void checkThread() {
        if (mThread != Thread.currentThread()) {
            throw new CalledFromWrongThreadException(
                    "Only the original thread that created a view hierarchy can touch its views.");
        }
}

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;
    }

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

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();
              }
    }

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);
}

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");
       }
    }
}

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) {
            }
        }
}