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