让我们看个实例吧：

package com.david.test.helloworld;

import android.os.Bundle;

import android.widget.Button;

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