当我们新建一个工程的时候，选择生成一个Activity,AS会自动给我们生成一个界面，那么这个界面是怎么生成的呢，下面我们就来分析一下：
Activity中的代码：
@Overrideprotected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main);}复制代码 XML中的代码：
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用Hierarchy viewer工具来查看一下结构图： ![@P8]6~ZC7~MJ~LJFG(DPBWA.png](http://upload-images.jianshu.io/upload_images/587163-e85daee233003243.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)
DecorView以及它的子View一目了然，先看下SetContentView()的实现：
@Overridepublic 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) { 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 { mLayoutInflater.inflate(layoutResID, mContentParent); } mContentParent.requestApplyInsets(); final Callback cb = getCallback(); if (cb != null && !isDestroyed()) { cb.onContentChanged(); }}复制代码 // This is the top-level view of the window, containing the window decor.private DecorView mDecor;// This is the view in which the window contents are placed. It is either// mDecor itself, or a child of mDecor where the contents go.//mContentParent 是放置窗体内容的容器，也就是我们 setContentView() //时所加入的 View 视图树private ViewGroup mContentParent;复制代码
DecorView是PhoneWindow的一个内部类，同时DecorView也是Activiy的顶级View，一般来说DecorView的内部包括导航栏（NavigationBar）和状态栏（StatusBar）,但这个会随着主题的变化而发生改变。刚开始mContentParent的值是null，所以会走installDecor()：
if (mDecor == null) { mDecor = generateDecor(); mDecor.setDescendantFocusability(ViewGroup.FOCUS_AFTER_DESCENDANTS); mDecor.setIsRootNamespace(true); if (!mInvalidatePanelMenuPosted && mInvalidatePanelMenuFeatures != 0) { mDecor.postOnAnimation(mInvalidatePanelMenuRunnable); }}if (mContentParent == null) { mContentParent = generateLayout(mDecor); // Set up decor part of UI to ignore fitsSystemWindows if appropriate. mDecor.makeOptionalFitsSystemWindows(); final DecorContentParent decorContentParent = (DecorContentParent) mDecor.findViewById(R.id.decor_content_parent);复制代码 protected DecorView generateDecor() { return new DecorView(getContext(), -1);}复制代码 在installDecor()里面通过调用generateDecor()方法来初始化DecorView,此时DecorView什么都没有，只是一个空的FrameLayout,往下走，来到generateLayout(mDecor)，跳过一些主题布局设置，直接来到关键代码：
mDecor.startChanging();View in = mLayoutInflater.inflate(layoutResource, null);decor.addView(in, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));mContentRoot = (ViewGroup) in;ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);if (contentParent == null) { throw new RuntimeException("Window couldn't find content container view");}复制代码 /** * The ID that the main layout in the XML layout file should have. */public static final int ID_ANDROID_CONTENT = com.android.internal.R.id.content;复制代码
DecorView通过generateLayout(mDecor)加载到具体的布局文件，具体的布局文件和系统版本以及主题有关，ID_ANDROID_CONTENT对应的即是R.id.content，也是我们通过SetContentView()中设置的布局id,走完installDecor()，我们继续看PhoneWindow中的SetContentView()方法，
if (hasFeature(FEATURE_CONTENT_TRANSITIONS)) { final Scene newScene = Scene.getSceneForLayout(mContentParent, layoutResID,getContext()); transitionTo(newScene); } else { mLayoutInflater.inflate(layoutResID, mContentParent); }复制代码 接下来就简单了，mLayoutInflater.inflate(layoutResID, mContentParent)，因为前面通过installDecor()创建了DecorView,因此这一步就是将Activity的布局（layoutResID）添加到mContentParent中了，到这里为止，Activity的布局文件就已经添加到DecorView里面了，继续看SetcontentView():
mContentParent.requestApplyInsets();final Callback cb = getCallback();if (cb != null && !isDestroyed()) { cb.onContentChanged();}复制代码 由于Activity实现了Window的Callback接口，