Android 里子线程真的不能刷新UI吗？

如果你在网上搜索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。
让我们一起看看代码吧！
首先，CalledFromWrongThreadException这个异常是有下面的代码抛出的：
view source
    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？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);
}
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！！！

原文出处：http://blog.csdn.net/imyfriend/article/details/6877959