Android UI操作并不是线程安全的，并且这些操作必须在UI线程执行

Activity.runOnUiThread(Runnable)
View.post(Runnable)
View.postDelayed(Runnable, long)
Handler

采用一个专门的线程从队列中抽取事件，并把他们转发给应用程序定义的事件处理器

public void postInvalidate() {postInvalidateDelayed(0);
}public void postInvalidateDelayed(long delayMilliseconds) {// We try only with the AttachInfo because there's no point in invalidating// if we are not attached to our windowif (mAttachInfo != null) {Message msg = Message.obtain();msg.what = AttachInfo.INVALIDATE_MSG;msg.obj = this;mAttachInfo.mHandler.sendMessageDelayed(msg, delayMilliseconds);}
}

android.view.ViewRoot$CalledFromWrongThreadException: Only the original thread that created a view hierarchy can touch its views.

Activity.runOnUiThread(Runnable)
View.post(Runnable)
View.postDelayed(Runnable, long)
Handler

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

public ViewRootImpl(Context context, Display display) {mContext = context;mWindowSession = WindowManagerGlobal.getWindowSession();mDisplay = display;mBasePackageName = context.getBasePackageName();mDisplayAdjustments = display.getDisplayAdjustments();mThread = Thread.currentThread();......
}

@Override
public void invalidateChild(View child, Rect dirty) {invalidateChildInParent(null, dirty);
}@Override
public ViewParent invalidateChildInParent(int[] location, Rect dirty) {checkThread();......
}

class NonUiThread extends Thread{@Overridepublic void run() {Looper.prepare();TextView tx = new TextView(MainActivity.this);tx.setText("non-UiThread update textview");WindowManager windowManager = MainActivity.this.getWindowManager();WindowManager.LayoutParams params = new WindowManager.LayoutParams(
             200, 200, 200, 200, WindowManager.LayoutParams.FIRST_SUB_WINDOW,WindowManager.LayoutParams.TYPE_TOAST,PixelFormat.OPAQUE);windowManager.addView(tx, params); Looper.loop();}}

@Override
public void addView(@NonNull View view, @NonNull ViewGroup.LayoutParams params) {applyDefaultToken(params);mGlobal.addView(view, params, mDisplay, mParentWindow);
}

private final WindowManagerGlobal mGlobal = WindowManagerGlobal.getInstance();

public void addView(View view, ViewGroup.LayoutParams params,Display display, Window parentWindow) {if (view == null) {throw new IllegalArgumentException("view must not be null");}if (display == null) {throw new IllegalArgumentException("display must not be null");}if (!(params instanceof WindowManager.LayoutParams)) {throw new IllegalArgumentException("Params must be WindowManager.LayoutParams");}final WindowManager.LayoutParams wparams = (WindowManager.LayoutParams) params;if (parentWindow != null) {parentWindow.adjustLayoutParamsForSubWindow(wparams);} else {// If there's no parent, then hardware acceleration for this view is// set from the application's hardware acceleration setting.final Context context = view.getContext();if (context != null&& (context.getApplicationInfo().flags& ApplicationInfo.FLAG_HARDWARE_ACCELERATED) != 0) {wparams.flags |= WindowManager.LayoutParams.FLAG_HARDWARE_ACCELERATED;}}ViewRootImpl root;View panelParentView = null;synchronized (mLock) {// Start watching for system property changes.if (mSystemPropertyUpdater == null) {mSystemPropertyUpdater = new Runnable() {@Override public void run() {synchronized (mLock) {for (int i = mRoots.size() - 1; i >= 0; --i) {mRoots.get(i).loadSystemProperties();}}}};SystemProperties.addChangeCallback(mSystemPropertyUpdater);}int index = findViewLocked(view, false);if (index >= 0) {if (mDyingViews.contains(view)) {// Don't wait for MSG_DIE to make it's way through root's queue.
                    mRoots.get(index).doDie();} else {throw new IllegalStateException("View " + view+ " has already been added to the window manager.");}// The previous removeView() had not completed executing. Now it has.
            }// 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.size();for (int i = 0; i < count; i++) {if (mRoots.get(i).mWindow.asBinder() == wparams.token) {panelParentView = mViews.get(i);}}}root = new ViewRootImpl(view.getContext(), display);view.setLayoutParams(wparams);mViews.add(view);mRoots.add(root);mParams.add(wparams);}// do this last because it fires off messages to start doing thingstry {root.setView(view, wparams, panelParentView);} catch (RuntimeException e) {// BadTokenException or InvalidDisplayException, clean up.synchronized (mLock) {final int index = findViewLocked(view, false);if (index >= 0) {removeViewLocked(index, true);}}throw e;}}

final void handleResumeActivity(IBinder token,boolean clearHide, boolean isForward, boolean reallyResume) {// If we are getting ready to gc after going to the background, well// we are back active so skip it.
        unscheduleGcIdler();mSomeActivitiesChanged = true;// TODO Push resumeArgs into the activity for considerationActivityClientRecord r = performResumeActivity(token, clearHide);......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;}......
}