前言：在Activity的启动流程中我们分析到了ActivityThread类中的handleResumeActivity中的PerformResumeActivity方法市区实现Activity的onResume的生命周期
在该方法中我们接着往下看
@Override
public void handleResumeActivity(IBinder token, boolean finalStateRequest, boolean isForward,
String reason) {
final ActivityClientRecord r = performResumeActivity(token, finalStateRequest, reason);
...
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();
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) {
if (!a.mWindowAdded) {
a.mWindowAdded = true;
//把DecorView的ViewRootImpl传进去
wm.addView(decor, l);
} else {
// The activity will get a callback for this {@link LayoutParams} change
// earlier. However, at that time the decor will not be set (this is set
// in this method), so no action will be taken. This call ensures the
// callback occurs with the decor set.
a.onWindowAttributesChanged(l);
}
}
}
可以看到通过ViewManager wm = a.getWindowManager();a就是Activity,getWindowManager创建出WindowManager,我们进入该类，发现是个接口，老规矩我们去寻找实现类，直接查找WindowManagerImpl，找到addView方法
@Override
public void addView(@NonNull View view, @NonNull ViewGroup.LayoutParams params) {
applyDefaultToken(params);
mGlobal.addView(view, params, mContext.getDisplay(), mParentWindow);
}
发现调用了mGlobal.addView，mGlobal是WindowManagerGlobal，跟进去
public void addView(View view, ViewGroup.LayoutParams params,
Display display, Window parentWindow) {
...
ViewRootImpl root;
View panelParentView = null;
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.
if (index >= 0) {
removeViewLocked(index, true);
}
throw e;
}
}
看到mViews,mRoots,mParams是三个集合，分别添加进DecorView,DecorView的ViewRootImpl，以及DecorView的layoutParams,通过panelParentView = mViews.get(i)可以看到panelParentView其实就是DecorView本身，然后下面执行root.setView(view, wparams, panelParentView)，跟进去
public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) {
...
//寻找关键代码，找啊找
...
// Schedule the first layout -before- adding to the window
// manager, to make sure we do the relayout before receiving
// any other events from the system.
requestLayout();
}
突然发现熟悉的方法requestLayout(),就是我们调用该方法去重新刷新布局，跟进去
@Override
public void requestLayout() {
if (!mHandlingLayoutInLayoutRequest) {
checkThread();
mLayoutRequested = true;
scheduleTraversals();
}
}
checkThread()方法就是检查当前线程是不是主线程，然后我们看到scheduleTraversals()方法在ActivityThread中有该方法，跟进去
void scheduleTraversals() {
if (!mTraversalScheduled) {
mTraversalScheduled = true;
mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
mChoreographer.postCallback(
Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
if (!mUnbufferedInputDispatch) {
scheduleConsumeBatchedInput();
}
notifyRendererOfFramePending();
pokeDrawLockIfNeeded();
}
}
关键代码mChoreographer.postCallback(Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);发现传进来一个mTraversalRunnable，点击发现是一个TraversalRunnable类，并且实现了Runnable
final class TraversalRunnable implements Runnable {
@Override
public void run() {
doTraversal();
}
}
final TraversalRunnable mTraversalRunnable = new TraversalRunnable();
我们看run方法中的doTraversal()做了什么
void doTraversal() {
if (mTraversalScheduled) {
mTraversalScheduled = false;
mHandler.getLooper().getQueue().removeSyncBarrier(mTraversalBarrier);
if (mProfile) {
Debug.startMethodTracing("ViewAncestor");
}
//关键代码
performTraversals();
if (mProfile) {
Debug.stopMethodTracing();
mProfile = false;
}
}
}
发现调用了performTraversals，继续跟进，然后我们看到一些跟布局绘制有关的信息WindowManager.LayoutParams，Rect，我们继续往下找返现找到了我们熟悉的
private void performTraversals() {
...
// Ask host how big it wants to be
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
...
performLayout(lp, mWidth, mHeight);
...
performDraw();
}
总结下流程
