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?