Android 子线程更新UI那些事儿

Posted 2021-04-27 Coder随笔

android 子线程真的不能更新UI么

如果在其他线程更新UI，Android系统提供了以下几种方式:

1. Activity.runonUIthread(Runnable)

2. view.post(Runnable)

3. view.postDelay(Runnable,long)

4. Handler

为什么在子线程就不能更新UI呢。

我们知道在子线程更新UI会抛出一个

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

当碰到这个异常的时候，很多人会用上面的4种方式中的一种来解决。那么我们来查看下这个异常是在哪里抛出的呢？

从异常堆栈中，我们可以定位到，是ViewRootImpl.invalidateChildInParent（）方法抛出的这个异常。

2019-03-31 20:52:30.234 18524-18970/com.msxf.cash.internal.dev E/AndroidRuntime: FATAL EXCEPTION: testthread 
    Process: com.msxf.cash.internal.dev, PID: 18524
    android.view.ViewRootImpl$CalledFromWrongThreadException: Only the original thread that created a view hierarchy can touch its views.
        at android.view.ViewRootImpl.checkThread(ViewRootImpl.java:6920)
        at android.view.ViewRootImpl.invalidateChildInParent(ViewRootImpl.java:1116)
        at android.view.ViewGroup.invalidateChild(ViewGroup.java:5244)
        at android.view.View.invalidateInternal(View.java:13593)
        at android.view.View.invalidate(View.java:13557)
        at android.view.View.invalidate(View.java:13541)
        at android.widget.TextView.checkForRelayout(TextView.java:7357)
        at android.widget.TextView.setText(TextView.java:4482)
        at android.widget.TextView.setText(TextView.java:4339)
        at android.widget.TextView.setText(TextView.java:4314)
        at com.msxf.cash.ui.account.LoginActivity$1.handleMessage(LoginActivity.java:149)
        at android.os.Handler.dispatchMessage(Handler.java:98)
        at android.os.Looper.loop(Looper.java:163)
        at android.os.HandlerThread.run(HandlerThread.java:61)

再来查看checkThread方法的实现，

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

然后在ViewRootImpl的构造方法里 可以找到mThread 的初始化

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

    mDisplayAdjustments = display.getDisplayAdjustments();

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

那么，我们能通过某种方式绕过这个异常么。答案是肯定的。

我们可以给要刷新的View绑定自己的ViewRoot。

就是通过windowManager.addView创建了ViewRoot.

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();
     }
 }

WindowManagerImpl.java

@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();

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。

ps：

通过自己实验，发现在oncreate方法中启动线程操作ui并不会报异常，这是为什么呢？

既然Android的UI相关操作是单线程模型，那么先找到UI线程实现类ActivityThread来看看，是在哪里addview的。

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;
        }
    ......
}

扩展阅读：

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。

当程序启动的时候，Android会同时启动一个主线程，这个主线程就是UI线程。UI线程主要负责与UI相关的计算和事件，比如点击事件，控件绘制。

那么为什么要选择单线程模型呢？

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。