Audio系列之音量键

根据按键派发策略，派发顺序是Activity，然后是PhoneWindow。

如果activity需要处理音量键，可以在应用中重写Activity 的onKeyDown（）方法以截获音量键将其用作其他功能，如相机的音量快捷键。Activity#onKeyDown()方法源码

/**
     * Called when a key was pressed down and not handled by any of the views
     * inside of the activity. So, for example, key presses while the cursor
     * is inside a TextView will not trigger the event (unless it is a navigation
     * to another object) because TextView handles its own key presses.
     *
     * 
If the focused view didn't want this event, this method is called.
     *
     * 
The default implementation takes care of {@link KeyEvent#KEYCODE_BACK}
     * by calling {@link #onBackPressed()}, though the behavior varies based
     * on the application compatibility mode: for
     * {@link android.os.Build.VERSION_CODES#ECLAIR} or later applications,
     * it will set up the dispatch to call {@link #onKeyUp} where the action
     * will be performed; for earlier applications, it will perform the
     * action immediately in on-down, as those versions of the platform
     * behaved.
     *
     * 
Other additional default key handling may be performed
     * if configured with {@link #setDefaultKeyMode}.
     *
     * @return Return true to prevent this event from being propagated
     * further, or false to indicate that you have not handled
     * this event and it should continue to be propagated.
     * @see #onKeyUp
     * @see android.view.KeyEvent
     */
    public boolean onKeyDown(int keyCode, KeyEvent event)  {
        if (keyCode == KeyEvent.KEYCODE_BACK) {
            if (getApplicationInfo().targetSdkVersion
                    >= Build.VERSION_CODES.ECLAIR) {
                event.startTracking();
            } else {
                onBackPressed();
            }
            return true;
        }

        if (mDefaultKeyMode == DEFAULT_KEYS_DISABLE) {
            return false;
        } else if (mDefaultKeyMode == DEFAULT_KEYS_SHORTCUT) {
            Window w = getWindow();
            if (w.hasFeature(Window.FEATURE_OPTIONS_PANEL) &&
                    w.performPanelShortcut(Window.FEATURE_OPTIONS_PANEL, keyCode, event,
                            Menu.FLAG_ALWAYS_PERFORM_CLOSE)) {
                return true;
            }
            return false;
        } else {
            // Common code for DEFAULT_KEYS_DIALER & DEFAULT_KEYS_SEARCH_*
            boolean clearSpannable = false;
            boolean handled;
            if ((event.getRepeatCount() != 0) || event.isSystem()) {
                clearSpannable = true;
                handled = false;
            } else {
                handled = TextKeyListener.getInstance().onKeyDown(
                        null, mDefaultKeySsb, keyCode, event);
                if (handled && mDefaultKeySsb.length() > 0) {
                    // something useable has been typed - dispatch it now.

                    final String str = mDefaultKeySsb.toString();
                    clearSpannable = true;

                    switch (mDefaultKeyMode) {
                    case DEFAULT_KEYS_DIALER:
                        Intent intent = new Intent(Intent.ACTION_DIAL,  Uri.parse("tel:" + str));
                        intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
                        startActivity(intent);
                        break;
                    case DEFAULT_KEYS_SEARCH_LOCAL:
                        startSearch(str, false, null, false);
                        break;
                    case DEFAULT_KEYS_SEARCH_GLOBAL:
                        startSearch(str, false, null, true);
                        break;
                    }
                }
            }
            if (clearSpannable) {
                mDefaultKeySsb.clear();
                mDefaultKeySsb.clearSpans();
                Selection.setSelection(mDefaultKeySsb,0);
            }
            return handled;
        }
    }

Activity.onKeyDown()对一般按键有默认的一些处理，但是我们可以复写这个方法以处理我们想要处理的按键。如果不做处理，会让PhoneWindow的onKeyDown处理，下面是PhoneWindow#onKeyDown()方法源码：

protected boolean onKeyDown(int featureId, int keyCode, KeyEvent event) {
        /* ****************************************************************************
         * HOW TO DECIDE WHERE YOUR KEY HANDLING GOES.
         *
         * If your key handling must happen before the app gets a crack at the event,
         * it goes in PhoneWindowManager.
         *
         * If your key handling should happen in all windows, and does not depend on
         * the state of the current application, other than that the current
         * application can override the behavior by handling the event itself, it
         * should go in PhoneFallbackEventHandler.
         *
         * Only if your handling depends on the window, and the fact that it has
         * a DecorView, should it go here.
         * ****************************************************************************/

        final KeyEvent.DispatcherState dispatcher =
                mDecor != null ? mDecor.getKeyDispatcherState() : null;
        //Log.i(TAG, "Key down: repeat=" + event.getRepeatCount()
        //        + " flags=0x" + Integer.toHexString(event.getFlags()));

        switch (keyCode) {
            case KeyEvent.KEYCODE_VOLUME_UP:
            case KeyEvent.KEYCODE_VOLUME_DOWN:
            case KeyEvent.KEYCODE_VOLUME_MUTE: {
                int direction = 0;
                switch (keyCode) {
                    case KeyEvent.KEYCODE_VOLUME_UP:
                        direction = AudioManager.ADJUST_RAISE;
                        break;
                    case KeyEvent.KEYCODE_VOLUME_DOWN:
                        direction = AudioManager.ADJUST_LOWER;
                        break;
                    case KeyEvent.KEYCODE_VOLUME_MUTE:
                        direction = AudioManager.ADJUST_TOGGLE_MUTE;
                        break;
                }
                // If we have a session send it the volume command, otherwise
                // use the suggested stream.
                if (mMediaController != null) {
                    mMediaController.adjustVolume(direction, AudioManager.FLAG_SHOW_UI);
                } else {
                    MediaSessionLegacyHelper.getHelper(getContext()).sendAdjustVolumeBy(
                            mVolumeControlStreamType, direction,
                            AudioManager.FLAG_SHOW_UI | AudioManager.FLAG_VIBRATE
                                    | AudioManager.FLAG_FROM_KEY);
                }
                return true;
            }
            // These are all the recognized media key codes in
            // KeyEvent.isMediaKey()
            case KeyEvent.KEYCODE_MEDIA_PLAY:
            case KeyEvent.KEYCODE_MEDIA_PAUSE:
            case KeyEvent.KEYCODE_MEDIA_PLAY_PAUSE:
            case KeyEvent.KEYCODE_MUTE:
            case KeyEvent.KEYCODE_HEADSETHOOK:
            case KeyEvent.KEYCODE_MEDIA_STOP:
            case KeyEvent.KEYCODE_MEDIA_NEXT:
            case KeyEvent.KEYCODE_MEDIA_PREVIOUS:
            case KeyEvent.KEYCODE_MEDIA_REWIND:
            case KeyEvent.KEYCODE_MEDIA_RECORD:
            case KeyEvent.KEYCODE_MEDIA_FAST_FORWARD: {
                if (mMediaController != null) {
                    if (mMediaController.dispatchMediaButtonEvent(event)) {
                        return true;
                    }
                }
                return false;
            }

            case KeyEvent.KEYCODE_MENU: {
                onKeyDownPanel((featureId < 0) ? FEATURE_OPTIONS_PANEL : featureId, event);
                return true;
            }

            case KeyEvent.KEYCODE_BACK: {
                if (event.getRepeatCount() > 0) break;
                if (featureId < 0) break;
                // Currently don't do anything with long press.
                if (dispatcher != null) {
                    dispatcher.startTracking(event, this);
                }
                return true;
            }

        }

        return false;
    }

在PhoneWindow#onKeyDown（）中最终调用mMediaController.adjustVolume(direction, AudioManager.FLAG_SHOW_UI);调节音量，源码如下：

/**
     * Adjust the volume of the output this session is playing on. The direction
     * must be one of {@link AudioManager#ADJUST_LOWER},
     * {@link AudioManager#ADJUST_RAISE}, or {@link AudioManager#ADJUST_SAME}.
     * The command will be ignored if the session does not support
     * {@link VolumeProvider#VOLUME_CONTROL_RELATIVE} or
     * {@link VolumeProvider#VOLUME_CONTROL_ABSOLUTE}. The flags in
     * {@link AudioManager} may be used to affect the handling.
     *
     * @see #getPlaybackInfo()
     * @param direction The direction to adjust the volume in.
     * @param flags Any flags to pass with the command.
     */
    public void adjustVolume(int direction, int flags) {
        try {
            mSessionBinder.adjustVolume(direction, flags, mContext.getPackageName());
        } catch (RemoteException e) {
            Log.wtf(TAG, "Error calling adjustVolumeBy.", e);
        }
    }

在PhoneWindow#onKeyDown（）的flag有几个，其中AudioManager.FLAG_SHOW_UI告诉AudioService需要弹出一个音量控制板。而PhoneWindow#onKeyUp（）中对应的有AudioManager.FLAG_PLAY_SOUND,这是告诉AudioService松开音量键后有提示音，还有更多的flag自己去看。

每个流类型（StreamType）都有独立的音量值。而PhoneWindow最终设置的音量所属类型由PhoneWindow#mVolumeControlStream决定，在Activity#setVolumeControlStream（）可以设置绑定的PhoneWindow的mVolumeControlStream。

音量按键有三个值，分别是KeyEvent.KEYCODE_VOLUME_UP(加), KeyEvent.KEYCODE_VOLUME_DOWN（减）, KeyEvent.KEYCODE_VOLUME_MUTE（静音）

音量改变后，会发广播，下面是AudioService#setIndex（）源码：

mVolumeChanged = new Intent(AudioManager.VOLUME_CHANGED_ACTION);
mVolumeChanged.putExtra(AudioManager.EXTRA_VOLUME_STREAM_VALUE, index);
                mVolumeChanged.putExtra(AudioManager.EXTRA_PREV_VOLUME_STREAM_VALUE, oldIndex);
                mVolumeChanged.putExtra(AudioManager.EXTRA_VOLUME_STREAM_TYPE_ALIAS,
                        mStreamVolumeAlias[mStreamType]);
                sendBroadcastToAll(mVolumeChanged);

sendVolumnUpdate( )用于发送广播通知外界音量发生了变化.在使用setVolumeStream()设置音量的时候也会了发送广播.