence ringer".  We handle these keys here, rather than
                                // in the InCallScreen, to make sure we'll respond to them
                                // even if the InCallScreen hasn't come to the foreground yet.
                                // Look for the DOWN event here, to agree with the "fallback"
                                // behavior in the InCallScreen.
                                Log.i(TAG, "interceptKeyBeforeQueueing:"
                                      + " VOLUME key-down while ringing: Silence ringer!");

                                // Silence the ringer.  (It's safe to call this
                                // even if the ringer has already been silenced.)
                                telephonyService.silenceRinger();

                                // And *don't* pass this key thru to the current activity
                                // (which is probably the InCallScreen.)
                                result &= ~ACTION_PASS_TO_USER;
                                break;
                            }
   .......
}

主要是调用了ITelephony接口的silenceRinger()方法，ITelephony接口是由Phone进程中的PhoneInterfaceManager这个类实现并在Phone进程启动的时候向SystemManager（系统服务大总管）注册名称为“phone”的系统服务。silenceRinger这个方法就是发送了一个名为“CMD_SILENCE_RINGER”的message。

    public void silenceRinger() {
        if (DBG) log("silenceRinger...");
        // TODO: find a more appropriate permission to check here.
        // (That can probably wait till the big TelephonyManager API overhaul.
        // For now, protect this call with the MODIFY_PHONE_STATE permission.)
        enforceModifyPermission();
        sendRequestAsync(CMD_SILENCE_RINGER);
    }

处理CMD_SILENCE_RINGER消息如下：

  case CMD_SILENCE_RINGER:
        silenceRingerInternal();
        break;

最终是调用了silenceRingerInternal()方法。

    private void silenceRingerInternal() {
        if ((mCM.getState() == Phone.State.RINGING)
            && mApp.notifier.isRinging()) {
            // Ringer is actually playing, so silence it.
            if (DBG) log("silenceRingerInternal: silencing...");
            mApp.notifier.silenceRinger();
        }
    }

会调用mApp.notifier.silenceRinger()方法，mApp就是PhoneApp实例，notifier就是CallNotifier的实例，在silenceRinger方法中的实现其实就是调用了Ringer的stopRing()方法停止响铃的。

    void silenceRinger() {
        mSilentRingerRequested = true;
        if (DBG) log("stopRing()... (silenceRinger)");
        mRinger.stopRing();
    }

根据上面的分析再看一下当有来电振铃时按电源键时会做什么呢

......
case KeyEvent.KEYCODE_POWER: {
result &= ~ACTION_PASS_TO_USER;
if (down) {
    if (isScreenOn && !mPowerKeyTriggered
            && (event.getFlags() & KeyEvent.FLAG_FALLBACK) == 0) {
        mPowerKeyTriggered = true;
        mPowerKeyTime = event.getDownTime();
        interceptScreenshotChord();
    }

    ITelephony telephonyService = getTelephonyService();
    boolean hungUp = false;
    if (telephonyService != null) {
        try {
            if (telephonyService.isRinging()) {
                // Pressing Power while there's a ringing incoming
                // call should silence the ringer.
                telephonyService.silenceRinger();
.....
可以看到加粗的部分也是调用了ITelephony.silenceRinger()方法静音。