我們都知道在Activity中通過setContentView(layoutId)來加載布局文件流妻，使我們的布局文件能夠顯示在手機(jī)上，那么系統(tǒng)是如何將我們的布局文件加載到界面上的呢？

setContentView

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

通過setContentView的源碼可以發(fā)現(xiàn)其實(shí)是調(diào)用了Window的setContentView方法尚蝌，而Window是一個抽象類遭庶，PhoneWindow是Window的實(shí)現(xiàn)類。

// 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;
}

代碼中首先通過installDecor()創(chuàng)建出DecorView蓖乘。
之后再通過mLayoutInflater.inflate(layoutResID, mContentParent)將我們的布局加載進(jìn)內(nèi)存锤悄。

DecorView

private void installDecor() {
   mForceDecorInstall = false;
   if (mDecor == null) {
    // 創(chuàng)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) {
    // 將android.R.id.content解析出來
       mContentParent = generateLayout(mDecor);
  }
}

installDecor主要有2個作用：

1. 創(chuàng)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. 將android.R.id.content解析出來

   protected ViewGroup generateLayout(DecorView decor) {
       // Inflate the window decor.
      int layoutResource;
      int features = getLocalFeatures();
      // 各種判斷加載系統(tǒng)布局
      if ((features & (1 << FEATURE_SWIPE_TO_DISMISS)) != 0) {
          layoutResource = R.layout.screen_swipe_dismiss;
      } else {
          layoutResource = R.layout.screen_simple;
      }
   
      mDecor.startChanging();
      // 將得到的布局文件加載到DecorView中
      mDecor.onResourcesLoaded(mLayoutInflater, layoutResource);
       // 獲取android.R.id.content
      ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);
   }
   

   generateLayout的作用就是：

   1. 通過條件獲取到系統(tǒng)需要加載的布局文件，這里我們以最簡單的R.layout.screen_simple來看：

      <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. 將得到的布局文件加載到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. 獲取android.R.id.content

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

   至此系統(tǒng)的布局就加載好了嘉抒，大概的結(jié)構(gòu)如下：

   

inflate加載我們的布局

當(dāng)系統(tǒng)的布局通過installDecor()加載完成之后零聚，就會通過mLayoutInflater.inflate(layoutResID, mContentParent)加載我們自己設(shè)置進(jìn)去的布局。

而inflate的源碼分析請參考：

1. 【Android源碼】LayoutInflater 分析
2. 【Android源碼】View的創(chuàng)建流程