AppWidget实现机制分析--应用更新时更新AppWidget深入分析_mappwidgethost 监听更新情况

前不久看了博客主 stonecao 关于AppWidget的两篇文章，在读本篇博客之前，请务必先读这两篇博客：
AppWidget实现机制分析–什么是桌面插件
AppWidget实现机制分析–launcher添加和删除appwidget深入分析
这篇文章是对这两篇文章的一个补充，主要是描述下当桌面挂件应用在更新之后，桌面挂件是怎么更新的

luancher注册监听
在launcher主界面(一个Activity)起来的时候，会在onCreate执行如下一段代码：
Launcher.java

    //mAppWidgetHost是launcher承载AppWidgetView的宿主
    mAppWidgetHost = new LauncherAppWidgetHost(this, APPWIDGET_HOST_ID);
    mAppWidgetHost.startListening();
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AppWidgetHost.java : startListening

    try {
           //mCallbacks是比较重要的回调，它是在AppWidgetHost初始化的时候创建的
           updatedIds = sService.startListening(mCallbacks, 
                          mContextOpPackageName,mHostId,updatedViews);
        }
        catch (RemoteException e) {
            throw new RuntimeException("system server dead?", e);
        }
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AppWidgetHost.java : Callbacks

static class Callbacks extends IAppWidgetHost.Stub {
        private final WeakReference<Handler> mWeakHandler;

        public Callbacks(Handler handler) {
            mWeakHandler = new WeakReference<>(handler);
        }
        // 更新Widget
        public void updateAppWidget(int appWidgetId, RemoteViews views) {
            if (isLocalBinder() && views != null) {
                views = views.clone();
            }
            Handler handler = mWeakHandler.get();
            if (handler == null) {
                return;
            }
            Message msg = handler.obtainMessage(HANDLE_UPDATE, appWidgetId, 0, views);
            msg.sendToTarget();
        }
        // 当桌面挂件应用的privider发生变化的时候，会回调该方法
        public void providerChanged(int appWidgetId, AppWidgetProviderInfo info) {
            if (isLocalBinder() && info != null) {
                info = info.clone();
            }
            Handler handler = mWeakHandler.get();
            if (handler == null) {
                return;
            }
            Message msg = handler.obtainMessage(HANDLE_PROVIDER_CHANGED,
                    appWidgetId, 0, info);
            msg.sendToTarget();
        }

        public void providersChanged() {
            Handler handler = mWeakHandler.get();
            if (handler == null) {
                return;
            }
            handler.obtainMessage(HANDLE_PROVIDERS_CHANGED).sendToTarget();
        }

        public void viewDataChanged(int appWidgetId, int viewId) {
            Handler handler = mWeakHandler.get();
            if (handler == null) {
                return;
            }
            Message msg = handler.obtainMessage(HANDLE_VIEW_DATA_CHANGED,
                    appWidgetId, viewId);
            msg.sendToTarget();
        }

        private static boolean isLocalBinder() {
            
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