【Android】【Dialog】CalledFromWrongThreadException 深入分析

先上结论

因为在非 UI 线程中创建了 Dialog，而在 UI 线程中调用了 show() 方法

问题还原

在使用 dialog 的时候，因为线程问题，在调用 dismiss() 方法的时候，出现 crash，即 CalledFromWrongThreadException，堆栈信息如下：

threadName:main,threadPriority:5,threadState:RUNNABLE
Caused by: android.view.ViewRootImpl$CalledFromWrongThreadException: Only the original thread that created a view hierarchy can touch its views.
	at android.view.ViewRootImpl.checkThread(ViewRootImpl.java:7020)
	at android.view.ViewRootImpl.doDie(ViewRootImpl.java:6035)
	at android.view.ViewRootImpl.die(ViewRootImpl.java:6020)
	at android.view.WindowManagerGlobal.removeViewLocked(WindowManagerGlobal.java:486)
	at android.view.WindowManagerGlobal.removeView(WindowManagerGlobal.java:424)
	at android.view.WindowManagerImpl.removeViewImmediate(WindowManagerImpl.java:126)
	at android.app.Dialog.dismissDialog(Dialog.java:363)
	....省略部分堆栈

关键：Caused by: android.view.ViewRootImpl$CalledFromWrongThreadException: Only the original thread that created a view hierarchy can touch its views. 显然，大家的第一反应，觉得开发在非 ui 线程进行了 ui 操作。

Demo 还原

public class MainActivity extends BaseActivity {
    private static final String TAG = "MainActivity";
    private ProgressDialog dialog;

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

        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("zhangrr", "Dialog onCancel thread: " + getThreadInfo());
                    }
                });
                dialog.setOnDismissListener(new DialogInterface.OnDismissListener() {
                    @Override
                    public void onDismiss(DialogInterface dialog) {
                        Log.d("zhangrr", "Dialog onDismiss thread: " + getThreadInfo());
                    }
                });
                dialog.setMessage("dialog test");
                Log.d("zhangrr", "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 线程中 show，然后点击空白区域 dismiss
                dialog.show();
                Log.d("zhangrr", "Dialog show thread: " + getThreadInfo());
            }
        });
    }

    private String getThreadInfo(){
        return "[" + Thread.currentThread().getId() + "]" +
                ((Looper.myLooper() == Looper.getMainLooper())? " is UI-Thread" : "");
    }
}

demo 很简单，在 Activity 创建的时候，在子线程中创建了一个 ProgressDialog 实例，在 UI 线程显示 Dialog。再点击界面空白处，dismiss Dialog 结果直接 crash 了，抛出了 CalledFromWrongThreadException 的异常。我们来看看打印的 log：

Dialog create thread: [3953] 
Dialog show thread: [1] is UI-Thread
Dialog onCancel thread: [3953]
..crash...如上堆栈信息

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？

再来根据堆栈信息，我们将深入源码一步步的一探究竟

threadName:main,threadPriority:5,threadState:RUNNABLE
Caused by: android.view.ViewRootImpl$CalledFromWrongThreadException: Only the original thread that created a view hierarchy can touch its views.
	at android.view.ViewRootImpl.checkThread(ViewRootImpl.java:7020)
	at android.view.ViewRootImpl.doDie(ViewRootImpl.java:6035)
	at android.view.ViewRootImpl.die(ViewRootImpl.java:6020)
	at android.view.WindowManagerGlobal.removeViewLocked(WindowManagerGlobal.java:486)
	at android.view.WindowManagerGlobal.removeView(WindowManagerGlobal.java:424)
	at android.view.WindowManagerImpl.removeViewImmediate(WindowManagerImpl.java:126)
	at android.app.Dialog.dismissDialog(Dialog.java:363)
	....省略部分堆栈

源码还原

根据上面的出错堆栈信息，我们从下往上一步步追踪源码：

1、Dialog.dismiss()

	/**
     * 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);
        }
    }

官方的解释是，dismiss() 方法可以在任意线程中调用，不用担心线程安全问题。也就是说，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 所在的线程队列中等待执行。那么这个 mHandler 又是哪里来的呢？

public class Dialog implements DialogInterface, Window.Callback,
        KeyEvent.Callback, OnCreateContextMenuListener, Window.OnWindowDismissedCallback {

		private final Handler mHandler = new Handler();
}

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。因此，可以断定，该 mHandler 直接关联的就是我们 new Dialog() 即创建 Dialog 的线程。也就是说，最终真正执行 dismissDialog() 方法销毁 Dialog 的线程也是 new Dialog() 的线程。

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();
        }
    }

可以发现最终调用了 mWindowManager.removeViewImmediate(mDecor); 来销毁 Dialog，继续跟进 removeViewImmediate() 这个方法

2、WindowManagerImpl.removeViewImmediate()

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

Dialog(@NonNull Context context, @StyleRes int themeResId, boolean createContextThemeWrapper) {
        /* 省略部分代码 */
        mWindowManager = (WindowManager) context.getSystemService(Context.WINDOW_SERVICE);

        final Window w = new PhoneWindow(mContext);
        mWindow = w;
        w.setCallback(this);
        w.setOnWindowDismissedCallback(this);
        w.setOnWindowSwipeDismissedCallback(() -> {
            if (mCancelable) {
                cancel();
            }
        });
        w.setWindowManager(mWindowManager, null, null);
        w.setGravity(Gravity.CENTER);

        mListenersHandler = new ListenersHandler(this);
    }

mWindowManager 是通过 context.getSystemService(Context.WINDOW_SERVICE); 得到的，再看 Activity 中的 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...
}

可以发现 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。

/**
     * Set the window manager for use by this Window to, for example,
     * display panels.  This is <em>not</em> used for displaying the
     * Window itself -- that must be done by the client.
     *
     * @param wm The window manager for adding new windows.
     */
    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);
    }

发现 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 Context mContext;
    private final Window mParentWindow;

    public WindowManagerImpl createLocalWindowManager(Window parentWindow) {
        return new WindowManagerImpl(mContext, parentWindow);
    }

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

    @Override
    public void removeView(View view) {
        mGlobal.removeView(view, false);
    }

    @Override
    public void removeViewImmediate(View view) {
        mGlobal.removeView(view, true);
    }
}

终于！！！看到了 removeViewImmediate() 函数的身影。因此，可以得出结论，在执行 Dialog 销毁的函数 dismissDialog() 时，最终调用了 mWindowManager.removeViewImmediate(mDecor); 来销毁Dialog，即 WindowManagerImpl 实例中的 removeViewImmediate() 方法。

再来插播一下之前的堆栈信息

threadName:main,threadPriority:5,threadState:RUNNABLE
Caused by: android.view.ViewRootImpl$CalledFromWrongThreadException: Only the original thread that created a view hierarchy can touch its views.
	at android.view.ViewRootImpl.checkThread(ViewRootImpl.java:7020)
	at android.view.ViewRootImpl.doDie(ViewRootImpl.java:6035)
	at android.view.ViewRootImpl.die(ViewRootImpl.java:6020)
	at android.view.WindowManagerGlobal.removeViewLocked(WindowManagerGlobal.java:486)
	at android.view.WindowManagerGlobal.removeView(WindowManagerGlobal.java:424)
	at android.view.WindowManagerImpl.removeViewImmediate(WindowManagerImpl.java:126)
	at android.app.Dialog.dismissDialog(Dialog.java:363)
	....省略部分堆栈

3、WindowManagerGlobal.removeView()

现在到了第3步，WindowManagerImpl 实例中的 removeViewImmediate() 方法，实际上调用的是 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);
        }
    }

4、WindowManagerGlobal.removeViewLocked()

马不停蹄，我们追踪到第4步了,

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);
            }
        }
    }

注意这两行代码：root 对象其实是个 ViewRootImpl 的实例，因此我们将追一下它的 die() 方法：

ViewRootImpl root = mRoots.get(index);
boolean deferred = root.die(immediate);

再来插播一下之前的堆栈信息

threadName:main,threadPriority:5,threadState:RUNNABLE
Caused by: android.view.ViewRootImpl$CalledFromWrongThreadException: Only the original thread that created a view hierarchy can touch its views.
	at android.view.ViewRootImpl.checkThread(ViewRootImpl.java:7020)
	at android.view.ViewRootImpl.doDie(ViewRootImpl.java:6035)
	at android.view.ViewRootImpl.die(ViewRootImpl.java:6020)
	at android.view.WindowManagerGlobal.removeViewLocked(WindowManagerGlobal.java:486)
	at android.view.WindowManagerGlobal.removeView(WindowManagerGlobal.java:424)
	at android.view.WindowManagerImpl.removeViewImmediate(WindowManagerImpl.java:126)
	at android.app.Dialog.dismissDialog(Dialog.java:363)
	....省略部分堆栈

5、ViewRootImpl.die()

	/**
     * @param immediate True, do now if not in traversal. False, put on queue and do later.
     * @return True, request has been queued. False, request has been completed.
     */
    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;
        }
    }

最终，执行到了 ViewRootImpl 类的 doDie() 方法。

6、ViewRootImpl.doDie()

马上就要拨开云雾见天日了，顶住顶住！！！

	void doDie() {
        checkThread();
        // 省略代码
    }

根据堆栈信息，我们知道最终抛出异常的位置就在 ViewRootImpl 代码中的 checkThread() 函数。

再来插播一下之前的堆栈信息

threadName:main,threadPriority:5,threadState:RUNNABLE
Caused by: android.view.ViewRootImpl$CalledFromWrongThreadException: Only the original thread that created a view hierarchy can touch its views.
	at android.view.ViewRootImpl.checkThread(ViewRootImpl.java:7020)
	at android.view.ViewRootImpl.doDie(ViewRootImpl.java:6035)
	at android.view.ViewRootImpl.die(ViewRootImpl.java:6020)
	at android.view.WindowManagerGlobal.removeViewLocked(WindowManagerGlobal.java:486)
	at android.view.WindowManagerGlobal.removeView(WindowManagerGlobal.java:424)
	at android.view.WindowManagerImpl.removeViewImmediate(WindowManagerImpl.java:126)
	at android.app.Dialog.dismissDialog(Dialog.java:363)
	....省略部分堆栈

7、ViewRootImpl.checkThread()

终于来到了关键之处

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

初步结论：根据文章开始的介绍，当调用 dialog 的 dismiss() 时，dialog 会自动抛到我们 new Dialog() 的线程中执行，也就是 Thread.currentThread 。之所以 crash，抛出该异常，是因为，ViewRootImpl 本身的 mThread 和我们 new Dialog() 的线程不一致。

因此，接下来问题的元凶就是 mThread 是怎么来的了！！！

在 ViewRootImpl 中寻找 mThread 赋值的地方：

public ViewRootImpl(Context context, Display display) {
    mThread = Thread.currentThread();

    // 省略部分代码 
}

那现在的问题就是，ViewRootImpl 什么时候调用这个构造函数创建实例的呢？

回到第4小部分，WindowManagerGlobal 的 removeView() 方法

ViewRootImpl root = mRoots.get(index);
boolean deferred = root.die(immediate);

发现 root 对象是从 mRoots 对象中取出来的，而 mRoots 是一个 ArrayList<ViewRootImpl>，因此，我们看看 mRoots 的生成

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);
    }
}

那 addView() 方法什么时候会调用呢？就是 WindowManagerImpl.addView()

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

我们在 Dialog 的源码中找一下 mWindowManager 对象调用 addView() 方法的地方，真的是，蓦然回首，那人却在灯火阑珊处啊：

	/**
     * Start the dialog and display it on screen.  The window is placed in the
     * application layer and opaque.  Note that you should not override this
     * method to do initialization when the dialog is shown, instead implement
     * that in {@link #onStart}.
     */
    public void show() {
        // 省略部分代码
        onStart();
        mDecor = mWindow.getDecorView();

        mWindowManager.addView(mDecor, l);

        sendShowMessage();
    }

看到这，估计也都看明白了，调用 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（也就是调用 Dialog 的 show() 方法的线程）。

再来插播一下之前的堆栈信息

threadName:main,threadPriority:5,threadState:RUNNABLE
Caused by: android.view.ViewRootImpl$CalledFromWrongThreadException: Only the original thread that created a view hierarchy can touch its views.
	at android.view.ViewRootImpl.checkThread(ViewRootImpl.java:7020)
	at android.view.ViewRootImpl.doDie(ViewRootImpl.java:6035)
	at android.view.ViewRootImpl.die(ViewRootImpl.java:6020)
	at android.view.WindowManagerGlobal.removeViewLocked(WindowManagerGlobal.java:486)
	at android.view.WindowManagerGlobal.removeView(WindowManagerGlobal.java:424)
	at android.view.WindowManagerImpl.removeViewImmediate(WindowManagerImpl.java:126)
	at android.app.Dialog.dismissDialog(Dialog.java:363)
	....省略部分堆栈

跟着堆栈信息，我们一步步走到了问题的根源，再来简单总结一下：

new Dialog：非 UI 线程（Thread.currentThread()）
show()：UI 主线程（mThread）
cancel()：非 UI 线程
dismiss()：非 UI 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 new 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()。

而在例子中，Dialog 的 show() 是在主线程执行，new Dialog() 是在非 UI 线程中执行的，所以抛出了 CalledFromWrongThreadException 的异常。

结论

Dialog 的 dismiss() 会首先被抛到 new Dialog() 的线程中执行。
只要保证创建 Dialog 和调用 show() 方法在同一个线程中执行，无论是否放到 UI 线程都可以。

参考文章

https://my.oschina.net/qixiaobo025/blog/195396

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