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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