【Android】Dialog异常CalledFromWrongThreadException深入分析

刺骨的言语ヽ痛彻心扉 2022-06-06 02:07 526阅读 0赞

### 问题 ###

在使用Dialog时，因为线程问题，在调用dismiss方法时出现了CalledFromWrongThreadException的Crash，如下：

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

抛出异常为`CalledFromWrongThreadException`，很明显第一反应就是出现了非ui线程进行了ui操作造成了此异常。通过分析工程代码，发现本质上是因为在非ui线程中创建了Dialog，而在主线程（即ui线程）中调用了`show()`以及`dismiss()`方法，我把问题模型写成测试代码如下：

public class MainActivity extends BaseActivity {
        
        private static final String TAG = "MainActivity test";
        private ProgressDialog dialog;
    
    
        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);
            EventBus.getDefault().register(this);
    
            new Thread(new Runnable() {
                @Override
                public void run() {
                    Looper.prepare();
    
                    //子线程中创建Dialog
                    dialog = new ProgressDialog(MainActivity.this);
                    dialog.setCanceledOnTouchOutside(true);
                    dialog.setOnCancelListener(new DialogInterface.OnCancelListener() {
                        @Override
                        public void onCancel(DialogInterface dialog) {
                            Log.d(TAG, "Dialog onCancel thread: " + getThreadInfo());
                        }
                    });
                    dialog.setOnDismissListener(new DialogInterface.OnDismissListener() {
                        @Override
                        public void onDismiss(DialogInterface dialog) {
                            Log.d(TAG, "Dialog onDismiss thread: " + getThreadInfo());
                        }
                    });
                    dialog.setMessage("正在加载...");
                    Log.d(TAG, "Dialog create thread: " + getThreadInfo());
    
                    Looper.loop();
                }
            }).start();
    
    
            Button btn = (Button) findViewById(R.id.btn_helloworld);
            btn.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {
                    //UI主线程中show，然后点击空白区域dismiss
                    dialog.show();
                    Log.d(TAG, "Dialog show thread: " + getThreadInfo());
                }
            });
        }
    
    
        /**
         * 输出线程信息
         */
        private String getThreadInfo(){
            return "[" + Thread.currentThread().getId() + "]" +
                    ((Looper.myLooper() == Looper.getMainLooper())? " is UI-Thread" : "");
        }
    }

就是Activity打开的时候，使用work子线程创建了一个Dialog，然后手动点击按钮的时候，显示Dialog。再点击空白处，dialog本应该dismiss的，但是直接crash了。抛出了`CalledFromWrongThreadException`的异常。

在上面的代码中，我顺便输出了Dialog每个操作的线程ID，同时会判定是不是ui主线程。我们来看看log：

> 10-26 16:11:07.836 7405-7652/com.cuc.myandroidtest D/MainActivity test: Dialog create thread: \[3953\]  
> 10-26 16:11:27.763 7405-7405/com.cuc.myandroidtest D/MainActivity test: Dialog show thread: \[1\] is UI-Thread  
> 10-26 16:11:35.642 7405-7652/com.cuc.myandroidtest D/MainActivity test: Dialog onCancel thread: \[3953\]
> 
> ——– beginning of crash

可以看到，以上出现的问题中执行Dialog操作的线程信息如下：

*  创建Dialog：work子线程
 *  show()：ui主线程
 *  cancel()：work子线程
 *  dismiss()：因为crash没有执行到，未知

如果说**只有创建这个控件的线程才能去更新该控件的内容**。那么在调用show方法的时候为什么不会crash，然后dismiss的时候才会崩溃？

另外，到底是不是所有的操作都必须放到ui线程中执行才对？带着疑问我们深入Dialog源码一看究竟。

### 源码分析 ###

我们先看Dialog的dismiss方法：

/**
     * Dismiss this dialog, removing it from the screen. This method can be
     * invoked safely from any thread.  Note that you should not override this
     * method to do cleanup when the dialog is dismissed, instead implement
     * that in {@link #onStop}.
     */
    @Override
    public void dismiss() {
        if (Looper.myLooper() == mHandler.getLooper()) {
            dismissDialog();
        } else {
            mHandler.post(mDismissAction);
        }
    }
    
    
    private final Runnable mDismissAction = new Runnable() {
        public void run() {
            dismissDialog();
        }
    };

我们先看注释，意思是**dismiss()这个函数可以在任意线程中调用，不用担心线程安全问题**。

很明显，dialog对于ui操作做了特别处理。如果当前执行dismiss操作的线程和mHandler所依附的线程不一致的话那么就会将dismiss操作丢到对应的mHandler的线程队列中等待执行。那么这个Handler又是哪里来的呢？

我们开始调查，可以看到`mHandler`对象是Dialog类中私有的，会在new Dialog的时候自动初始化：

public class Dialog implements DialogInterface, Window.Callback,
            KeyEvent.Callback, OnCreateContextMenuListener, Window.OnWindowDismissedCallback {
        
    
        private final Handler mHandler = new Handler();
    
        //...省略其余代码...
    }

可以分析得出，该mHandler直接关联的就是new Dialog的线程。也就能得出以下结论：

> **结论一**：最终真正执行`dismissDialog()`方法销毁Dialog的线程就是new Dialog的线程。

然后我们跟进去`dismissDialog()`看看到底如何销毁Dialog的：

void dismissDialog() {
        if (mDecor == null || !mShowing) {
            return;
        }
    
        if (mWindow.isDestroyed()) {
            Log.e(TAG, "Tried to dismissDialog() but the Dialog's window was already destroyed!");
            return;
        }
    
        try {
            mWindowManager.removeViewImmediate(mDecor);
        } finally {
            if (mActionMode != null) {
                mActionMode.finish();
            }
            mDecor = null;
            mWindow.closeAllPanels();
            onStop();
            mShowing = false;
    
            sendDismissMessage();
        }
    }

可以看出最终调用了`mWindowManager.removeViewImmediate(mDecor);`来销毁Dialog，继续跟进`removeViewImmediate()`这个方法。发现`mWindowManager`的类WindowManager是个abstract的类，我们来找找本尊。

#### **Dialog中mWindowManager对象的来历** ####

发现`mWindowManager`这个对象的初始化是在Dialog的构造函数中：

Dialog(Context context, int theme, boolean createContextThemeWrapper) {
        if (createContextThemeWrapper) {
            if (theme == 0) {
                TypedValue outValue = new TypedValue();
                context.getTheme().resolveAttribute(com.android.internal.R.attr.dialogTheme,
                        outValue, true);
                theme = outValue.resourceId;
            }
            mContext = new ContextThemeWrapper(context, theme);
        } else {
            mContext = context;
        }
    
        mWindowManager = (WindowManager)context.getSystemService(Context.WINDOW_SERVICE);
        Window w = PolicyManager.makeNewWindow(mContext);
        mWindow = w;
        w.setCallback(this);
        w.setOnWindowDismissedCallback(this);
        w.setWindowManager(mWindowManager, null, null);
        w.setGravity(Gravity.CENTER);
        mListenersHandler = new ListenersHandler(this);
    }

它是通过`context.getSystemService(Context.WINDOW_SERVICE);`得到的，这里的context肯定就是Activity了，我们去Activity中找`getSystemService()`函数：

@Override
    public Object getSystemService(@ServiceName @NonNull String name) {
        if (getBaseContext() == null) {
            throw new IllegalStateException(
                    "System services not available to Activities before onCreate()");
        }
    
        if (WINDOW_SERVICE.equals(name)) {
            return mWindowManager;
        } else if (SEARCH_SERVICE.equals(name)) {
            ensureSearchManager();
            return mSearchManager;
        }
        return super.getSystemService(name);
    }
    
    
    final void attach(Context context, ActivityThread aThread,
            Instrumentation instr, IBinder token, int ident,
            Application application, Intent intent, ActivityInfo info,
            CharSequence title, Activity parent, String id,
            NonConfigurationInstances lastNonConfigurationInstances,
            Configuration config, IVoiceInteractor voiceInteractor) {
    
        mWindow.setWindowManager((WindowManager)context.getSystemService(Context.WINDOW_SERVICE),
                mToken, mComponent.flattenToString(),
                (info.flags & ActivityInfo.FLAG_HARDWARE_ACCELERATED) != 0);
    
        mWindowManager = mWindow.getWindowManager();
    
        //...省略其他代码...
    }

我们看到`mWindowManager`这个对象是在Activity被创建之后调用attach函数的时候通过`mWindow.setWindowManager()`初始化的，而这个函数里干了什么呢？

public void setWindowManager(WindowManager wm, IBinder appToken, String appName,
            boolean hardwareAccelerated) {
        mAppToken = appToken;
        mAppName = appName;
        mHardwareAccelerated = hardwareAccelerated
                || SystemProperties.getBoolean(PROPERTY_HARDWARE_UI, false);
        if (wm == null) {
            wm = (WindowManager)mContext.getSystemService(Context.WINDOW_SERVICE);
        }
        mWindowManager = ((WindowManagerImpl)wm).createLocalWindowManager(this);
    }

可以看到`mWindowManager`这个对象最终来源于`WindowManagerImpl`类：

public final class WindowManagerImpl implements WindowManager {
        
        private final WindowManagerGlobal mGlobal = WindowManagerGlobal.getInstance();
        private final Display mDisplay;
        private final Window mParentWindow;
    
    
        public WindowManagerImpl createLocalWindowManager(Window parentWindow) {
            return new WindowManagerImpl(mDisplay, parentWindow);
        }
    
        @Override
        public void addView(View view, ViewGroup.LayoutParams params) {
            mGlobal.addView(view, params, mDisplay, mParentWindow);
        }
    
        @Override
        public void removeView(View view) {
            mGlobal.removeView(view, false);
        }
    
        @Override
        public void removeViewImmediate(View view) {
            mGlobal.removeView(view, true);
        }
    
        //...省略其余代码...
    }

在其中我们终于看到了`removeViewImmediate()`函数的身影，也就是说，在执行Dialog销毁的函数`dismissDialog()`中，最终调用了`mWindowManager.removeViewImmediate(mDecor);`来销毁Dialog。实际上调用的就是`WindowManagerImpl`实例中的`removeViewImmediate()`方法。

而它又调用的是`WindowManagerGlobal`的`removeView()`函数：

public void removeView(View view, boolean immediate) {
        if (view == null) {
            throw new IllegalArgumentException("view must not be null");
        }
    
        synchronized (mLock) {
            int index = findViewLocked(view, true);
            View curView = mRoots.get(index).getView();
            removeViewLocked(index, immediate);
            if (curView == view) {
                return;
            }
    
            throw new IllegalStateException("Calling with view " + view
                    + " but the ViewAncestor is attached to " + curView);
        }
    }
    
    
    private void removeViewLocked(int index, boolean immediate) {
        ViewRootImpl root = mRoots.get(index);
        View view = root.getView();
    
        if (view != null) {
            InputMethodManager imm = InputMethodManager.getInstance();
            if (imm != null) {
                imm.windowDismissed(mViews.get(index).getWindowToken());
            }
        }
        boolean deferred = root.die(immediate);
        if (view != null) {
            view.assignParent(null);
            if (deferred) {
                mDyingViews.add(view);
            }
        }
    }

注意这句`boolean deferred = root.die(immediate);`，其中root对象是个`ViewRootImpl`的实例，我们看看它的`die()`方法：

boolean die(boolean immediate) {
        // Make sure we do execute immediately if we are in the middle of a traversal or the damage
        // done by dispatchDetachedFromWindow will cause havoc on return.
        if (immediate && !mIsInTraversal) {
            doDie();
            return false;
        }
    
         //...省略其余代码...
    }
    
    
    
    void doDie() {
        checkThread();
    
        //...省略其余代码...
    }

最终，执行到了`ViewRootImpl`类的`doDie()`方法，这个方法的第一句就是`checkThread()`，根据[Android4.4DialogUI线程CalledFromWrongThreadExcection][Android4.4DialogUI_CalledFromWrongThreadExcection]这篇文章，我们知道最终抛出异常的位置就是是在`ViewRootImpl`代码中的`checkThread`函数：

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

也就是说，当调用Dialog的`dismiss()`方法时，Dialog会自动抛到new Dialog的线程中执行，就是上面代码中的`Thread.currentThread()`。之所以抛出这个异常是因为**ViewRootImpl本身的这个`mThread`和这个`Thread.currentThread()`（即new Dialog的线程）不是同一个线程**。然后我们看看这个ViewRootImpl本身的mThread的来源在何处。

#### **ViewRootImpl中mThread的来历** ####

在ViewRootImpl的构造函数中发现了mThread赋值的地方：

public ViewRootImpl(Context context, Display display) {
        mThread = Thread.currentThread();
    
        //...省略其余代码...
    }

那这个ViewRootImpl什么时候调用这个构造函数创建实例的呢？我们刚才在`WindowManagerGlobal`的`removeView()`函数中，看到了`root`对象是从`mRoots`对象中取出来的，而`mRoots`是一个`ArrayList<ViewRootImpl>`。

所以我们来`WindowManagerGlobal`中找找`mRoots.add()`的地方，发现是在它的`addView()`函数中创建了一个`ViewRootImpl`对象并添加到了`mRoots`这个list中：

public void addView(View view, ViewGroup.LayoutParams params,
            Display display, Window parentWindow) {
    
        //...省略其余代码.....
    
        ViewRootImpl root;
        synchronized (mLock) {
    
           //...省略其余代码.....
    
            root = new ViewRootImpl(view.getContext(), display);
            mRoots.add(root);
        }
    
        // 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;
        }
    }

而这个`addView`方法什么时候会调用呢？就是`WindowManagerImpl`。

就是刚才分析Dialog中`mWindowManager`对象的来历时，知道了它其实是`WindowManagerImpl`类的一个实例，WindowManagerImpl会通过`WindowManagerGlobal`的`removeView()`方法去实现removeView。同理，此处`WindowManagerGlobal`的`addView()`方法也是被WindowManagerImpl调用的。

我们在Dialog的源码中找一下`mWindowManager`对象调用`addView()`方法的地方，很让人惊喜，它竟然在Dialog的`show()`方法中出现了：

public void show() {
    
        //...省略其余代码.....
        onStart();
        mDecor = mWindow.getDecorView();
    
        try {
            mWindowManager.addView(mDecor, l);
            mShowing = true;
    
            sendShowMessage();
        } finally {
        }
    }

也就是说，Dialog的`show()`方法，会通过`mWindowManager.addView(mDecor, l);`创建一个`ViewRootImpl`的对象，这个对象会在创建的时候保存一个当前线程的Thread对象。也就是调用Dialog的`show()`方法的线程。

而在调用Dialog的`dismiss()`方法时，会首先把它抛到new Dialog的线程中执行，最后通过调用`mWindowManager.removeViewImmediate()`来销毁View，此时也就自然调用到了`ViewRootImpl`对象的`doDie()`方法，这个方法中会`checkThread();`，此时会检查当前线程（也就是调用new Dialog的线程）是不是创建`ViewRootImpl`的对象的线程（也就是Dialog的`show()`方法的线程）。

到这里，本文的bug根源也就找到了说通了。我们再来熟悉一下这个异常的场景。

*  创建Dialog：work子线程
 *  show()：ui主线程
 *  cancel()：work子线程
 *  dismiss()：因为crash没有执行到，未知（其实是抛到了work子线程）

现在就明确了，执行`show()`方法的时候`ViewRootImpl`没有`checkThread()`，所以不会出现crash。而在执行`dismiss()`的时候，它首先被抛到创建Dialog的线程中执行，而后真正销毁View时`ViewRootImpl`会`checkThread()`，保证addView的线程才能removeView。而在文章开头出错的例子中，Dialog的`show()`是在主线程执行，`new Dialog()`是在work子线程中执行的，所以抛出了`CalledFromWrongThreadException`的异常。

### 结论 ###

1.  Dialog的`dismiss()`会首先被抛到new Dialog的线程中执行。
2.  只要保证创建Dialog和`show()`方法在同一个线程中执行，无论是在放到ui线程还是work子线程都可以。

比如，把文章开头的例子中的`show()`方法同样放到work线程中，可以正常执行，输出log如下：

> 10-26 19:23:02.603 27689-27760/com.cuc.myandroidtest D/MainActivity test: Dialog create thread: \[4213\]  
> 10-26 19:23:02.686 27689-27760/com.cuc.myandroidtest D/MainActivity test: Dialog show thread: \[4213\]  
> 10-26 19:23:07.243 27689-27760/com.cuc.myandroidtest D/MainActivity test: Dialog onCancel thread: \[4213\]  
> 10-26 19:23:07.243 27689-27760/com.cuc.myandroidtest D/MainActivity test: Dialog onDismiss thread: \[4213\]

### 版本差异 ###

注意，本文的这个`CalledFromWrongThreadException`异常，是在4.4版本及以上才会出现的。具体区别可以参考这篇文章：[Android4.4DialogUI线程CalledFromWrongThreadExcection][Android4.4DialogUI_CalledFromWrongThreadExcection]

4.2中Dialog的dismissDialog和4.4中Dialog的dismissDialog区别如下：

//4.2中Dialog的dismissDialog
    try {
        mWindowManager.removeView(mDecor);
    }

//4.4中Dialog的dismissDialog
    try {
        mWindowManager.removeViewImmediate(mDecor);
    }

### 参考资料 ###

*  [Android4.4DialogUI线程CalledFromWrongThreadExcection][Android4.4DialogUI_CalledFromWrongThreadExcection]
 *  [Android异常：android.view.ViewRootImpl$CalledFromWrongThreadException: Only the original][Android_android.view.ViewRootImpl_CalledFromWrongThreadException_ Only the original]
 *  [Activity WMS ViewRootImpl三者关系（Activity创建窗口 按键分发等）][Activity WMS ViewRootImpl_Activity_]

[Android4.4DialogUI_CalledFromWrongThreadExcection]: https://my.oschina.net/qixiaobo025/blog/195396
[Android_android.view.ViewRootImpl_CalledFromWrongThreadException_ Only the original]: http://blog.csdn.net/qq_32059827/article/details/51689309
[Activity WMS ViewRootImpl_Activity_]: http://blog.csdn.net/kc58236582/article/details/52088224