Activityの生成
ActivityThread类中有个handleLaunchActivity方法。
public Activity handleLaunchActivity(ActivityClientRecord r,
PendingTransactionActions pendingActions, Intent customIntent){
...
final Activity a = performLaunchActivity(r, customIntent);
return a;
}
Activity对象便是在这个方法中通过反射创建了。该方法内部紧接着又调用了activity.attach方法，以及callActivityOnCreate方法
private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
...
Activity activity = null;
try {
java.lang.ClassLoader cl = appContext.getClassLoader();
// 内部实现
// Class.forName(className, false, cl).asSubclass(Activity.class)
// .getDeclaredConstructor().newInstance();
activity = mInstrumentation.newActivity(
cl, component.getClassName(), r.intent);
StrictMode.incrementExpectedActivityCount(activity.getClass());
r.intent.setExtrasClassLoader(cl);
r.intent.prepareToEnterProcess();
if (r.state != null) {
r.state.setClassLoader(cl);
}
} catch (Exception e) {
//
}
...
activity.attach(appContext, this, getInstrumentation(), r.token,
r.ident, app, r.intent, r.activityInfo, title, r.parent,
r.embeddedID, r.lastNonConfigurationInstances, config,
r.referrer, r.voiceInteractor, window, r.configCallback,
r.assistToken);
...
//显然下面这个方法是奔着activity.onCreate()去的。
if (r.isPersistable()) {
mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
} else {
mInstrumentation.callActivityOnCreate(activity, r.state);
}
return activity;
}
Activity.attach()中创建了mWindow和mWindowManager两个对象。其中mWindow是PhoneWindow()。
final void attach(Context context, ...){
mWindow = new PhoneWindow(this, window, activityConfigCallback);
mWindow.setCallback(this);
...
mWindow.setWindowManager(
(WindowManager)context.getSystemService(Context.WINDOW_SERVICE),
mToken, mComponent.flattenToString(),
(info.flags & ActivityInfo.FLAG_HARDWARE_ACCELERATED) != 0);
mWindowManager = mWindow.getWindowManager();
...
}
SetWindowManager的中告诉我们mWindowManager实现类是WindowManagerImpl。
public void setWindowManager(WindowManager wm, IBinder appToken, String appName,
boolean hardwareAccelerated) {
mAppToken = appToken;
mAppName = appName;
mHardwareAccelerated = hardwareAccelerated;
if (wm == null) {
wm = (WindowManager)mContext.getSystemService(Context.WINDOW_SERVICE);
}
mWindowManager = ((WindowManagerImpl)wm).createLocalWindowManager(this);
}
Activity关联布局
在onCreate的方法中，我们一般会setContentView。而setContentView内部是用的window.setContentView()。所以可以直接看PhoneWindow的setContentView方法；
public void setContentView(@LayoutRes int layoutResID) {
getWindow().setContentView(layoutResID);
initWindowDecorActionBar();
}
@Override
public void setContentView(int layoutResID) {
// Note: FEATURE_CONTENT_TRANSITIONS may be set in the process of installing the window
// decor, when theme attributes and the like are crystalized. Do not check the feature
// before this happens.
if (mContentParent == null) {
//内部做了3件事：
//new DecorView()并添加布局
//mContentParent = findViewById(R.id.content)
installDecor();
} else if (!hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
mContentParent.removeAllViews();
}
if (hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
final Scene newScene = Scene.getSceneForLayout(mContentParent, layoutResID,
getContext());
transitionTo(newScene);
} else {
//把layoutResId inflate到mContentParent上去
mLayoutInflater.inflate(layoutResID, mContentParent);
}
mContentParent.requestApplyInsets();
final Callback cb = getCallback();
if (cb != null && !isDestroyed()) {
cb.onContentChanged();
}
mContentParentExplicitlySet = true;
}
installDecor中为我们生成了一个DecorView，也就是所有Activity的顶层Veiw。【DecorView继承自FramLayout】
private void installDecor() {
if (mDecor == null) {
mDecor = generateDecor(-1);
} else {
mDecor.setWindow(this);
}
if (mContentParent == null) {
//generateLayout中为mDecor选定一个布局（根据feature）并addView上去
//并返回这个布局的ID为R.id.content的ViewGroup--findViewById(ID_ANDROID_CONTENT)
mContentParent = generateLayout(mDecor);
}
...
}
为顶层Decor添加parent
然后跟着生命周期走到了ActivityThread.handleResumeActivity()方法：
public void handleResumeActivity(...){
//间接调用Activity.onResume
final ActivityClientRecord r = performResumeActivity(token, finalStateRequest, reason);
...
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;
...
//重点在这
wm.addView(decor, l);
}
...
}
windowManager就是第一步里面我加粗了的那个对象。windowManagerImpl
private final WindowManagerGlobal mGlobal = WindowManagerGlobal.getInstance();
@Override
public void addView(@NonNull View view, @NonNull ViewGroup.LayoutParams params) {
applyDefaultToken(params);
mGlobal.addView(view, params, mContext.getDisplayNoVerify(), mParentWindow,
mContext.getUserId());
}
所以直接开WindowManagerGlobal.addView():这里传入的view就是decorView，也就是Activity顶层View。
public void addView(View view, ViewGroup.LayoutParams params,
Display display, Window parentWindow, int userId) {
ViewRootImpl root = new ViewRootImpl(view.getContext(), display);
...
//decor成了root(ViewRootImpl)的子view;
//想点进去看的话可以进去后找 view.assignParent(this);方法
root.setView(view, wparams, panelParentView, userId);
}
至此，ViewRootImpl就成了所有view的祖宗view了。
view绘制三部曲（之旅）
如果读者还有兴致，可以点开root.setView方法，方法中在assignParent之前有一行：
public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView,
int userId) {
synchronized (this) {
...
requestLayout();
...
view.assignParent(this);
...
}
}
@Override
public void requestLayout() {
if (!mHandlingLayoutInLayoutRequest) {
checkThread();
mLayoutRequested = true;
scheduleTraversals();
}
}
void checkThread() {
//著名的线程检查的地方，流传多年的名句“子线程不能更新UI”的判断就在这里。
//其实按上面的逻辑，root(也就是ViewRootImpl)在resume之前还未成为decorView的parent，所以可以推理出：
//在onResume之前，其实是可以子线程更新UI的，感兴趣的可以试一试。
if (mThread != Thread.currentThread()) {
throw new CalledFromWrongThreadException(
"Only the original thread that created a view hierarchy can touch its views.");
}
}
这里的requestLayout就是整个View Tree第一次开始绘制的地方了。scheduleTraversals方法也在ViewRootImpl类中：
final TraversalRunnable mTraversalRunnable = new TraversalRunnable();
void scheduleTraversals() {
if (!mTraversalScheduled) {
mTraversalScheduled = true;
mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
mChoreographer.postCallback(
Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
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;
}
}
}
//默认的super(LayoutParams.MATCH_PARENT, LayoutParams.MATCH_PARENT);
public final WindowManager.LayoutParams mWindowAttributes = new WindowManager.LayoutParams();
private void performTraversals() {
...
WindowManager.LayoutParams lp = mWindowAttributes;
int desiredWindowWidth;
int desiredWindowHeight;
...
Rect frame = mWinFrame;//屏幕大小
if (mFirst) {
...
if (...) {
...
} else {
//宽高默认为屏幕大小
desiredWindowWidth = frame.width();
desiredWindowHeight = frame.height();
}
...
//把mAttachInfo向下传个子view，所以子view中的mAttachInfo，都是ViewRootImpl中的mAttachInfo。
//view.post（）中其实就是用到的mAttachInfo.mHandler,这个mHandler就是ViewRootImpl构造时new的。
host.dispatchAttachedToWindow(mAttachInfo, 0);
mAttachInfo.mTreeObserver.dispatchOnWindowAttachedChange(true);
//状态栏和导航栏在下面这个方法设置
dispatchApplyInsets(host);
}
...
//view.post如果在resume之前执行，由于viewRootImpl还未成为decor的parent，所以拿不到mHandler，所以就会存到队列中，此时再执行
getRunQueue().executeActions(mAttachInfo.mHandler);
if (layoutRequested) {
...
// Ask host how big it wants to be
windowSizeMayChange |= measureHierarchy(host, lp, res,
desiredWindowWidth, desiredWindowHeight);
}
...
performLayout(lp, mWidth, mHeight);
mAttachInfo.mTreeObserver.dispatchOnGlobalLayout();
...
boolean cancelDraw = mAttachInfo.mTreeObserver.dispatchOnPreDraw() || !isViewVisible;
performDraw()
}
measureHierarchy也在该类下：
private boolean measureHierarchy(final View host, final WindowManager.LayoutParams lp,
final Resources res, final int desiredWindowWidth, final int desiredWindowHeight){
boolean goodMeasure = false;
if (!goodMeasure) {
childWidthMeasureSpec = getRootMeasureSpec(desiredWindowWidth, lp.width);
childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height);
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
if (mWidth != host.getMeasuredWidth() || mHeight != host.getMeasuredHeight()) {
windowSizeMayChange = true;
}
}
...
}
private static int getRootMeasureSpec(int windowSize, int rootDimension) {
int measureSpec;
switch (rootDimension) {
case ViewGroup.LayoutParams.MATCH_PARENT:
// Window can't resize. Force root view to be windowSize.
measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);
break;
case ViewGroup.LayoutParams.WRAP_CONTENT:
// Window can resize. Set max size for root view.
measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);
break;
default:
// Window wants to be an exact size. Force root view to be that size.
measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);
break;
}
return measureSpec;
}
private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {
if (mView == null) {
return;
}
//mview也就是viewRootImpl
mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
到这里，decor开始它的测量，由此遍历整颗view Tree。