Android裡子線程真的不能刷新UI嗎？

如果你在網上搜索CalledFromWrongThreadException：Only the original thread that created a view hierarchy can touch its views. 那麼你肯定能看到很多文章說Android裡子線程不能刷新UI。這句話不能說錯，只是有些不太嚴謹。其實線程能否刷新UI的關鍵在於ViewRoot是否屬於該線程。

       讓我們一起看看代碼吧！

       首先，CalledFromWrongThreadException這個異常是有下面的代碼拋出的：

    void checkThread() {

        if (mThread != Thread.currentThread()) {

            throw new CalledFromWrongThreadException(

                    "Only the original thread that created a view hierarchy can touch its views.");

        }

}

該段代碼出自 framework/base/core/java/android/view/ViewRoot.java

       其次，看看RootView的構造函數：

    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;

    }

       最後，我們看看ViewRoot.checkThread的調用順序：

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