Android子线程真的不能更新UI么

  Android单线程模型是这样描述的:

Android UI操作并不是线程安全的,并且这些操作必须在UI线程执行

  如果在其它线程访问UI线程,Android提供了以下的方式:

Activity.runOnUiThread(Runnable)
View.post(Runnable)
View.postDelayed(Runnable, long)
Handler

  为什么呢?在子线程中就不能操作UI么?

  当一个程序第一次启动的时候,Android会同时启动一个对应的主线程,这个主线程就是UI线程,也就是ActivityThread。UI线程主要负责处理与UI相关的事件,如用户的按键点击、用户触摸屏幕以及屏幕绘图等。系统不会为每个组件单独创建一个线程,在同一个进程里的UI组件都会在UI线程里实例化,系统对每一个组件的调用都从UI线程分发出去。所以,响应系统回调的方法永远都是在UI线程里运行,如响应用户动作的onKeyDown()的回调。

  那为什么选择一个主线程干这些活呢?换个说法,Android为什么使用单线程模型,它有什么好处?

  先让我们看下单线程化的事件队列模型是怎么定义的:

采用一个专门的线程从队列中抽取事件,并把他们转发给应用程序定义的事件处理器

  这看起来就是Android的消息队列、Looper和Handler嘛。类似知识请参考:深入理解Message, MessageQueue, Handler和Looper

  其实现代GUI框架就是使用了类似这样的模型:模型创建一个专门的线程,事件派发线程来处理GUI事件。单线程化也不单单存在Android中,Qt、XWindows等都是单线程化。当然,也有人试图用多线程的GUI,最终由于竞争条件和死锁导致的稳定性问题等,又回到单线程化的事件队列模型老路上来。单线程化的GUI框架通过限制来达到线程安全:所有GUI中的对象,包括可视组件和数据模型,都只能被事件线程访问。

  这就解释了Android为什么使用单线程模型。

  那Android的UI操作并不是线程安全的又是怎么回事?

  Android实现View更新有两组方法,分别是invalidate和postInvalidate。前者在UI线程中使用,后者在非UI线程中使用。换句话说,Android的UI操作不是线程安全可以表述为invalidate在子线程中调用会导致线程不安全。作一个假设,现在我用invalidate在子线程中刷新界面,同时UI线程也在用invalidate刷新界面,这样会不会导致界面的刷新不能同步?既然刷新不同步,那么invalidate就不能在子线程中使用。这就是invalidate不能在子线程中使用的原因。

  postInvalidate可以在子线程中使用,它是怎么做到的?

  看看源码是怎么实现的:

public void postInvalidate() {
    postInvalidateDelayed(0);
}

public void postInvalidateDelayed(long delayMilliseconds) {
    // We try only with the AttachInfo because there's no point in invalidating
    // if we are not attached to our window
    if (mAttachInfo != null) {
        Message msg = Message.obtain();
        msg.what = AttachInfo.INVALIDATE_MSG;
        msg.obj = this;
        mAttachInfo.mHandler.sendMessageDelayed(msg, delayMilliseconds);
    }
}

  说到底还是通过Handler的sendMessageDelayed啊,还是逃不过消息队列,最终还是交给UI线程处理。所以View的更新只能由UI线程处理。

  如果我非要在子线程中更新UI,那会出现什么情况呢?

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

  抛了一个CalledFromWrongThreadException异常。

  相信很多人遇到这个异常后,就会通过前面的四种方式中的其中一种解决:

Activity.runOnUiThread(Runnable)
View.post(Runnable)
View.postDelayed(Runnable, long)
Handler

  说到底还没触发到根本,为什么会出现这个异常呢?这个异常在哪里抛出来的呢?

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

  该代码出自 framework/base/core/java/android/view/ViewRootImpl.java

  再看下ViewRootImpl的构造函数,mThread就是在这初始化的:

public ViewRootImpl(Context context, Display display) {
    mContext = context;
    mWindowSession = WindowManagerGlobal.getWindowSession();
    mDisplay = display;
    mBasePackageName = context.getBasePackageName();

    mDisplayAdjustments = display.getDisplayAdjustments();

    mThread = Thread.currentThread();
    ......
}

  再研究一下这个CalledFromWrongThreadException异常的堆栈,会发现最后到了invalidateChild和invalidateChildInParent方法中:

@Override
public void invalidateChild(View child, Rect dirty) {
    invalidateChildInParent(null, dirty);
}

@Override
public ViewParent invalidateChildInParent(int[] location, Rect dirty) {
    checkThread();
    ......
}

  最终通过checkThread形成了这个异常。说到底,非UI线程是可以刷新UI的呀,前提是它要拥有自己的ViewRoot。如果想直接创建ViewRoot实例,你会发现找不到这个类。那怎么做呢?通过WindowManager。

class NonUiThread extends Thread{
      @Override
      public void run() {
         Looper.prepare();
         TextView tx = new TextView(MainActivity.this);
         tx.setText("non-UiThread update textview");
 
         WindowManager windowManager = MainActivity.this.getWindowManager();
         WindowManager.LayoutParams params = new WindowManager.LayoutParams(
             200, 200, 200, 200, WindowManager.LayoutParams.FIRST_SUB_WINDOW,
                 WindowManager.LayoutParams.TYPE_TOAST,PixelFormat.OPAQUE);
         windowManager.addView(tx, params); 
         Looper.loop();
     }
 }

  就是通过windowManager.addView创建了ViewRoot,WindowManagerImpl.java中的addView方法:

@Override
public void addView(@NonNull View view, @NonNull ViewGroup.LayoutParams params) {
    applyDefaultToken(params);
    mGlobal.addView(view, params, mDisplay, mParentWindow);
}
private final WindowManagerGlobal mGlobal = WindowManagerGlobal.getInstance();

  mGlobal是一个WindowManagerGlobal实例,代码在 frameworks/base/core/java/android/view/WindowManagerGlobal.java中,具体实现如下:

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);
        } else {
            // If there's no parent, then hardware acceleration for this view is
            // set from the application's hardware acceleration setting.
            final Context context = view.getContext();
            if (context != null
                    && (context.getApplicationInfo().flags
                            & ApplicationInfo.FLAG_HARDWARE_ACCELERATED) != 0) {
                wparams.flags |= WindowManager.LayoutParams.FLAG_HARDWARE_ACCELERATED;
            }
        }

        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;
        }
    }

  所以,非UI线程能更新UI,只要它有自己的ViewRoot。

  延伸一下:Android Activity本身是在什么时候创建ViewRoot的呢?

  既然是单线程模型,就要先找到这个UI线程实现类ActivityThread,看里面哪里addView了。没错,是在onResume里面,对应ActivityThread就是handleResumeActivity这个方法:

final void handleResumeActivity(IBinder token,
            boolean clearHide, boolean isForward, boolean reallyResume) {
        // If we are getting ready to gc after going to the background, well
        // we are back active so skip it.
        unscheduleGcIdler();
        mSomeActivitiesChanged = true;

        // TODO Push resumeArgs into the activity for consideration
        ActivityClientRecord r = performResumeActivity(token, clearHide);
        ......
        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;
        }
    ......
}

  所以,如果在onCreate中通过子线程直接更新UI,并不会抛CalledFromWrongThreadException异常。但是一般情况下,我们不会在onCreate中做这样的事情。

  这就是Android为我们设计的单线程模型,核心就是一句话:Android UI操作并不是线程安全的,并且这些操作必须在UI线程执行。但这一句话背后,却隐藏着我们平时看不见的代码实现,只有搞懂这些,我们才能知其然知其所以然。

 

参考:Android子线程在没有ViewRoot的情况下能刷新UI吗?

posted @ 2016-01-07 11:42 LeoLiang 阅读(...) 评论(...) 编辑 收藏