大家都知道在Activity的onCreate()中调用Activity.setContent()方法可以加载布局文件以设置该Activity的显示界面。本文将从setContentView()的源码谈起，分析布局文件加载所涉及到的调用链。本文所用的源码为android-19.

Step 1  、Activity.setContentView(intresId)

public void setContentView(int layoutResID) {
	getWindow().setContentView(layoutResID);
	initActionBar();
}

public Window getWindow() {
	return mWindow;
}

Step 2  、PhoneWindow.setContentView()

@Override
public void setContentView(int layoutResID) {
	if (mContentParent == null) {
		installDecor();
	} else {
		mContentParent.removeAllViews();
	}
	// 将布局文件添加到mContentParent中
	mLayoutInflater.inflate(layoutResID, mContentParent);
	final Callback cb = getCallback();
	if (cb != null && !isDestroyed()) {
		cb.onContentChanged();
	}
}

Step 3、 PhoneWindow.installDecor() 
这段代码比较长，下面的伪代码只介绍逻辑，读者可自行查看源码。

private void installDecor() {
	if (mDecor == null) {
		/*创建一个DecorView对象并设置相应的属性。DecorView是所有View的根View*/
		mDecor = generateDecor();
		mDecor.setDescendantFocusability(ViewGroup.FOCUS_AFTER_DESCENDANTS);
		mDecor.setIsRootNamespace(true);
		if (!mInvalidatePanelMenuPosted && mInvalidatePanelMenuFeatures != 0) {
			mDecor.postOnAnimation(mInvalidatePanelMenuRunnable);
		}
	}
	if (mContentParent == null) {
		/*创建mContentParent对象*/
		mContentParent = generateLayout(mDecor);

		// Set up decor part of UI to ignore fitsSystemWindows if appropriate.
		mDecor.makeOptionalFitsSystemWindows();
		
		mTitleView = (TextView)findViewById(com.android.internal.R.id.title);
		if (mTitleView != null) {
	
			mTitleView.setLayoutDirection(mDecor.getLayoutDirection());
			// 根据features值，设置Title的相关属性
			if ((getLocalFeatures() & (1 << FEATURE_NO_TITLE)) != 0) {
				View titleContainer = findViewById(com.android.internal.R.id.title_container);
				if (titleContainer != null) {
					titleContainer.setVisibility(View.GONE);
				} else {
					mTitleView.setVisibility(View.GONE);
				}
				if (mContentParent instanceof FrameLayout) {
					((FrameLayout)mContentParent).setForeground(null);
				}
			} else {
				mTitleView.setText(mTitle);
			}
		} else {
			//若没有Title，则设置ActionBar的相关属性，如回调函数、风格属性
			mActionBar = (ActionBarView) findViewById(com.android.internal.R.id.action_bar);
			if (mActionBar != null) {
				//.......
				
				// 推迟调用invalidate，放置onCreateOptionsMenu在 onCreate的时候被调用
				mDecor.post(new Runnable() {
					public void run() {
						// Invalidate if the panel menu hasn't been created before this.
						PanelFeatureState st = getPanelState(FEATURE_OPTIONS_PANEL, false);
						if (!isDestroyed() && (st == null || st.menu == null)) {
							invalidatePanelMenu(FEATURE_ACTION_BAR);
						}
					}
				});
			}
		}
	}
}

protected ViewGroup generateLayout(DecorView decor) {
	// Apply data from current theme.
	//获取当前Window主题的属性数据，相关字段的文件位置为：sdk\platforms\android-19\data\res\values\attrs.xml
	//这些属性值可以在AndroidManifest.xml中设置Activityandroid:theme="",也可以在Activity的onCreate中通过
	//requestWindowFeature()来设置.注意，该方法一定要在setContentView之前被调用
	TypedArray a = getWindowStyle(); 
	//获取android:theme=""中设置的theme
	//根据主题属性值来设置PhoneWindow的特征与布局，包括Title、ActionBar、ActionBar的模式、Window的尺寸等属性。
	//........
	

	// Inflate the window decor.
	// 根据上面设置的Window feature来确定布局文件
	// Android SDK内置布局文件夹位置为：sdk\platforms\android-19\data\res\layout
	典型的窗口布局文件有：
		  R.layout.dialog_titile_icons                          R.layout.screen_title_icons
		  R.layout.screen_progress                             R.layout.dialog_custom_title
		  R.layout.dialog_title   
		  R.layout.screen_title         // 最常用的Activity窗口修饰布局文件
		  R.layout.screen_simple    //全屏的Activity窗口布局文件
	
	int layoutResource;
	int features = getLocalFeatures(); //会调用requesetWindowFeature()
	// ......
	mDecor.startChanging();
	// 将布局文件转换成View数，然后添加到DecorView中
	View in = mLayoutInflater.inflate(layoutResource, null);
	decor.addView(in, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));
	//取出作为Content的ViewGroup， android:id="@android:id/content",是一个FrameLayout
	ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);
	if (contentParent == null) {
		throw new RuntimeException("Window couldn't find content container view");
	}

	if ((features & (1 << FEATURE_INDETERMINATE_PROGRESS)) != 0) {
		ProgressBar progress = getCircularProgressBar(false);
		if (progress != null) {
			progress.setIndeterminate(true);
		}
	}

	// 将顶层Window的背景、标题、Frame等属性设置成以前的
	if (getContainer() == null) {
		//......
	}

	mDecor.finishChanging();

	return contentParent;
}

//系统resume一个Activity时，调用此方法  
final void handleResumeActivity(IBinder token, boolean clearHide, boolean isForward) {  
    ActivityRecord r = performResumeActivity(token, clearHide);  
    //...  
     if (r.activity.mVisibleFromClient) {  
         r.activity.makeVisible();  
     }  
} 

void makeVisible() {  
    if (!mWindowAdded) {  
        ViewManager wm = getWindowManager();   // 获取WindowManager对象  
        wm.addView(mDecor, getWindow().getAttributes());  
        mWindowAdded = true;  
    }  
    mDecor.setVisibility(View.VISIBLE); //使其处于显示状况  
}