16.源码阅读（View的绘制-android api-26）
今天带着一个问题来看Android View的绘制流程 View的绘制入口在哪？ 很多时候，在进入到一个页面的时候，会需要动态的获取到布局中某一个view的宽度或者高度，但是我们发现如果直接在onCreate方法或者onResume方法中通过这种方式去取高度值得到的是0 int measuredHeight = mTextView.getMeasuredHeight(); 而调用post方法才可以得到正确的值 mTextView.post(new Runnable() { @Override public void run() { int measuredHeight1 = mTextView.getMeasuredHeight(); System.out.println("post measuredHeight:"+measuredHeight1); } }); 所以回到我们的第一个问题，view的绘制入口在哪里，只有view绘制完成经过测量才能得到宽高值，是否在onCreate和onResume方法中，view还没有完成绘制测量呢？ 在Activity的启动流程中已经了解到，最终要启动Activity并且开始执行Activity生命周期的位置是在ActivityThread的handleLaunchActivity中，我们直接来到这里看，对Activity启动流程不熟悉可以看另一篇https://www.jianshu.com/p/bd5208574430 handleLaunchActivity private void handleLaunchActivity(ActivityClientRecord r, Intent customIntent, String reason) { ...... //这里会回调onCreate方法 Activity a = performLaunchActivity(r, customIntent); if (a != null) { r.createdConfig = new Configuration(mConfiguration); reportSizeConfigurations(r); Bundle oldState = r.state; //这里会回调onResume方法 handleResumeActivity(r.token, false, r.isForward, !r.activity.mFinished && !r.startsNotResumed, r.lastProcessedSeq, reason); if (!r.activity.mFinished && r.startsNotResumed) { // The activity manager actually wants this one to start out paused, because it // needs to be visible but isn't in the foreground. We accomplish this by going // through the normal startup (because activities expect to go through onResume() // the first time they run, before their window is displayed), and then pausing it. // However, in this case we do -not- need to do the full pause cycle (of freezing // and such) because the activity manager assumes it can just retain the current // state it has. performPauseActivityIfNeeded(r, reason); // We need to keep around the original state, in case we need to be created again. // But we only do this for pre-Honeycomb apps, which always save their state when // pausing, so we can not have them save their state when restarting from a paused // state. For HC and later, we want to (and can) let the state be saved as the // normal part of stopping the activity. if (r.isPreHoneycomb()) { r.state = oldState; } } } else { //如果启动Activity失败，交给ActivityManager处理 // If there was an error, for any reason, tell the activity manager to stop us. try { ActivityManager.getService() .finishActivity(r.token, Activity.RESULT_CANCELED, null, Activity.DONT_FINISH_TASK_WITH_ACTIVITY); } catch (RemoteException ex) { throw ex.rethrowFromSystemServer(); } } } performLaunchActivity ClassLoader加载出Activity后执行了callActivityOnCreate，会回调onCreate方法 private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) { ...... Activity activity = null; try { java.lang.ClassLoader cl = appContext.getClassLoader(); activity = mInstrumentation.newActivity( cl, component.getClassName(), r.intent); ...... activity.mCalled = false; if (r.isPersistable()) { mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState); } else { mInstrumentation.callActivityOnCreate(activity, r.state); } return activity; } public void callActivityOnCreate(Activity activity, Bundle icicle) { prePerformCreate(activity); //调用activity中的performCreate，onCreate方法就在里面 activity.performCreate(icicle); postPerformCreate(activity); } 可以看到，activity创建出来后就开始回调onCreate，而onCreate中做了什么呢，调用了setContentView方法加载我们的布局文件，setContentView源码分析https://www.jianshu.com/p/2f87ebe77f4e 从源码中可以看到，setContentView做了以下的操作： 1.new了一个DecorView 2.加载了一个id为android.R.id.content的布局并把它加入DecorView 3.我们设置的布局文件被加载到id为android.R.id.content的ViewGroup中 也就是说，setContentView后只是加载了布局，但是还没有进行测量绘制，那么在onResum方法调用时是否完成了测量绘制呢？ performLaunchActivity之后系统开始执行handleResumeActivity final void handleResumeActivity(IBinder token, boolean clearHide, boolean isForward, boolean reallyResume, int seq, String reason) { .... //回调onResum方法 r = performResumeActivity(token, clearHide, reason); if (r != null) { ...... //获取到WindowManager，将包含了我们我们自己的布局的 //DecorView加入到WindowManager中(ViewManager是一个 //接口，WindowManager实现了它) 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 (r.mPreserveWindow) { a.mWindowAdded = true; r.mPreserveWindow = false; // Normally the ViewRoot sets up callbacks with the Activity // in addView->ViewRootImpl#setView. If we are instead reusing // the decor view we have to notify the view root that the // callbacks may have changed. ViewRootImpl impl = decor.getViewRootImpl(); if (impl != null) { impl.notifyChildRebuilt(); } } //如果Activity已经可见了，就将DecorView加入Window if (a.mVisibleFromClient) { if (!a.mWindowAdded) { a.mWindowAdded = true; wm.addView(decor, l); } } ...... // The window is now visible if it has been added, we are not // simply finishing, and we are not starting another activity. if (!r.activity.mFinished && willBeVisible ...... WindowManager.LayoutParams l = r.window.getAttributes(); if ((l.softInputMode & WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION) != forwardBit) { l.softInputMode = (l.softInputMode & (~WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION)) | forwardBit; if (r.activity.mVisibleFromClient) { ViewManager wm = a.getWindowManager(); View decor = r.window.getDecorView(); //window可见之后，开始真正的绘制view了，这个方法很关键 wm.updateViewLayout(decor, l); } } ...... r.onlyLocalRequest = false; // Tell the activity manager we have resumed. if (reallyResume) { try { ActivityManager.getService().activityResumed(token); } catch (RemoteException ex) { throw ex.rethrowFromSystemServer(); } } } } 也就是说在onCreate和onResume方法之心的时候，还没有完成View的测量，所以在这两个方法中无法得到实际的高度，具体绘制的代码先不看，我们看看为什么post方法中可以得到高度 public boolean post(Runnable action) { final AttachInfo attachInfo = mAttachInfo; if (attachInfo != null) { return attachInfo.mHandler.post(action); } // Postpone the runnable until we know on which thread it needs to run. // Assume that the runnable will be successfully placed after attach. //把Runnable加入了队列 getRunQueue().post(action); return true; } public void post(Runnable action) { postDelayed(action, 0); } public void postDelayed(Runnable action, long delayMillis) { final HandlerAction handlerAction = new HandlerAction(action, delayMillis); //看得出这是一个数组实现的队列，加入队列后也没有执行 //那么是什么时候执行的 synchronized (this) { if (mActions == null) { mActions = new HandlerAction[4]; } mActions = GrowingArrayUtils.append(mActions, mCount, handlerAction); mCount++; } } public static <T> T[] append(T[] array, int currentSize, T element) { assert currentSize <= array.length; if (currentSize + 1 > array.length) { T[] newArray = (T[]) Array.newInstance(array.getClass().getComponentType(), growSize(currentSize)); System.arraycopy(array, 0, newArray, 0, currentSize); array = newArray; } array[currentSize] = element; return array; } dispatchAttachToWindow,这个队列是在这个方法中执行的，后边我们会找到这个方法执行的时机，然后就会知道为什么这个方法中执行就可以得到高度了 void dispatchAttachedToWindow(AttachInfo info, int visibility) { ...... if (mRunQueue != null) { mRunQueue.executeActions(info.mHandler); mRunQueue = null; } performCollectViewAttributes(mAttachInfo, visibility); onAttachedToWindow(); ...... } 我们再回到上边ViewManager的addView和updateViewLayout方法，ViewManager和WindowManager都是接口，找到它的实现类WindowManagerImpl（可以通过PhoneWindow找到WindowManager的实现类是他） @Override public void addView(@NonNull View view, @NonNull ViewGroup.LayoutParams params) { applyDefaultToken(params); mGlobal.addView(view, params, mContext.getDisplay(), mParentWindow); } @Override public void updateViewLayout(@NonNull View view, @NonNull ViewGroup.LayoutParams params) { applyDefaultToken(params); mGlobal.updateViewLayout(view, params); } 看一下WindowManagerGlobal这个类中做了什么,这里创建了一个ViewRootImpl，并把DecorView加入了这个View中 public void addView(View view, ViewGroup.LayoutParams params, Display display, Window parentWindow) { ...... ViewRootImpl root; View panelParentView = null; ...... root = new ViewRootImpl(view.getContext(), display); ...... // do this last because it fires off messages to start doing things try { root.setView(view, wparams, panelParentView); } catch (RuntimeException e) { } } } //这个方法是给view设置参数的，所以关键还是在addView方法 public void updateViewLayout(View view, ViewGroup.LayoutParams params) { if (view == null) { throw new IllegalArgumentException("view must not be null"); } if (!(params instanceof WindowManager.LayoutParams)) { throw new IllegalArgumentException("Params must be WindowManager.LayoutParams"); } final WindowManager.LayoutParams wparams = (WindowManager.LayoutParams)params; view.setLayoutParams(wparams); synchronized (mLock) { int index = findViewLocked(view, true); ViewRootImpl root = mRoots.get(index); mParams.remove(index); mParams.add(index, wparams); root.setLayoutParams(wparams, false); } } setView public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) { synchronized (this) { ...... requestLayout(); } } 这里又来到了曾经在invalidate方法中看到的源码，刷新view @Override public void requestLayout() { if (!mHandlingLayoutInLayoutRequest) { checkThread(); mLayoutRequested = true; scheduleTraversals(); } } void scheduleTraversals() { if (!mTraversalScheduled) { mTraversalScheduled = true; mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier(); mChoreographer.postCallback( //执行这个Runnable Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null); if (!mUnbufferedInputDispatch) { scheduleConsumeBatchedInput(); } notifyRendererOfFramePending(); pokeDrawLockIfNeeded(); } } final class TraversalRunnable implements Runnable { @Override public void run() { doTraversal(); } } void doTraversal() { if (mTraversalScheduled) { mTraversalScheduled = false; mHandler.getLooper().getQueue().removeSyncBarrier(mTraversalBarrier); if (mProfile) { Debug.startMethodTracing("ViewAncestor"); } performTraversals(); if (mProfile) { Debug.stopMethodTracing(); mProfile = false; } } } private void performTraversals() { ....省略大量代码 // Ask host how big it wants to be 回调到measure了 performMeasure(childWidthMeasureSpec, childHeightMeasureSpec); .... //回调到layout了 if (didLayout) { performLayout(lp, mWidth, mHeight); } ...... //回调draw了 performDraw(); } 总结一下： 1.onCreate和onResume方法调用时还没有进行view的测量 2.onResume方法调用之后将包含着当前布局文件的DecorView交给了WindowManager（实现类WindowManagerImpl）处理 3.WindowManager中将DecorView设置到ViewRootImpl中，在ViewRootImpl中调用requestLayout开始刷新View,执行measure layout 和 draw方法 4.post方法中能得到高度是因为post将runnable加入队列，在view attachToWindow时后才去获取高度，此时view已经完成了测量
