Android导航栏音量调节

导航栏音量加减其实质是模拟实体按键

android系统定义了音量up和down的keyevent,如下：
./core/java/android/view/KeyEvent.java

    /** Key code constant: Volume Up key.
     * Adjusts the speaker volume up. */
    public static final int KEYCODE_VOLUME_UP       = 24;                                                                                         
    /** Key code constant: Volume Down key.
     * Adjusts the speaker volume down. */
    public static final int KEYCODE_VOLUME_DOWN     = 25;

事件被触发的代码如下：
./services/core/java/com/android/server/policy/PhoneWindowManager.java

        int flags = AudioManager.FLAG_SHOW_UI | AudioManager.FLAG_PLAY_SOUND
                | AudioManager.FLAG_FROM_KEY;
        String pkgName = mContext.getOpPackageName();
        switch (keyCode) {
            case KeyEvent.KEYCODE_VOLUME_UP:   
                try {
                    getAudioService().adjustSuggestedStreamVolume(AudioManager.ADJUST_RAISE,
                            AudioManager.USE_DEFAULT_STREAM_TYPE, flags, pkgName, TAG);
                } catch (Exception e) {
                    Log.e(TAG, "Error dispatching volume up in dispatchTvAudioEvent.", e);
                }
                break;
            case KeyEvent.KEYCODE_VOLUME_DOWN: 
                try {
                    getAudioService().adjustSuggestedStreamVolume(AudioManager.ADJUST_LOWER,
                            AudioManager.USE_DEFAULT_STREAM_TYPE, flags, pkgName, TAG);
                } catch (Exception e) {
                    Log.e(TAG, "Error dispatching volume down in dispatchTvAudioEvent.", e);
                }
                break;

最终通过AudioService去设置音量大小

因此在导航栏创建虚拟音量键时，需要在xml中加上keyCode值，如下

<com.android.systemui.statusbar.policy.KeyButtonView
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:systemui="http://schemas.android.com/apk/res-auto"
    android:id="@+id/volume_up"
    android:layout_width="@dimen/navigation_key_width"
    android:layout_height="match_parent"
    android:layout_weight="0"
    systemui:keyCode="24"
    android:scaleType="center"
    android:contentDescription="@string/accessibility_home"
    android:paddingStart="@dimen/navigation_key_padding"
    android:paddingEnd="@dimen/navigation_key_padding"
    />

<com.android.systemui.statusbar.policy.KeyButtonView
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:systemui="http://schemas.android.com/apk/res-auto"
    android:id="@+id/volume_down"
    android:layout_width="@dimen/navigation_key_width"
    android:layout_height="match_parent"
    android:layout_weight="0"
    systemui:keyCode="25"
    android:scaleType="center"
    android:contentDescription="@string/accessibility_home"
    android:paddingStart="@dimen/navigation_key_padding"
    android:paddingEnd="@dimen/navigation_key_padding"
    />