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：
"1.0" encoding="utf-8"?>
"http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent">
"match_parent"
android:layout_height="match_parent"
android:layout_margin="15dp"
android:gravity="top"
android:hint="这是一个EditTextView"
android:inputType="textMultiLine"
android:maxLines="20"
android:minLines="6"
android:textSize="30sp" />
复制代码用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：
@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) {
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，
并且Activity的布局文件已经添加到DecorView里的mContentParent里了，当执行cb.onContentChanged()后，Activity就会回调onContentChanged()方法，由于Activity的onContentChanged()是个空实现，我们可以在子Activity中处理这个回调处理相应逻辑，到这里为止DecorView就已经被创建并初始化完毕。