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Android -- ViewRoot，关于子线程刷新UI
Android在4.0之后执行线程更新UI操作会报异常：CalledFromWrongThreadException：Only the original thread that created a view hierarchy can touch its views.那么你肯定能看到很多文章说android里子线程不能刷新UI。这句话不能说错，只是有些不太严谨。其实线程能否刷新UI的关键在于ViewRoot是否属于该线程。
首先，CalledFromWrongThreadException这个异常是有下面的代码抛出的：
void checkThread() { if (mThread != Thread.currentThread()) { throw new CalledFromWrongThreadException( "Only the original thread that created a view hierarchy can touch its views."); }}
该段代码出自 framework/base/core/java/android/view/ViewRoot.java
其次，看看RootView的构造函数：
public ViewRoot(Context context) { super(); if (MEASURE_LATENCY && lt == null) { lt = new LatencyTimer(100, 1000); } // For debug only //++sInstanceCount; // Initialize the statics when this class is first instantiated. This is // done here instead of in the static block because Zygote does not // allow the spawning of threads. getWindowSession(context.getMainLooper()); mThread = Thread.currentThread(); mLocation = new WindowLeaked(null); mLocation.fillInStackTrace(); mWidth = -1; mHeight = -1; mDirty = new Rect(); mTempRect = new Rect(); mVisRect = new Rect(); mWinFrame = new Rect(); mWindow = new W(this, context); mInputMethodCallback = new InputMethodCallback(this); mViewVisibility = View.GONE; mTransparentRegion = new Region(); mPreviousTransparentRegion = new Region(); mFirst = true; // true for the first time the view is added mAdded = false; mAttachInfo = new View.AttachInfo(sWindowSession, mWindow, this, this); mViewConfiguration = ViewConfiguration.get(context); mDensity = context.getResources().getDisplayMetrics().densityDpi; }
最后，我们看看ViewRoot.checkThread的调用顺序：
com.david.test.helloworld.MainActivity$TestThread2.run -> android.widget.TextView.setText -> android.widget.TextView.checkForRelayout -> android.view.View.invalidate -> android.view.ViewGroup.invalidateChild -> android.view.ViewRoot.invalidateChildInParent -> android.view.ViewRoot.invalidateChild -> android.view.ViewRoot.checkThread
到这里相信网友已经明白CalledFromWrongThreadException为什么出现了。那到底非主线程以外的线程能否刷新UI呢？答案当然是能，前提条件是它要拥有自己的ViewRoot。如果你要直接创建ViewRoot的实例的话，你会失望的发现不能找到这个类。那么我们要如何做呢？让我们用实例来说说吧，代码如下：
class TestThread1 extends Thread{ @Override public void run() { Looper.prepare(); TextView tx = new TextView(MainActivity.this); tx.setText("test11111111111111111"); WindowManager wm = MainActivity.this.getWindowManager(); WindowManager.LayoutParams params = new WindowManager.LayoutParams(250, 250, 200, 200, WindowManager.LayoutParams.FIRST_SUB_WINDOW, WindowManager.LayoutParams.TYPE_TOAST,PixelFormat.OPAQUE); wm.addView(tx, params); Looper.loop(); } }
MainActivity是建立android工程时生成的入口类，TestThread1是MainActivity的内部类。感兴趣的话，试试吧！看看是不是在屏幕上看到了"test11111111111111111"？
最后，说说那里创建了ViewRoot，这里：wm.addView(tx, params)。还是看看具体流程吧：
WindowManagerImpl.addView(View view, ViewGroup.LayoutParams params)
-> WindowManagerImpl.addView(View view, ViewGroup.LayoutParams params, boolean nest)
奥妙就在这里，具体看看代码吧！
private void addView(View view, ViewGroup.LayoutParams params, boolean nest){ if (Config.LOGV) Log.v("WindowManager", "addView view=" + view); if (!(params instanceof WindowManager.LayoutParams)) { throw new IllegalArgumentException( "Params must be WindowManager.LayoutParams"); } final WindowManager.LayoutParams wparams = (WindowManager.LayoutParams)params; ViewRoot root; View panelParentView = null; synchronized (this) { // Here‘s an odd/questionable case: if someone tries to add a // view multiple times, then we simply bump up a nesting count // and they need to remove the view the corresponding number of // times to have it actually removed from the window manager. // This is useful specifically for the notification manager, // which can continually add/remove the same view as a // notification gets updated. int index = findViewLocked(view, false); if (index >= 0) { if (!nest) { throw new IllegalStateException("View " + view + " has already been added to the window manager."); } root = mRoots[index]; root.mAddNesting++; // Update layout parameters. view.setLayoutParams(wparams); root.setLayoutParams(wparams, true); return; } // If this is a panel window, then find the window it is being // attached to for future reference. if (wparams.type >= WindowManager.LayoutParams.FIRST_SUB_WINDOW && wparams.type <= WindowManager.LayoutParams.LAST_SUB_WINDOW) { final int count = mViews != null ? mViews.length : 0; for (int i=0; i<count; i++) { if (mRoots[i].mWindow.asBinder() == wparams.token) { panelParentView = mViews[i]; } } } root = new ViewRoot(view.getContext()); root.mAddNesting = 1; view.setLayoutParams(wparams); if (mViews == null) { index = 1; mViews = new View[1]; mRoots = new ViewRoot[1]; mParams = new WindowManager.LayoutParams[1]; } else { index = mViews.length + 1; Object[] old = mViews; mViews = new View[index]; System.arraycopy(old, 0, mViews, 0, index-1); old = mRoots; mRoots = new ViewRoot[index]; System.arraycopy(old, 0, mRoots, 0, index-1); old = mParams; mParams = new WindowManager.LayoutParams[index]; System.arraycopy(old, 0, mParams, 0, index-1); } index--; mViews[index] = view; mRoots[index] = root; mParams[index] = wparams; // do this last because it fires off messages to start doing things root.setView(view, wparams, panelParentView);}
出自：frameworks/base/core/java/android/view/WindowManagerImpl.java
Ok，相信到了这里，大家都已经明白了：子线程是能够刷新UI的！！！
public class TestActivity extends Activity { Button btn = null; /** Called when the activity is first created. */ public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.main); btn = (Button) findViewById(R.id.Button01); TestThread2 t = new TestThread2(btn); t.start(); } class TestThread2 extends Thread { Button btn = null; public TestThread2(Button btn) { this.btn = btn; } @Override public void run() { btn.setText("TestThread2.run"); } }}
建立一个工程，将上述代码拷贝进去，运行看看吧! Btn的文本一定改变为"TestThread2.run"了。
那么这到底是怎么回事呢？当我发现这个问题时，也困惑了。经过一番调查后，真相大白。现在和大家分享一下。奥秘在于ViewRoot的建立时间，它是在ActivityThread.java的final void handleResumeActivity(IBinder token, boolean clearHide, boolean isForward)里创建的。
看看代码吧：
final void handleResumeActivity(IBinder token, boolean clearHide, boolean isForward) { // If we are getting ready to gc after going to the background, well // we are back active so skip it. unscheduleGcIdler(); ActivityRecord r = performResumeActivity(token, clearHide); if (r != null) { final Activity a = r.activity; if (localLOGV) Slog.v( TAG, "Resume " + r + " started activity: " + a.mStartedActivity + ", hideForNow: " + r.hideForNow + ", finished: " + a.mFinished); final int forwardBit = isForward ? WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION : 0; // If the window hasn‘t yet been added to the window manager, // and this guy didn‘t finish itself or start another activity, // then go ahead and add the window. boolean willBeVisible = !a.mStartedActivity; if (!willBeVisible) { try { willBeVisible = ActivityManagerNative.getDefault().willActivityBeVisible( a.getActivityToken()); } catch (RemoteException e) { } } if (r.window == null && !a.mFinished && willBeVisible) { r.window = r.activity.getWindow(); View decor = r.window.getDecorView(); decor.setVisibility(View.INVISIBLE); ViewManager wm = a.getWindowManager(); WindowManager.LayoutParams l = r.window.getAttributes(); a.mDecor = decor; l.type = WindowManager.LayoutParams.TYPE_BASE_APPLICATION; l.softInputMode |= forwardBit; if (a.mVisibleFromClient) { a.mWindowAdded = true; wm.addView(decor, l); } // If the window has already been added, but during resume // we started another activity, then don‘t yet make the // window visible. } else if (!willBeVisible) { if (localLOGV) Slog.v( TAG, "Launch " + r + " mStartedActivity set"); r.hideForNow = true; } // The window is now visible if it has been added, we are not // simply finishing, and we are not starting another activity. if (!r.activity.mFinished && willBeVisible && r.activity.mDecor != null && !r.hideForNow) { if (r.newConfig != null) { if (DEBUG_CONFIGURATION) Slog.v(TAG, "Resuming activity " + r.activityInfo.name + " with newConfig " + r.newConfig); performConfigurationChanged(r.activity, r.newConfig); r.newConfig = null; } if (localLOGV) Slog.v(TAG, "Resuming " + r + " with isForward=" + isForward); WindowManager.LayoutParams l = r.window.getAttributes(); if ((l.softInputMode & WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION) != forwardBit) { l.softInputMode = (l.softInputMode & (~WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION)) | forwardBit; if (r.activity.mVisibleFromClient) { ViewManager wm = a.getWindowManager(); View decor = r.window.getDecorView(); wm.updateViewLayout(decor, l); } } r.activity.mVisibleFromServer = true; mNumVisibleActivities++; if (r.activity.mVisibleFromClient) { r.activity.makeVisible(); } } r.nextIdle = mNewActivities; mNewActivities = r; if (localLOGV) Slog.v( TAG, "Scheduling idle handler for " + r); Looper.myQueue().addIdleHandler(new Idler()); } else { // If an exception was thrown when trying to resume, then // just end this activity. try { ActivityManagerNative.getDefault() .finishActivity(token, Activity.RESULT_CANCELED, null); } catch (RemoteException ex) { } }}
在Activity.onResume前，ViewRoot实例没有建立，所以没有ViewRoot.checkThread检查。而btn.setText时设定的文本却保留了下来，所以当ViewRoot真正去刷新界面时，就把"TestThread2.run"刷了出来！
注意onStart，因为onStart在oncreate->onStart->onResume过程中，子线程刷新UI没问题的，但是在onPause->onRestart->onStart过程中，就有问题了。
我是天王盖地虎的分割线
参考：http://blog.csdn.net/imyfriend/article/details/6877962
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