AppWidget深入 AndroidMusic中的MediaAppWidgetProvider注释

怀着沉痛的心情发表这篇博客，不知道是不是我在CSDN上发表的最后一篇。这两天打算研究framework层和Linux的一些东西，顺便总结11月份和12月份的一些知识！！

 // 本来以为AppWidget是通过AIDL和MediaPlaybackService通信了，后来看了看不是
 // 如果一些基础的看不懂的话可以看看我上一篇
public class MediaAppWidgetProvider extends AppWidgetProvider {
    static final String TAG = "MusicAppWidgetProvider";
    
    public static final String CMDAPPWIDGETUPDATE = "appwidgetupdate";

    private static MediaAppWidgetProvider sInstance;
    
	// 单例模式，当你点击一个播放时所有的AppWidget都播放
    static synchronized MediaAppWidgetProvider getInstance() {
        if (sInstance == null) {
            sInstance = new MediaAppWidgetProvider();
        }
        return sInstance;
    }

	// 每次在创建AppWiget的时候或者到了更新频率的时候执行此函数
    @Override
    public void onUpdate(Context context, AppWidgetManager appWidgetManager, int[] appWidgetIds) {
	    // 设置AppWidget的一些默认设置，并且传递一些相关参数
        defaultAppWidget(context, appWidgetIds);
        
        // Send broadcast intent to any running MediaPlaybackService so it can
        // wrap around with an immediate update.
		// 发送广播消息通知正在运行的MediaPlaybackService以响应AppWidget
        Intent updateIntent = new Intent(MediaPlaybackService.SERVICECMD);
        updateIntent.putExtra(MediaPlaybackService.CMDNAME,
                MediaAppWidgetProvider.CMDAPPWIDGETUPDATE);
        updateIntent.putExtra(AppWidgetManager.EXTRA_APPWIDGET_IDS, appWidgetIds);
        updateIntent.addFlags(Intent.FLAG_RECEIVER_REGISTERED_ONLY);
		// 发送广播
        context.sendBroadcast(updateIntent);
    }
    
    /**
     * Initialize given widgets to default state, where we launch Music on default click
     * and hide actions if service not running.
     */
	 // 初始化widget的默认状态,在onUpdate()里就开始执行
    private void defaultAppWidget(Context context, int[] appWidgetIds) {
        final Resources res = context.getResources();
		// 获得AppWidget上的布局视图，然后通过views获得控件id
        final RemoteViews views = new RemoteViews(context.getPackageName(), R.layout.album_appwidget);
        
        views.setViewVisibility(R.id.title, View.GONE);
        views.setTextViewText(R.id.artist, res.getText(R.string.widget_initial_text));

		// 开始创建Widget的时候默认是不播放音乐的
        linkButtons(context, views, false /* not playing */);
        pushUpdate(context, appWidgetIds, views);
    }
    
	// 刷新Widget的状态
    private void pushUpdate(Context context, int[] appWidgetIds, RemoteViews views) {
        // Update specific list of appWidgetIds if given, otherwise default to all
        final AppWidgetManager gm = AppWidgetManager.getInstance(context);
        if (appWidgetIds != null) {
            gm.updateAppWidget(appWidgetIds, views);
        } else {
            gm.updateAppWidget(new ComponentName(context, this.getClass()), views);
        }
    }
    
    /**
     * Check against {@link AppWidgetManager} if there are any instances of this widget.
     */
	// 通过AppWidgetManager检查是否有存在AppWidget
    private boolean hasInstances(Context context) {	    
        AppWidgetManager appWidgetManager = AppWidgetManager.getInstance(context);
        int[] appWidgetIds = appWidgetManager.getAppWidgetIds(
                new ComponentName(context, this.getClass()));
        return (appWidgetIds.length > 0);
    }

    /**
     * Handle a change notification coming over from {@link MediaPlaybackService}
     */
	 // 这个函数是在MediaPlaybackService中调用,作用是检查播放状态是否发生变化
    void notifyChange(MediaPlaybackService service, String what) {
        if (hasInstances(service)) {
            if (MediaPlaybackService.META_CHANGED.equals(what) ||
                    MediaPlaybackService.PLAYSTATE_CHANGED.equals(what)) {
				// 如果状态有改变，那么更新AppWidget的状态
                performUpdate(service, null);
            }
        }
    }
    
    /**
     * Update all active widget instances by pushing changes 
     */
	// 更新所有AppWidget的状态
    void performUpdate(MediaPlaybackService service, int[] appWidgetIds) {
		// 获取所有的相关资源
        final Resources res = service.getResources();
		// RemoteViews用于获取AppWidget的相关控件Id，及更新相关控件
        final RemoteViews views = new RemoteViews(service.getPackageName(), R.layout.album_appwidget);
        
		// 通过MediaPlaybackService获取歌曲相关信息，歌曲名，歌手名，SD卡的状态
        CharSequence titleName = service.getTrackName();
        CharSequence artistName = service.getArtistName();
        CharSequence errorState = null;
        
        // Format title string with track number, or show SD card message
		// 获取扩展介质(一般指SD卡)状态以更新变量 errorState
		// 各种状态可以查看string中值自己确定
        String status = Environment.getExternalStorageState();
        if (status.equals(Environment.MEDIA_SHARED) ||
                status.equals(Environment.MEDIA_UNMOUNTED)) {
            if (android.os.Environment.isExternalStorageRemovable()) {
                errorState = res.getText(R.string.sdcard_busy_title);
            } else {
                errorState = res.getText(R.string.sdcard_busy_title_nosdcard);
            }
        } else if (status.equals(Environment.MEDIA_REMOVED)) {
            if (android.os.Environment.isExternalStorageRemovable()) {
                errorState = res.getText(R.string.sdcard_missing_title);
            } else {
                errorState = res.getText(R.string.sdcard_missing_title_nosdcard);
            }
        } else if (titleName == null) {
            errorState = res.getText(R.string.emptyplaylist);
        }
        
        if (errorState != null) {
            // Show error state to user
			// 如果SD卡状态不符，歌曲名的TextView隐藏，在艺术家TextView中显示errorState
            views.setViewVisibility(R.id.title, View.GONE);
            views.setTextViewText(R.id.artist, errorState);
            
        } else {
            // No error, so show normal titles
			// 正常显示
            views.setViewVisibility(R.id.title, View.VISIBLE);
            views.setTextViewText(R.id.title, titleName);
            views.setTextViewText(R.id.artist, artistName);
        }
        
        // Set correct drawable for pause state
		// 判断歌曲是否在播放，如果在播放是播放按钮显示为暂停，反之
        final boolean playing = service.isPlaying();
        if (playing) {
            views.setImageViewResource(R.id.control_play, R.drawable.ic_appwidget_music_pause);
        } else {
            views.setImageViewResource(R.id.control_play, R.drawable.ic_appwidget_music_play);
        }

        // Link actions buttons to intents
		// 根据Widget的布局事件，发送相应的Intent
        linkButtons(service, views, playing);
        
		// 更新所有AppWidget的状态
        pushUpdate(service, appWidgetIds, views);
    }

    /**
     * Link up various button actions using {@link PendingIntents}.
     * 
     * @param playerActive True if player is active in background, which means
     *            widget click will launch {@link MediaPlaybackActivity},
     *            otherwise we launch {@link MusicBrowserActivity}.
     */
    private void linkButtons(Context context, RemoteViews views, boolean playerActive) {
        // Connect up various buttons and touch events
        Intent intent;
		// 关于PendingIntent的相关说明请看我上一篇说明
        PendingIntent pendingIntent;
        
		// 获取MediaPlaybackService的组件名称
        final ComponentName serviceName = new ComponentName(context, MediaPlaybackService.class);
        
        if (playerActive) {
		    // 如果正在播放，则启动播放器Activity 即MediaPlaybackActivity
            intent = new Intent(context, MediaPlaybackActivity.class);
            pendingIntent = PendingIntent.getActivity(context,
                    0 /* no requestCode */, intent, 0 /* no flags */);
            views.setOnClickPendingIntent(R.id.album_appwidget, pendingIntent);
        } else {
			// 如果没有在播放则启动音乐浏览Activity 即MusicBrowserActivity
            intent = new Intent(context, MusicBrowserActivity.class);
            pendingIntent = PendingIntent.getActivity(context,
                    0 /* no requestCode */, intent, 0 /* no flags */);
            views.setOnClickPendingIntent(R.id.album_appwidget, pendingIntent);
        }
        
        intent = new Intent(MediaPlaybackService.TOGGLEPAUSE_ACTION);
        intent.setComponent(serviceName);
        pendingIntent = PendingIntent.getService(context,
                0 /* no requestCode */, intent, 0 /* no flags */);
        views.setOnClickPendingIntent(R.id.control_play, pendingIntent);
        
		// 获取动作，播放下一首歌曲
        intent = new Intent(MediaPlaybackService.NEXT_ACTION);
        intent.setComponent(serviceName);
        pendingIntent = PendingIntent.getService(context,
                0 /* no requestCode */, intent, 0 /* no flags */);
        views.setOnClickPendingIntent(R.id.control_next, pendingIntent);
    }
} 

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