Window是我们进行交互的窗口，那么窗口里的那些形形色色的控件，也就是View是怎么展示到这个Window上的呢？
查看源码，Activity启动的时候会执行ActivityThread.handleResumeActivity()，就是这个方法把DecorView与ViewRootImpl关联起来的，我们来看看它的内部实现
final void handleResumeActivity(IBinder token,
boolean clearHide, boolean isForward, boolean reallyResume, int seq, String reason) {
ActivityClientRecord r = mActivities.get(token);
if (r != null) {
final Activity a = r.activity;
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();
}
}
if (a.mVisibleFromClient) {
if (!a.mWindowAdded) {
a.mWindowAdded = true;
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);
}
}
}
只看关键代码wm.addView(decor, l);wm就是ViewManager他是一个接口，它的实现类是WindowManager，WindowManager也是一个接口，它的实现类是WindowManagerImpl也就是最终调用WindowManagerImpl.addView
@Override
public void addView(@NonNull View view, @NonNull ViewGroup.LayoutParams params) {
applyDefaultToken(params);
mGlobal.addView(view, params, mContext.getDisplay(), mParentWindow);
}
可以看到最终走到了WindowManagerGlobal.addView
public void addView(View view, ViewGroup.LayoutParams params,
Display display, Window parentWindow) {
// 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;
}
}
}
root.setView(view, wparams, panelParentView);
于是就到了ViewRootImpl的setView
public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) {
synchronized (this) {
if (mView == null) {
mView = view;
mAdded = true;
int res; /* = WindowManagerImpl.ADD_OKAY; */
requestLayout();
}
}
只看关键代码requestLayout();
@Override
public void requestLayout() {
if (!mHandlingLayoutInLayoutRequest) {
checkThread();
mLayoutRequested = true;
scheduleTraversals();
}
}
再看scheduleTraversals();
void scheduleTraversals() {
if (!mTraversalScheduled) {
mTraversalScheduled = true;
mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
mChoreographer.postCallback(
Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
if (!mUnbufferedInputDispatch) {
scheduleConsumeBatchedInput();
}
notifyRendererOfFramePending();
pokeDrawLockIfNeeded();
}
}
可以看到Handler发送了一条异步分隔栏消息开启遍历线程
final class TraversalRunnable implements Runnable {
@Override
public void run() {
doTraversal();
}
}
doTraversal()内部代码
void doTraversal() {
if (mTraversalScheduled) {
mTraversalScheduled = false;
mHandler.getLooper().getQueue().removeSyncBarrier(mTraversalBarrier);
if (mProfile) {
Debug.startMethodTracing("ViewAncestor");
}
performTraversals();
if (mProfile) {
Debug.stopMethodTracing();
mProfile = false;
}
}
}
performTraversals();执行遍历
private void performTraversals() {
if (mFirst || windowShouldResize || insetsChanged ||
viewVisibilityChanged || params != null || mForceNextWindowRelayout) {
try {
if (!mStopped || mReportNextDraw) {
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
int width = host.getMeasuredWidth();
int height = host.getMeasuredHeight();
boolean measureAgain = false;
if (lp.horizontalWeight > 0.0f) {
width += (int) ((mWidth - width) * lp.horizontalWeight);
childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(width,
MeasureSpec.EXACTLY);
measureAgain = true;
}
if (lp.verticalWeight > 0.0f) {
height += (int) ((mHeight - height) * lp.verticalWeight);
childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(height,
MeasureSpec.EXACTLY);
measureAgain = true;
}
if (measureAgain) {
if (DEBUG_LAYOUT) Log.v(mTag,
"And hey let's measure once more: width=" + width
+ " height=" + height);
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
}
layoutRequested = true;
}
}
}
if (didLayout) {
performLayout(lp, mWidth, mHeight);
if ((host.mPrivateFlags & View.PFLAG_REQUEST_TRANSPARENT_REGIONS) != 0) {
host.getLocationInWindow(mTmpLocation);
mTransparentRegion.set(mTmpLocation[0], mTmpLocation[1],
mTmpLocation[0] + host.mRight - host.mLeft,
mTmpLocation[1] + host.mBottom - host.mTop);
host.gatherTransparentRegion(mTransparentRegion);
if (mTranslator != null) {
mTranslator.translateRegionInWindowToScreen(mTransparentRegion);
}
}
}
if (regainedFocus) {
mLostWindowFocus = false;
} else if (!hasWindowFocus && mHadWindowFocus) {
mLostWindowFocus = true;
}
if (!cancelDraw && !newSurface) {
if (mPendingTransitions != null && mPendingTransitions.size() > 0) {
for (int i = 0; i < mPendingTransitions.size(); ++i) {
mPendingTransitions.get(i).startChangingAnimations();
}
mPendingTransitions.clear();
}
performDraw();
} else {
if (isViewVisible) {
// Try again
scheduleTraversals();
} else if (mPendingTransitions != null && mPendingTransitions.size() > 0) {
for (int i = 0; i < mPendingTransitions.size(); ++i) {
mPendingTransitions.get(i).endChangingAnimations();
}
mPendingTransitions.clear();
}
}
mIsInTraversal = false;
}
这个方法代码很多，剔除一些代码我们只关注performMeasure，performLayout，performDraw，这三个方法正好对应view绘制的三个阶段
至此对于view的绘制这个过程我们已经有了一个大概的了解了，梳理一下
ActivityThread.handleResumeActivity()—>(ViewManager的addView—>WindowManagerd的addView)WindowManagerImpl.addView—>WindowManagerGlobal.addView—>ViewRootImpl.setView—>ViewRootImpl.requestLayout()—>ViewRootImpl.scheduleTraversals() —>ViewRootImpl.performTraversals()—>performMeasure,performLayout,performDraw
参考自：深入理解Android之View的绘制流程 https://www.jianshu.com/p/060b5f68da79