Android 从SetContentView()谈起

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);}
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XML中的代码：

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"   
 android:layout_width="match_parent"    
 android:layout_height="match_parent">  
  <EditText     
     android:layout_width="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" />
  </RelativeLayout>
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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：

@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();   
 }}
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// 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;
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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);
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protected DecorView generateDecor() {  
  return new DecorView(getContext(), -1);
}
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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");
}
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/** * 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;
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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);
 }
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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();
}
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由于Activity实现了Window的Callback接口，