通过之前的文章了解了setContentView和inflate方面的知识下面就可以正式开始讲解View的绘制流程了，而DecorView作为Activity的跟布局链接了整个View的绘制流程现在我们就来看看DecorView是怎样添加到Activity中的吧
我们从ActivityThread的handleLaunchActivity()方法开始
private void handleLaunchActivity(ActivityClientRecord r, Intent customIntent, String reason) {
// Make sure we are running with the most recent config.
...省略无关代码...
//创建activity在该方法中会创建PhoneWindow并且调用activity的onCreat方法
Activity a = performLaunchActivity(r, customIntent);
if (a != null) {
r.createdConfig = new Configuration(mConfiguration);
reportSizeConfigurations(r);
Bundle oldState = r.state;
//调用handleResumeActivity()这个方法开始添加DecorView
handleResumeActivity(r.token, false, r.isForward,
!r.activity.mFinished && !r.startsNotResumed, r.lastProcessedSeq, reason);
if (!r.activity.mFinished && r.startsNotResumed) {
performPauseActivityIfNeeded(r, reason);
if (r.isPreHoneycomb()) {
r.state = oldState;
}
}
} else {
try {
ActivityManagerNative.getDefault()
.finishActivity(r.token, Activity.RESULT_CANCELED, null,
Activity.DONT_FINISH_TASK_WITH_ACTIVITY);
} catch (RemoteException ex) {
throw ex.rethrowFromSystemServer();
}
}
}
上述代码中我们主要关注两个点第一activity的创建方法performLaunchActivity该方法主要是创建activity创建Phonewindow并且调用activity的生命周期方法onCreate另外一个handleResumeActivity方法看方法名字我们就知道这个肯定和Activity的onResume方法有关
final void handleResumeActivity(IBinder token,
boolean clearHide, boolean isForward, boolean reallyResume, int seq, String reason) {
...
if (r.window == null && !a.mFinished && willBeVisible) {
//回调onResume方法
r = performResumeActivity(token, clearHide, reason);
r.window = r.activity.getWindow();
//创建decorView
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();
}
}
if (a.mVisibleFromClient && !a.mWindowAdded) {
a.mWindowAdded = true;
//添加DecorView到
wm.addView(decor, l);
}
...
}
...
}
getDecorView方法在上篇博客中已经分析过了，这里我们主要分析addView()方法点进去会发现WindowManager是个接口并没有实现addView()方法，具体实现addView方法的是WindowManagerImpl类
public void addView(@NonNull View view, @NonNull ViewGroup.LayoutParams params) {
applyDefaultToken(params);
mGlobal.addView(view, params, mContext.getDisplay(), mParentWindow);
}
继续追踪
public void addView(View view, ViewGroup.LayoutParams params,
Display display, Window parentWindow) {
...
//创建ViewRootImpl 这个很重要之后我们分析View的测量摆放绘制流程都是从这个方法开始的
ViewRootImpl root;
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 {
//将decorView添加到ViewRootImpl中
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;
}
}
跟踪root.setView(view, wparams, panelParentView);代码
public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) {
synchronized (this) {
if (mView == null) {
mView = view;
//发起绘制流程
requestLayout();
...
//设置ViewRootImpl为DecorView的父控件
view.assignParent(this);
...
}
}
}
上述有两个很重要的代码第一个是requestLayout()方法，该方法 我们在开发过程中应该经常遇到，一般调用他进行View重绘，第二个view.assignParent(this);方法，设置DecorView的parent为ViewRootImpl。为什么要这样做呢我们来看一下View的requestLayout()方法是怎样调用的吧
public void requestLayout() {
if (mMeasureCache != null) mMeasureCache.clear();
if (mAttachInfo != null && mAttachInfo.mViewRequestingLayout == null) {
// Only trigger request-during-layout logic if this is the view requesting it,
// not the views in its parent hierarchy
ViewRootImpl viewRoot = getViewRootImpl();
if (viewRoot != null && viewRoot.isInLayout()) {
if (!viewRoot.requestLayoutDuringLayout(this)) {
return;
}
}
mAttachInfo.mViewRequestingLayout = this;
}
mPrivateFlags |= PFLAG_FORCE_LAYOUT;
mPrivateFlags |= PFLAG_INVALIDATED;
if (mParent != null && !mParent.isLayoutRequested()) {
//调用parent的requestLayout方法
mParent.requestLayout();
}
if (mAttachInfo != null && mAttachInfo.mViewRequestingLayout == this) {
mAttachInfo.mViewRequestingLayout = null;
}
}
View的requestLayout()会调用Parent的requestLayout()这样层级调用最终会调用到ViewRootImpl的reuqestLayout()方法，跟踪一下该方法
public void requestLayout() {
if (!mHandlingLayoutInLayoutRequest) {
checkThread();
mLayoutRequested = true;
scheduleTraversals();
}
}
void scheduleTraversals() {
if (!mTraversalScheduled) {
mTraversalScheduled = true;
mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
//回调mTraversalRunnable
mChoreographer.postCallback(
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");
}
//View的绘制发起方法
performTraversals();
if (mProfile) {
Debug.stopMethodTracing();
mProfile = false;
}
}
}
终于走到这里了上面调用了performTraversals();方法。它是我们整个View绘制流程的核心方法也是绘制流程的起始方法
private void performTraversals() {
// Ask host how big it wants to be
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
performLayout(lp, mWidth, mHeight);
performDraw();
}
performTraversals()的内容太多了我们这里只看上面三个方法。分别引导了View的测量、布局和绘制流程。具体每个方法的作用我们就在下个章节来讲解。