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.

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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!!

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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.