一起养成写作习惯！这是我参与「掘金日新计划 · 4 月更文挑战」的第28天，点击查看活动详情。
源码调用流程如下：
1.ActivityThread
final void handleResumeActivity(IBinder token,
boolean clearHide, boolean isForward, boolean reallyResume, int seq, String reason) {
......
//回调到activity的onResume()方法
r = performResumeActivity(token, clearHide, reason);
if (r != null) {
final Activity a = r.activity;
......
if (r.window == null && !a.mFinished && willBeVisible) {
//获取到PhoneWindow
r.window = r.activity.getWindow();
//获取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) {
if (!a.mWindowAdded) {
a.mWindowAdded = true;
//通过WindowManager.addView加载DecorView
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);
}
}
......
}
}
wm是一个接口ViewManager对象，而wm是通过Activity的getWindowManager()获取的，会调用到WindowManagerImpl.addView()，最终会调用到WindowManagerGlobal.addView()：
2.WindowManagerGlobal
public void addView(View view, ViewGroup.LayoutParams params,
Display display, Window parentWindow) {
......
ViewRootImpl root;
View panelParentView = null;
......
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;
}
}
在addView()内部会创建ViewRootImpl对象，然后执行ViewRootImpl的setView()方法：
3.ViewRootImpl
public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) {
synchronized (this) {
.......
// 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();
......
}
public void requestLayout() {
if (!mHandlingLayoutInLayoutRequest) {
//check是否为主线程
checkThread();
mLayoutRequested = true;
scheduleTraversals();
}
}
void scheduleTraversals() {
if (!mTraversalScheduled) {
mTraversalScheduled = true;
mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
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");
}
performTraversals();
if (mProfile) {
Debug.stopMethodTracing();
mProfile = false;
}
}
}
private void performTraversals() {
.......
.......
.......
int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);
int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height);
......
// Ask host how big it wants to be
//Measure
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
......
//layout
performLayout(lp, mWidth, mHeight);
......
//Draw
performDraw();
}
在setView()执行后，经过一步一步的调用，最终执行到performTraversals()，在该方法内部执行view的测量、放置、绘制工作。
private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {
if (mView == null) {
return;
}
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "measure");
try {
mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);
} finally {
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
}
通过performMeasure()从根节点向下遍历View树，完成所有ViewGroup和View的测量工作，计算出所有ViewGroup和View显示出来需要的高度和宽度；
private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth,
int desiredWindowHeight) {
......
try {
host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
......
}
......
}
通过performLayout()从根节点向下遍历View树，完成所有ViewGroup和View的布局计算工作，根据测量出来的宽高及自身属性，计算出所有ViewGroup和View显示在屏幕上的区域；
private void performDraw() {
......
try {
draw(fullRedrawNeeded);
} finally {
mIsDrawing = false;
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
.......
.......
.......
}
private void draw(boolean fullRedrawNeeded) {
.......
.......
.......
}
通过performDraw()从根节点向下遍历View树，完成所有ViewGroup和View的绘制工作，根据布局过程计算出的显示区域，将所有View的当前需显示的内容画到屏幕上；
三.其他
a.是否只能在主线程更新UI？
通过上面的分析可以看到，View的显示是在onResume()之后才进行绘制的，所以在onCreate()内部可以通过new Thread()来更新UI；
当绘制完成后，就只能通过主线程更新了，为什么？
1. 为了解决多线程并发的问题！防止各个线程同时更新UI，就会出现界面错乱的问题。
2. 硬性判断checkThread()，在更新UI前都会判断当前是否为主线程，否则的话直接抛异常；
b.在onResume()中能否获取到view的宽高？
通过上面的分析可以看到，View的显示是在onResume()之后才进行绘制的，从而执行onMeasure()，只有绘制完成后，才能获取到view的宽高；
c.如何在onCreate()确保View获取到宽高后进行UI逻辑处理？
可以通过View.post(Runnable)，在Runnable内的run()方法内进行UI的逻辑处理
