关于RemoteView的一点愚见（RemoteView在AppWidget中的工作流程）

前言

由于公司环境恶劣，小菜鸟我本来想画UML图来显示类与类之间的关系，可惜这个念头无法达成，也只好用Word文档来完成。待菜鸟我辞职了，再自己画上UML图和Gif动态图，来显示类于类之间的关系。所以如果有看客请谅解谅解本人的情况。

正文

之前复习了实现桌面小部件的时候,知道了其中运用到了RemoteView这个特殊的类。这个类，顾名思义就是远程视图。可以跨越进程的显示View。
这也就诞生了一些特殊应用，比如说，我们完成远程的显示视图。之前复习的AIDL也可以办到这一点，但是相比于直接使用RemoteView来说的确是复杂了那么一点。
不过RemoteView的缺点也是很明显的，为了提高RemoteView的进程间通讯的速度，RemoteView只支持一下几种View以及 ViewGroup：

Layout:
FrameLayout,LinearLayout,RelativeLayout，GridLayout
View：
AnalogClock，Button，Chronometer，ImageButton，ImageView，ProgressBar
，TextView，ViewFlipper，ListView，GridView，StackView，AdapterViewFilter，
ViewStub。

RemoteView在系统中主要运用的场景有：通知和桌面部件。之前对通知有了一定的研究了，这一次我们来分析桌面部件中RemoteView的应用。

首先，上一章我们提到了，在传输RemoteView之前,需要现在RemoteView中设置好相关的资源：

RemoteViews remoteView = new RemoteViews(context.getPackageName(),R.layout.appwidget);remoteView.setImageViewBitmap(R.id.imageView1,rotateBitmap(context,srcBitmap,degree));

那么我们就从方法setImageViewBitmap作为研究入口开始探索。

 public void setImageViewBitmap(int viewId, Bitmap bitmap) {setBitmap(viewId, "setImageBitmap", bitmap);}

这里很简单又调用了setBitmap

public void setBitmap(int viewId, String methodName, Bitmap value) {addAction(new BitmapReflectionAction(viewId, methodName, value));}

接着这里又调用了addAction的方法。在这里我先提一提RemoteView的工作原理，这样才好继续理解。
前文提到了，RemoteView同样可以向AIDL实现那样用View去访问远程进程，但是原理却完全不一样。AIDL是通过另一端实现.Stub内部类，在其中调用远程View数据来实现。

然而在RemoteView中，使用的策略是将实现了Parcelable接口的内部类Action传送到远程进程。

private abstract static class Action implements Parcelable

我们将操作的对象操作封装到Action中，接着在远程端依次操作Action，最后再一次跨进程回到RemoteView中让RemoteView去修改View。这么做的高明之处在于，省去了每一种View定义一个Binder接口,提高了程序的性能。

RemoteView中的set工作

了解RemoteView的原理之后，让我们看看addAction是如何实现的。

 private void addAction(Action a) {if (hasLandscapeAndPortraitLayouts()) {throw new RuntimeException("RemoteViews specifying separate landscape and portrait" +" layouts cannot be modified. Instead, fully configure the landscape and" +" portrait layouts individually before constructing the combined layout.");}if (mActions == null) {mActions = new ArrayList<Action>();}mActions.add(a);// update the memory usage statsa.updateMemoryUsageEstimate(mMemoryUsageCounter);}

从上面的源码，可以清晰的知道，addAction是将一系列Action封装到List中，这里就完成了Action的存储。

我们什么时候调用这里面的Action的list呢？我们之前写AppWidget接下来是这么写：

AppWidgetManager appWidgetManager = AppWidgetManager.getInstance(context);
appWidgetManager.updateAppWidget(new ComponentName(context, AppProvider.class),remoteView);

这么写其实就是AppWidgetManager调用AppWidgetProvider中的我们自己覆写的onUpdate()方法。

比如说，我们研究的setBitmap方法中，传入了方法名setImageBitmap，接着在addAction中通过反射调用这个方法。
且看在BitmapReflectionAction这个内部类中的构造类。

BitmapReflectionAction(int viewId, String methodName, Bitmap bitmap) {this.bitmap = bitmap;this.viewId = viewId;this.methodName = methodName;bitmapId = mBitmapCache.getBitmapId(bitmap);}

可以知道的，这里将相关的数据全部存入到这个继承了Action类的BitmapReflectionAction。

AppWidgetManager工作

下面是AppWidgetManager的updateAppWidget方法。

public void updateAppWidget(int[] appWidgetIds, RemoteViews views) {if (mService == null) {return;}try {mService.updateAppWidgetIds(mPackageName, appWidgetIds, views);}catch (RemoteException e) {throw new RuntimeException("system server dead?", e);}}

这里的AppWidgetManager将会在SystemServer中运行起来，这里并不讨论，先放出一点证据：

private static final String APPWIDGET_SERVICE_CLASS ="com.android.server.appwidget.AppWidgetService";
if (mPackageManager.hasSystemFeature(PackageManager.FEATURE_APP_WIDGETS)) {
mSystemServiceManager.startService(APPWIDGET_SERVICE_CLASS);
}

上面那个SystemServer究竟如何启动这里暂不做讨论，我们继续。

在上面一段函数中mService同样调用了 updateAppWidget，但是在这里指的是IAppWidgetService这个Binder接口。而AppWidgetServiceImpl就是实现这个接口。

class AppWidgetServiceImpl extends IAppWidgetService.Stub implements WidgetBackupProvider,OnCrossProfileWidgetProvidersChangeListener

AppWidgetServiceImpl中工作

我们看看这个类中updateAppWidget中调用了updateAppWidgetIds：

private void updateAppWidgetIds(String callingPackage, int[] appWidgetIds,RemoteViews views, boolean partially) {final int userId = UserHandle.getCallingUserId();if (appWidgetIds == null || appWidgetIds.length == 0) {return;}// Make sure the package runs under the caller uid.mSecurityPolicy.enforceCallFromPackage(callingPackage);final int bitmapMemoryUsage = (views != null) ? views.estimateMemoryUsage() : 0;if (bitmapMemoryUsage > mMaxWidgetBitmapMemory) {throw new IllegalArgumentException("RemoteViews for widget update exceeds"+ " maximum bitmap memory usage (used: " + bitmapMemoryUsage+ ", max: " + mMaxWidgetBitmapMemory + ")");}synchronized (mLock) {ensureGroupStateLoadedLocked(userId);final int N = appWidgetIds.length;for (int i = 0; i < N; i++) {final int appWidgetId = appWidgetIds[i];// NOTE: The lookup is enforcing security across users by making// sure the caller can only access widgets it hosts or provides.Widget widget = lookupWidgetLocked(appWidgetId,Binder.getCallingUid(), callingPackage);if (widget != null) {updateAppWidgetInstanceLocked(widget, views, partially);}}}}

上一段代码就是整个AppWidget的核心逻辑，上面做的事情有以下几件：
1.final int userId = UserHandle.getCallingUserId();获取uid(用来识别程序的ID)

2.mSecurityPolicy.enforceCallFromPackage(callingPackage);保证package中运行的是我们的程序

ensureGroupStateLoadedLocked(userId);做的事情有点多，简单说就是从uid获取到程序的路径以及相关文件的路径的，获取到Widget的实例，并且将Widget实例添加到ArrayList中。
updateAppWidgetInstanceLocked(widget, views, partially);widget的实例不为空的时候，则更新或者载入View。

我们先看看ensureGroupStateLoadedLocked(userId)做了什么：

private void ensureGroupStateLoadedLocked(int userId) {final int[] profileIds = mSecurityPolicy.getEnabledGroupProfileIds(userId);// Careful lad, we may have already loaded the state for some// group members, so check before loading and read only the// state for the new member(s).int newMemberCount = 0;final int profileIdCount = profileIds.length;for (int i = 0; i < profileIdCount; i++) {final int profileId = profileIds[i];if (mLoadedUserIds.indexOfKey(profileId) >= 0) {profileIds[i] = LOADED_PROFILE_ID;} else {newMemberCount++;}}if (newMemberCount <= 0) {return;}int newMemberIndex = 0;final int[] newProfileIds = new int[newMemberCount];for (int i = 0; i < profileIdCount; i++) {final int profileId = profileIds[i];if (profileId != LOADED_PROFILE_ID) {mLoadedUserIds.put(profileId, profileId);newProfileIds[newMemberIndex] = profileId;newMemberIndex++;}}clearProvidersAndHostsTagsLocked();loadGroupWidgetProvidersLocked(newProfileIds);loadGroupStateLocked(newProfileIds);}

在这里面工作的事情主要有两个：
1.loadGroupWidgetProvidersLocked(newProfileIds):将从xml中通过标签读取Provider到list中
2.loadGroupStateLocked(newProfileIds):通过读取之前定义的appwidget_info中的信息后，实例化widget加入到widget的list中。

检查完组件的状态之后，我们就应该做出更新的相应update动作,updateAppWidgetIds里面调用了updateAppWidgetInstanceLocked：

private void updateAppWidgetInstanceLocked(Widget widget, RemoteViews views,boolean isPartialUpdate) {if (widget != null && widget.provider != null&& !widget.provider.zombie && !widget.host.zombie) {if (isPartialUpdate && widget.views != null) {// For a partial update, we merge the new RemoteViews with the old.widget.views.mergeRemoteViews(views);} else {// For a full update we replace the RemoteViews completely.widget.views = views;}scheduleNotifyUpdateAppWidgetLocked(widget, views);}}

判断是否是部分widget的刷新，假如是部分刷新以及widget中实例为空，则操作其中RemoteView中的Action的list,接着交给scheduleNotifyUpdateAppWidgetLocked做核心工作。

private void scheduleNotifyUpdateAppWidgetLocked(Widget widget, RemoteViews updateViews) {if (widget == null || widget.provider == null || widget.provider.zombie|| widget.host.callbacks == null || widget.host.zombie) {return;}SomeArgs args = SomeArgs.obtain();args.arg1 = widget.host;args.arg2 = widget.host.callbacks;args.arg3 = updateViews;args.argi1 = widget.appWidgetId;mCallbackHandler.obtainMessage(CallbackHandler.MSG_NOTIFY_UPDATE_APP_WIDGET,args).sendToTarget();}

接下来就发送信息CallbackHandler.MSG_NOTIFY_UPDATE_APP_WIDGET，将工作交给mCallbackHandler这个Handler对象工作。
让我们看看这个Handler中handleMessage究竟完成了什么：

case MSG_NOTIFY_UPDATE_APP_WIDGET: {SomeArgs args = (SomeArgs) message.obj;Host host = (Host) args.arg1;IAppWidgetHost callbacks = (IAppWidgetHost) args.arg2;RemoteViews views = (RemoteViews) args.arg3;final int appWidgetId = args.argi1;args.recycle();handleNotifyUpdateAppWidget(host, callbacks, appWidgetId, views);} break;

可以知道我们最后将工作交给handleNotifyUpdateAppWidget()方法。

private void handleNotifyUpdateAppWidget(Host host, IAppWidgetHost callbacks,int appWidgetId, RemoteViews views) {try {callbacks.updateAppWidget(appWidgetId, views);} catch (RemoteException re) {synchronized (mLock) {Slog.e(TAG, "Widget host dead: " + host.id, re);host.callbacks = null;}}}

最后还是调用callbacks.updateAppWidget，而callbacks就是IAppWidgetHost这个Binder接口。

AppWidgetHost中的工作

而IAppWidgetHost的具体实现是AppWidgetHost中的Callback内部类，这个时候我们已经从SystemServer的进程中回到了我们的自己的进程，接着再通过消息机制，调用函数：

void updateAppWidgetView(int appWidgetId, RemoteViews views) {AppWidgetHostView v;synchronized (mViews) {v = mViews.get(appWidgetId);}if (v != null) {v.updateAppWidget(views);}}

可以看见这里就调用AppWidgetHostView里面的update方法。

AppWidgetHostView中的流程

可以说，接下来这个类就是真正执行更新的类。我们先去看看这个类中的update方法。
先声明AppWidgetHostView就是remoteView父容器，它是继承于FrameLayout,也就是说，它拥有FrameLayout中的特性，这样我们其实可以做很多事情了。

public class AppWidgetHostView extends FrameLayout

我们继续看看updateAppWidget中的方法：

public void updateAppWidget(RemoteViews remoteViews) {if (LOGD) Log.d(TAG, "updateAppWidget called mOld=" + mOld);boolean recycled = false;View content = null;Exception exception = null;// 插入以前的view到bitmap让我们可以办到淡入淡出效果if (CROSSFADE) {if (mFadeStartTime < 0) {if (mView != null) {final int width = mView.getWidth();final int height = mView.getHeight();try {mOld = Bitmap.createBitmap(width, height, Bitmap.Config.ARGB_8888);} catch (OutOfMemoryError e) {// we just won't do the fademOld = null;}if (mOld != null) {//mView.drawIntoBitmap(mOld);}}}}if (remoteViews == null) {if (mViewMode == VIEW_MODE_DEFAULT) {// We've already done this -- nothing to do.return;}content = getDefaultView();mLayoutId = -1;mViewMode = VIEW_MODE_DEFAULT;} else {// Prepare a local reference to the remote Context so we're ready to准备一个本地的应用给远程Context// inflate any requested LayoutParams.mRemoteContext = getRemoteContext();int layoutId = remoteViews.getLayoutId();// 大概是如果旧的布局和新的布局相匹配，则重新用回原来的remoteView// if (content == null && layoutId == mLayoutId) {try {remoteViews.reapply(mContext, mView, mOnClickHandler);content = mView;recycled = true;if (LOGD) Log.d(TAG, "was able to recycled existing layout");} catch (RuntimeException e) {exception = e;}}// Try normal RemoteView inflation尝试着加载远程视图remoteviewif (content == null) {try {content = remoteViews.apply(mContext, this, mOnClickHandler);if (LOGD) Log.d(TAG, "had to inflate new layout");} catch (RuntimeException e) {exception = e;}}mLayoutId = layoutId;mViewMode = VIEW_MODE_CONTENT;}if (content == null) {if (mViewMode == VIEW_MODE_ERROR) {// We've already done this -- nothing to do.return ;}Log.w(TAG, "updateAppWidget couldn't find any view, using error view", exception);content = getErrorView();mViewMode = VIEW_MODE_ERROR;}if (!recycled) {prepareView(content);addView(content);}if (mView != content) {removeView(mView);mView = content;}if (CROSSFADE) {if (mFadeStartTime < 0) {// if there is already an animation in progress, don't do anything --// the new view will pop in on top of the old one during the cross fade,// and that looks okay.mFadeStartTime = SystemClock.uptimeMillis();invalidate();}}}

上面的做的事情主要有两个：
1.如果过去的布局（layout）和新载入的布局（layout）相匹配则旧的重用，调用remoteView.reapply
2.如果过去的布局（layout）和新的不匹配,则调用remoteView.apply
我们这里只讨论第一次加载的情况，因此继续看apply方法。感兴趣的，可以起自行去看看reapply方法的内容。

回到RemoteView工作

在remoteViews.apply(mContext, this, mOnClickHandler)函数中调用了apply：

public View apply(Context context, ViewGroup parent, OnClickHandler handler) {RemoteViews rvToApply = getRemoteViewsToApply(context);View result;// RemoteViews may be built by an application installed in another// user. So build a context that loads resources from that user but// still returns the current users userId so settings like data / time formats// are loaded without requiring cross user persmissions.final Context contextForResources = getContextForResources(context);Context inflationContext = new ContextWrapper(context) {@Overridepublic Resources getResources() {return contextForResources.getResources();}@Overridepublic Resources.Theme getTheme() {return contextForResources.getTheme();}};LayoutInflater inflater = (LayoutInflater)context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);// Clone inflater so we load resources from correct context and// we don't add a filter to the static version returned by getSystemService.inflater = inflater.cloneInContext(inflationContext);inflater.setFilter(this);result = inflater.inflate(rvToApply.getLayoutId(), parent, false);rvToApply.performApply(result, parent, handler);return result;}

上面的代码可以看出我们是通过LayoutInfater动态加载RemoteView，加载布局文件可以动过rvToApply.getLayoutId()获得的。加载好文件之后，调用rvToApply.performApply(result, parent, handler)去执行具体的更新操作。

private void performApply(View v, ViewGroup parent, OnClickHandler handler) {if (mActions != null) {handler = handler == null ? DEFAULT_ON_CLICK_HANDLER : handler;final int count = mActions.size();for (int i = 0; i < count; i++) {Action a = mActions.get(i);a.apply(v, parent, handler);}}}

可以看到的是这个时候，我们获取list中的Action，在这里执行具体的对象对应具体的Action操作。这就完成了，我们不需要将View数据跨越进程的修改操作，而是在本线程进行真正的修改。
此时，存在队列中的ReflectionAction将会调用自身的apply，再通过反射去调用存在其中的方法名。

public void apply(View root, ViewGroup rootParent, OnClickHandler handler) {final View view = root.findViewById(viewId);if (view == null) return;Class<?> param = getParameterType();if (param == null) {throw new ActionException("bad type: " + this.type);}try {getMethod(view, this.methodName, param).invoke(view, wrapArg(this.value));} catch (ActionException e) {throw e;} catch (Exception ex) {throw new ActionException(ex);}}

这个时候我们发现将会调用ImageView中的setImageBitmap方法。
大致上，RemoteView的工作流程就完成了。同理在Notification中也是类似的思路，有兴趣的读者可以自己去看看。

RemoteView的使用以及意义

RemoteView可以作为一种简化后的可以跨进程UI更新的方案。下面是一个模拟通知框的远程修改UI的简单Demo，这一次我就借花献佛，借用任玉刚大神的Demo：

我们首先建立两个Activity，一个远程，一个本地，只需要在< activity >标签下添加属性“：remote”即可。
我们先看发送端：
DemoActivity_2.java:

public class DemoActivity_2 extends Activity {private static final String TAG = "DemoActivity_2";@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.demo_2);Log.d(TAG, "onCreate");Toast.makeText(this, getIntent().getStringExtra("sid"),Toast.LENGTH_SHORT).show();initView();}private void initView() {}public void onButtonClick(View v) {//加载RemoteView布局文件RemoteViews remoteViews = new RemoteViews(getPackageName(), R.layout.layout_simulated_notification);//加载资源文件remoteViews.setTextViewText(R.id.msg, "msg from process:" + Process.myPid());remoteViews.setImageViewResource(R.id.icon, R.drawable.icon1);//声明pendingintent是启动activityPendingIntent pendingIntent = PendingIntent.getActivity(this,0, new Intent(this, DemoActivity_1.class), PendingIntent.FLAG_UPDATE_CURRENT);PendingIntent openActivity2PendingIntent = PendingIntent.getActivity(this, 0, new Intent(this, DemoActivity_2.class), PendingIntent.FLAG_UPDATE_CURRENT);//给控件绑定pendingIntentremoteViews.setOnClickPendingIntent(R.id.item_holder, pendingIntent);remoteViews.setOnClickPendingIntent(R.id.open_activity2, openActivity2PendingIntent);Intent intent = new Intent(MyConstants.REMOTE_ACTION);intent.putExtra(MyConstants.EXTRA_REMOTE_VIEWS, remoteViews);//发送广播sendBroadcast(intent);}}

接下来是接收端MainActivity:

public class MainActivity extends Activity {private static final String TAG = "MainActivity";private LinearLayout mRemoteViewsContent;//类似像桌面小部件一样，做一个receiver来接受广播private BroadcastReceiver mRemoteViewsReceiver = new BroadcastReceiver() {@Overridepublic void onReceive(Context context, Intent intent) {RemoteViews remoteViews = intent.getParcelableExtra(MyConstants.EXTRA_REMOTE_VIEWS);if (remoteViews != null) {//发送来的remoteview不为空时更新updateUI(remoteViews);}}};@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);initView();}private void initView() {mRemoteViewsContent = (LinearLayout) findViewById(R.id.remote_views_content);IntentFilter filter = new IntentFilter(MyConstants.REMOTE_ACTION);registerReceiver(mRemoteViewsReceiver, filter);}private void updateUI(RemoteViews remoteViews) {
//        View view = remoteViews.apply(this, mRemoteViewsContent);//通过方法getIdentifier来加载相应名字的layout布局int layoutId = getResources().getIdentifier("layout_simulated_notification", "layout", getPackageName());View view = getLayoutInflater().inflate(layoutId, mRemoteViewsContent, false);//调用reapply更新remoteViewremoteViews.reapply(this, view);mRemoteViewsContent.addView(view);}@Overrideprotected void onDestroy() {unregisterReceiver(mRemoteViewsReceiver);super.onDestroy();}public void onButtonClick(View v) {if (v.getId() == R.id.button1) {Intent intent = new Intent(this, TestActivity.class);startActivity(intent);} else if (v.getId() == R.id.button2) {Intent intent = new Intent(this, DemoActivity_2.class);startActivity(intent);}}}

这样就完成一次跨进程的UI更新，是不是觉得比使用AIDL简单多了呢？注意这里要更新的话，必须使用remoteView支持的view和viewgroup

代码下载：Github

RemoteView流程图与机制

RemoteView机制：

RemoteView的View结构：

RemoteView工作流程图：

这样RemoteView的工作流程大致分析完了。当然里面不仅仅只有这么多，里面涉及到的Service不仅仅只有一个AppManagerService还有PackageService,UserService等等，更加详细的，读者感兴趣的可以去自行查看源码，这里只给出了大致脉络，以及主要流程。

感谢任玉刚大神的android开发探索艺术，帮助了我看代码。