我们都知道在Activity中通过setContentView(layoutId)来加载布局文件，使我们的布局文件能够显示在手机上，那么系统是如何将我们的布局文件加载到界面上的呢？

setContentView

// Activity.java
public void setContentView(@LayoutRes int layoutResID) {
   getWindow().setContentView(layoutResID);
   initWindowDecorActionBar();
}

通过setContentView的源码可以发现其实是调用了Window的setContentView方法，而Window是一个抽象类，PhoneWindow是Window的实现类。

// PhoneWindow.java
@Override
public void setContentView(int layoutResID) {
   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();
   }
   mContentParentExplicitlySet = true;
}

代码中首先通过installDecor()创建出DecorView。
之后再通过mLayoutInflater.inflate(layoutResID, mContentParent)将我们的布局加载进内存。

DecorView

private void installDecor() {
   mForceDecorInstall = false;
   if (mDecor == null) {
    // 创建DecorView
       mDecor = generateDecor(-1);
       mDecor.setDescendantFocusability(ViewGroup.FOCUS_AFTER_DESCENDANTS);
       mDecor.setIsRootNamespace(true);
       if (!mInvalidatePanelMenuPosted && mInvalidatePanelMenuFeatures != 0) {
           mDecor.postOnAnimation(mInvalidatePanelMenuRunnable);
       }
   } else {
       mDecor.setWindow(this);
   }
   if (mContentParent == null) {
    // 将android.R.id.content解析出来
       mContentParent = generateLayout(mDecor);
  }
}

installDecor主要有2个作用：

1. 创建DeorView

   protected DecorView generateDecor(int featureId) {
      Context context;
      if (mUseDecorContext) {
          Context applicationContext = getContext().getApplicationContext();
          if (applicationContext == null) {
              context = getContext();
          } else {
              context = new DecorContext(applicationContext, getContext().getResources());
              if (mTheme != -1) {
                  context.setTheme(mTheme);
              }
          }
      } else {
          context = getContext();
      }
      return new DecorView(context, featureId, this, getAttributes());
   }
   

2. 将android.R.id.content解析出来

   protected ViewGroup generateLayout(DecorView decor) {
       // Inflate the window decor.
      int layoutResource;
      int features = getLocalFeatures();
      // 各种判断加载系统布局
      if ((features & (1 << FEATURE_SWIPE_TO_DISMISS)) != 0) {
          layoutResource = R.layout.screen_swipe_dismiss;
      } else {
          layoutResource = R.layout.screen_simple;
      }
   
      mDecor.startChanging();
      // 将得到的布局文件加载到DecorView中
      mDecor.onResourcesLoaded(mLayoutInflater, layoutResource);
       // 获取android.R.id.content
      ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);
   }
   

   generateLayout的作用就是：

   1. 通过条件获取到系统需要加载的布局文件，这里我们以最简单的R.layout.screen_simple来看：

      <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
          android:layout_width="match_parent"
          android:layout_height="match_parent"
          android:fitsSystemWindows="true"
          android:orientation="vertical">
          <ViewStub android:id="@+id/action_mode_bar_stub"
                    android:inflatedId="@+id/action_mode_bar"
                    android:layout="@layout/action_mode_bar"
                    android:layout_width="match_parent"
                    android:layout_height="wrap_content"
                    android:theme="?attr/actionBarTheme" />
          <FrameLayout
               android:id="@android:id/content"
               android:layout_width="match_parent"
               android:layout_height="match_parent"
               android:foregroundInsidePadding="false"
               android:foregroundGravity="fill_horizontal|top"
               android:foreground="?android:attr/windowContentOverlay" />
      </LinearLayout>
      

   2. 将得到的布局文件加载到DecorView中：

      void onResourcesLoaded(LayoutInflater inflater, int layoutResource) {
      
         mDecorCaptionView = createDecorCaptionView(inflater);
         final View root = inflater.inflate(layoutResource, null);
         if (mDecorCaptionView != null) {
             if (mDecorCaptionView.getParent() == null) {
                 addView(mDecorCaptionView,
                         new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));
             }
             mDecorCaptionView.addView(root,
                     new ViewGroup.MarginLayoutParams(MATCH_PARENT, MATCH_PARENT));
         } else {
      
             // Put it below the color views.
             addView(root, 0, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));
         }
         mContentRoot = (ViewGroup) root;
         initializeElevation();
      

   }
   ```

   3. 获取android.R.id.content

      public static final int ID_ANDROID_CONTENT = com.android.internal.R.id.content;
      
      ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);
      

   至此系统的布局就加载好了，大概的结构如下：

   

inflate加载我们的布局

当系统的布局通过installDecor()加载完成之后，就会通过mLayoutInflater.inflate(layoutResID, mContentParent)加载我们自己设置进去的布局。

而inflate的源码分析请参考：

1. 【Android源码】LayoutInflater 分析
2. 【Android源码】View的创建流程