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一、概览

本源码基于Android 12
看代码前，我们先上一张Activity，Window, DecorView三者之间的关系图
在这里插入图片描述

DecorView是整个ViewTree的最顶层View，它是一个FrameLayout布局，代表了整个应用的界面。
在该布局下面，有标题view和内容view两个子元素。

Activity setContentView 核心就是PhoneWindow的setContentView方法，其主要干了两件事：
1.完成DecorView的创建与加载，这个DecorView会在后面onresume后添加到window中
2.将MainActivity的布局加载到DecorView内的一个ViewGroup中

创建DecorView，即installDecor方法，其内部用到了两个核心的方法：
1.generateDecor方法创建出DecorView对象
2.generateLayout方法完成这个DecorView对象的布局加载，并完成了MainActivity的父容器的赋值（即contentParent变量）

先上一张流程图
在这里插入图片描述

二、setContentView（）

我们跟踪一下源码，看看这个方法是怎么做的

    public void setContentView(View view) {getWindow().setContentView(view);initWindowDecorActionBar();}

这里window即为 PhoneWindow，
window的初始化是在 Acticity 创建的时候初始化，在Acticity对象创建后，会调用attach方法

@UnsupportedAppUsage(maxTargetSdk = Build.VERSION_CODES.R, trackingBug = 170729553)final void attach(Context context, ActivityThread aThread, ...) {attachBaseContext(context);mFragments.attachHost(null /*parent*/);mWindow = new PhoneWindow(this, window, activityConfigCallback);mWindow.setWindowControllerCallback(mWindowControllerCallback);mWindow.setCallback(this);mWindow.setWindowManager((WindowManager)context.getSystemService(Context.WINDOW_SERVICE),mToken, mComponent.flattenToString(),(info.flags & ActivityInfo.FLAG_HARDWARE_ACCELERATED) != 0);}

PhoneWindow.java

@Overridepublic void setContentView(int layoutResID) {根view 为空，则初始 mDecor viewif (mContentParent == null) {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 {// 将布局文件添加到 mContentParentmLayoutInflater.inflate(layoutResID, mContentParent);}mContentParent.requestApplyInsets();final Callback cb = getCallback();if (cb != null && !isDestroyed()) {cb.onContentChanged();}mContentParentExplicitlySet = true;}调用installDecor()进行DecorView的初始化private void installDecor() {mForceDecorInstall = false;if (mDecor == null) {// 创建出一个DecorView并返回mDecor = generateDecor(-1);} else {mDecor.setWindow(this);}if (mContentParent == null) {//对mContentParent进行赋值,作为Activity布局的父容器，mContentParent = generateLayout(mDecor);}}

首先判断了mContentParent是否为null，如果为空则执行installDecor（）方法，同时初始化一个mContentParent，这个就是Activity布局的父容器

三、inflate

LayoutInflater.java

    public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot) {final Resources res = getContext().getResources();View view = tryInflatePrecompiled(resource, res, root, attachToRoot);if (view != null) {return view;}XmlResourceParser parser = res.getLayout(resource);try {return inflate(parser, root, attachToRoot);} finally {}}

private @NullableView tryInflatePrecompiled(@LayoutRes int resource, Resources res, @Nullable ViewGroup root, boolean attachToRoot) {try {Class clazz = Class.forName("" + pkg + ".CompiledView", false, mPrecompiledClassLoader);Method inflater = clazz.getMethod(layout, Context.class, int.class);View view = (View) inflater.invoke(null, mContext, resource);if (view != null && root != null) {XmlResourceParser parser = res.getLayout(resource);try {AttributeSet attrs = Xml.asAttributeSet(parser);advanceToRootNode(parser);ViewGroup.LayoutParams params = root.generateLayoutParams(attrs);if (attachToRoot) {root.addView(view, params);} else {view.setLayoutParams(params);}} finally {parser.close();}}return view;} catch (Throwable e) {} finally {}return null;}

布局就是这么添加进mContentParent中的。
在这里插入图片描述

但是，view还是没有显示出来的，此时代码所做的事情仅仅只是加载了布局，并没有开始view的测量、布局、绘制工作。
对应方法是onMeasure, onLayout, onDraw，这些操作在后面

四、view的绘制展示

每一个Activity组件都有一个关联的Window对象，用来描述一个应用程序窗口。每一个应用程序窗口内部又包含一个View对象，用来描述应用程序窗口的视图。
我们再看下图：
在这里插入图片描述

Activity#onResume()之后才是布局由不可见变为可见的，我们看源码

4.1 Activity.onResume

ActivityThread.java
下面这个方法是在Activity onCreate创建后调用的，handleResumeActivity，不清楚的可以看前面app启动文章.

@Overridepublic void handleResumeActivity(ActivityClientRecord r, boolean finalStateRequest,boolean isForward, String reason) {// 这个方法会调用 activity的 onResume 方法if (!performResumeActivity(r, finalStateRequest, reason)) {return;}final Activity a = r.activity;// window 未被添加进 windowmanagerif (r.window == null && !a.mFinished && willBeVisible) {// windowr.window = r.activity.getWindow();// decorViewView decor = r.window.getDecorView();decor.setVisibility(View.INVISIBLE);ViewManager wm = a.getWindowManager();WindowManager.LayoutParams l = r.window.getAttributes();a.mDecor = decor;} else if (!willBeVisible){}if (a.mVisibleFromClient) {if (!a.mWindowAdded) {// DecorView 添加到 windowwm.addView(decor, l);} else {a.onWindowAttributesChanged(l);}}if (!r.activity.mFinished && willBeVisible && r.activity.mDecor != null && !r.hideForNow) {使布局可见if (r.activity.mVisibleFromClient) {r.activity.makeVisible();}}r.nextIdle = mNewActivities;}

在上面的代码中，会先调用Activity的onResume，然后再是view的绘制，最后将DecorView 设置可见;

4.2 WindowManager addView

WindowManagerImpl.java

private final WindowManagerGlobal mGlobal = WindowManagerGlobal.getInstance();...@Overridepublic void addView(View view, ViewGroup.LayoutParams params) {mGlobal.addView(view, params, mDisplay, mParentWindow);}

这里也是一个空壳代码，调用WindowManagerGlobal

WindowManagerGlobal.java

    public void addView(View view, ViewGroup.LayoutParams params,Display display, Window parentWindow, int userId) {final WindowManager.LayoutParams wparams = (WindowManager.LayoutParams) params;ViewRootImpl root;View panelParentView = null;// 加锁synchronized (mLock) {//实例化ViewRootImpl类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 thingstry {// //调用ViewRootImpl.setView方法，把DecorView作为参数传递进去root.setView(view, wparams, panelParentView, userId);} catch (RuntimeException e) {// BadTokenException or InvalidDisplayException, clean up.if (index >= 0) {removeViewLocked(index, true);}throw e;}}}

在方法内部，会通过跨进程方式向WMS（WindowManagerService）发起一个调用，从而将DecorView最终加到Window上，在这个过程中，ViewRootImpl、DecorView和WMS会彼此关联。
最后，WMS调用ViewRootImpl.performTraversals 方法开始View的测量、布局、绘制。

4.3 ViewRootImpl

一个 Window 对应着一个 ViewRootImpl 和一个 VIew。这个 View 就是被 ViewRootImpl 操作的.

从上面代码，我们可以看到，ViewRootImpl的初始化是在WindowManagerGlobal的addView中

ViewRootImpl.java


/*** We have one child*/public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView, int userId) {synchronized (this) {if (mView == null) {mView = view;mAdded = true;int res; /* = WindowManagerImpl.ADD_OKAY; */// 刷新布局的操作，触发view的measure -> layout -> draw 操作requestLayout();try {//将 View 添加到 WMS 中res = mWindowSession.addToDisplayAsUser(mWindow, mWindowAttributes, ...);} catch (RemoteException e) {} finally {}// Set up the input pipeline. 设置了一系列的输入通道CharSequence counterSuffix = attrs.getTitle();mSyntheticInputStage = new SyntheticInputStage();InputStage viewPostImeStage = new ViewPostImeInputStage(mSyntheticInputStage);}}}

首先会调用requestLayout方法来刷新布局，然后将 View 添加到 WMS 中，最后是view事件的处理；
view事件的处理，最后还是会回到了 PhoneWindow 中的 DecorView 来处理，剩下的就是从 DecorView 开始将事件层层传递给内部的子 View 中了
这里就不展开

ViewGroup.java@Overridepublic boolean dispatchTouchEvent(MotionEvent ev) {}

    @Overridepublic void requestLayout() {if (!mHandlingLayoutInLayoutRequest) {checkThread();mLayoutRequested = true;scheduleTraversals();}}final class TraversalRunnable implements Runnable {@Overridepublic void run() {doTraversal();}}void doTraversal() {if (mTraversalScheduled) {performTraversals();}}

requestLayout()最终会调用到performTraversals，在这个方法中会调用 View 的 measure() ，layout() ，draw() 方法。
我们看下面源码


private void performTraversals() {final View host = mView;if (mFirst || windowShouldResize || viewVisibilityChanged || params != null|| mForceNextWindowRelayout) {try {if (!mPendingMergedConfiguration.equals(mLastReportedMergedConfiguration)) {performConfigurationChange(new MergedConfiguration(mPendingMergedConfiguration),!mFirst, INVALID_DISPLAY /* same display */);}} catch (RemoteException e) {}if (!mStopped || wasReportNextDraw) {//View 的测量performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);if (measureAgain) {//View 的测量performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);}layoutRequested = true;}}} else {}if (didLayout) {// View 的布局performLayout(lp, mWidth, mHeight);}if (!cancelDraw) {// View 的绘制performDraw();} else {}mIsInTraversal = false;
}

4.4 addWindow & makeVisible

com.android.server.wm.Session.java

@Overridepublic int addToDisplay(IWindow window, WindowManager.LayoutParams attrs,int viewVisibility, int displayId, InsetsVisibilities requestedVisibilities,InputChannel outInputChannel, InsetsState outInsetsState,InsetsSourceControl[] outActiveControls) {return mService.addWindow(this, window, attrs, viewVisibility, displayId,UserHandle.getUserId(mUid), requestedVisibilities, outInputChannel, outInsetsState,outActiveControls);}

Activity.java

把DecorView的状态设置为可见，那么布局也就可见了void makeVisible() {if (!mWindowAdded) {ViewManager wm = getWindowManager();wm.addView(mDecor, getWindow().getAttributes());mWindowAdded = true;}mDecor.setVisibility(View.VISIBLE);}