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？呵呵，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);}