从源码分析 对 非UI线程不允许访问UI元素的 理解

做Android开发初期大部分开发者都会遇到下面这个问题.

android.view.viewroot$calledfromwrongthreadexception: only the original thread that created a view hierarchy can touch its views.

原因大半都是 由于在异步线程中视图 更新界面中的元素

百度一搜,可以搜到大量解决办法.

结果提炼一下: 大致就是讲 UI元素/视图元素 必须在 UI线程(主线程)中更新修改.

新手这样理解就够了.

但是仔细看这个Exception说明

only
the original thread that created a view hierarchy can touch its views.

里面并没有提到 主线程/UI 线程,只是提到,必须是创建

View
hierarchy

的线程

!注意区分

创建View的线程和创建View hierarchy的不一定是同一个线程,不要理解错了,下面在源码中具体分析

观察ViewRoot源码中抛出calledfromwrongthreadexception的位置

void checkThread() {

if (mThread != Thread.currentThread()) {

throw new CalledFromWrongThreadException(
"Only the original thread that created a view hierarchy can touch its views.");
}
}

可以注意到 这里只检查了 当前执行线程和 mThread是否为同一个线程

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();

由ViewRoot的构造函数可以看到,mThread被绑定在了 创建

viewRoot

的线程上.

当然大部分情况下,

ViewRoot

都是在主线程中创建的,所以在异步线程中修改view会造成checkThread失败.

mTextView.setText(“123”);为例子.

跟踪一下调用栈

-- 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

可以看到 view会不断遍历去获取 ViewParent

一个视图上的View遍历到最后 就是 获取到了 ViewRoot

(ViewRoot本身就是ViewParent的一个子类)

然后就会调用到上面提到的 checkThread()

到这里有一个结论:

UI修改操作确实会验证操作的线程

* * *

回到标题上来说, 因为 ViewRoot是由UI线程创建的,所以所有被添加到ViewRoot下的子View都必须在UI线程中修改,看起来也很合理.

那么,有可能在非UI线程中创建ViewRoot吗?

其实是可以的.

这里就涉及到ViewRoot和WindowManager之间的关系

demo代码如下:

package com.dk.asyncui;

import android.app.Activity;
import android.graphics.PixelFormat;
import android.os.Bundle;
import android.os.Looper;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.WindowManager;
import android.widget.Button;

public class MainActivity extends Activity {
private Button mButton ;
private WindowManager mWindowManager ;

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout. activity_main);

mWindowManager = (WindowManager) getSystemService("window" );

mButton = new Button(this );
mButton.setText( "HelloWorld");

Thread mthread = new Thread(new Runnable() {

@Override
public void run() {
Looper. prepare();
WindowManager.LayoutParams wmParams = new WindowManager.LayoutParams();
wmParams. type = WindowManager.LayoutParams.TYPE_APPLICATION ;
wmParams. format = PixelFormat. RGBA_8888;
wmParams. flags = WindowManager.LayoutParams.FLAG_NOT_TOUCH_MODAL ;
wmParams. width = 320;
wmParams. height = 140;
mWindowManager.addView(mButton , wmParams);
Looper. loop();

}
});
mthread.start();
mButton.setOnClickListener( new View.OnClickListener() {

@Override
public void onClick(View v) {

runOnUiThread( new Runnable() {
public void run() {
mButton.setText( "22222222");
}
});
}
});
}

}

可以看到,我在异步线程中将 mButton添加到 WindowManager中

查看 WindowManangerImpl中addView的代码：

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);
}
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 things
try {
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;
}
}

可以发现在addView的过程中 WindowMananger创建了 ViewRoot

这就是上面提到的在 异步线程中创建 ViewRoot

上面Demo代码中的 runOnUiThread是为了保证下面的

mButton.setText( “22222222” );

是在主线程中执行

可以看到,这段代码即使是在主线程中执行,依然抛出了CalledFromWrongThreadException

在特意构造的这个特殊场景下,主线程/UI线程中 不能更新在异步线程中”创建/添加”的UI元素!!

从源码和设计的角度上分析,我倾向于认为这其实是设计上的一个漏洞,我通过这个漏洞创建了一个非主线程绑定的ViewRoot对象,才会导致上面的结果.