Android子线程真的不能更新UI么

jopen 6年前

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现场安全: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?

来自: http://www.cnblogs.com/lao-liang/p/5108745.html