从setContentView看Activity的构成

  常规地，我们会在Activity的onCreate()方法内调用setContentView(R.layout.xxx)以此来设置页面布局。那么，这个方法背后到底隐藏了什么操作呢？PS:以下源码是参照API 25

  首先，我们先进入Activity#setContentView(int layoutResID)

    public void setContentView(@LayoutRes int layoutResID) {        getWindow().setContentView(layoutResID);        initWindowDecorActionBar();    }    public Window getWindow() {        return mWindow;    }

  然后我们可以发现，实际是调用了getWindow()的setContentView()，而getWindow()返回的是成员变量mWindow，mWindow是Window类型，是个抽象类。而mWindow在Activity#attach时被赋值：

    mWindow = new PhoneWindow(this, window);

  所以实际调用的是PhoneWindow#setContentView()，进入其中：

    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;    }

  从第13行代码看出：从外部传进来的layoutResID会被LayoutInflater inflate的，然后被添加到mContentParent里。接下来我们一起探究mContentParent是什么妖怪吧。

    // 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.    ViewGroup mContentParent;

  注释上说，这个View是用来放window的内容的，可以是mDecor或者是mDecor的子View。而这个mDecor其实就是DecorView，而DecorView的解释是：

     // This is the top-level view of the window, containing the window decor.    private DecorView mDecor;

  它是Window最上层的View，它包含了Window的装饰。
  扯远了扯远了，我们回过头来看刚才的PhoneWindow#setContentView()，从上往下看，当mContentParent == null时，调用installDecor()：

    private void installDecor() {        mForceDecorInstall = false;        if (mDecor == null) {            //实例化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) {            //生成mContentParent            mContentParent = generateLayout(mDecor);        ...   }    protected DecorView generateDecor(int featureId) {        ...        return new DecorView(context, featureId, this, getAttributes());    }

  判断mDecor为空则new一个DecorView，并且把当前Window实例传给它。然后mContentParent为空则把DecorView传进generateLayout()里生成一个mContentParent。generateLayout这个方法巨长，省略一些不太重要的代码片段：

protected ViewGroup generateLayout(DecorView decor) {        ...        // Inflate the window decor.        int layoutResource;        int features = getLocalFeatures();        // System.out.println("Features: 0x" + Integer.toHexString(features));        if ((features & (1 << FEATURE_SWIPE_TO_DISMISS)) != 0) {            layoutResource = R.layout.screen_swipe_dismiss;        } else if ((features & ((1 << FEATURE_LEFT_ICON) | (1 << FEATURE_RIGHT_ICON))) != 0) {            if (mIsFloating) {                TypedValue res = new TypedValue();                getContext().getTheme().resolveAttribute(                        R.attr.dialogTitleIconsDecorLayout, res, true);                layoutResource = res.resourceId;            } else {                layoutResource = R.layout.screen_title_icons;            }            // XXX Remove this once action bar supports these features.            removeFeature(FEATURE_ACTION_BAR);            // System.out.println("Title Icons!");        } else if ((features & ((1 << FEATURE_PROGRESS) | (1 << FEATURE_INDETERMINATE_PROGRESS))) != 0                && (features & (1 << FEATURE_ACTION_BAR)) == 0) {            // Special case for a window with only a progress bar (and title).            // XXX Need to have a no-title version of embedded windows.            layoutResource = R.layout.screen_progress;            // System.out.println("Progress!");        } else if ((features & (1 << FEATURE_CUSTOM_TITLE)) != 0) {            // Special case for a window with a custom title.            // If the window is floating, we need a dialog layout            if (mIsFloating) {                TypedValue res = new TypedValue();                getContext().getTheme().resolveAttribute(                        R.attr.dialogCustomTitleDecorLayout, res, true);                layoutResource = res.resourceId;            } else {                layoutResource = R.layout.screen_custom_title;            }            // XXX Remove this once action bar supports these features.            removeFeature(FEATURE_ACTION_BAR);        } else if ((features & (1 << FEATURE_NO_TITLE)) == 0) {            // If no other features and not embedded, only need a title.            // If the window is floating, we need a dialog layout            if (mIsFloating) {                TypedValue res = new TypedValue();                getContext().getTheme().resolveAttribute(                        R.attr.dialogTitleDecorLayout, res, true);                layoutResource = res.resourceId;            } else if ((features & (1 << FEATURE_ACTION_BAR)) != 0) {                layoutResource = a.getResourceId(                        R.styleable.Window_windowActionBarFullscreenDecorLayout,                        R.layout.screen_action_bar);            } else {                layoutResource = R.layout.screen_title;            }            // System.out.println("Title!");        } else if ((features & (1 << FEATURE_ACTION_MODE_OVERLAY)) != 0) {            layoutResource = R.layout.screen_simple_overlay_action_mode;        } else {            // Embedded, so no decoration is needed.            layoutResource = R.layout.screen_simple;            // System.out.println("Simple!");        }        mDecor.startChanging();        mDecor.onResourcesLoaded(mLayoutInflater, layoutResource);        ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);        if (contentParent == null) {            throw new RuntimeException("Window couldn't find content container view");        }        ...        mDecor.finishChanging();        return contentParent;}

  前面一大段都在根据FEATURE值来决定到底是要用哪个layout文件，也就是layoutResource的值。然后调用mDecor.onResourcesLoaded()方法，再通过findViewById(ID_ANDROID_CONTENT)找到contentParent最后返回，赋值给PhoneWindow里的mContentParent。

  从前面一大段if else看出，layoutResource可以是layout.screen_title_icons，layout.screen_custom_title，layout.screen_title等。打开这些layout，可以发现都存在一个id为”@android:id/content”的FrameLayout。以layout.screen_simple为例：

<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>

  而ID_ANDROID_CONTENT的值其实就是

    /**     * 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;

  刚好能在上述layout里找到，所以能find到，是不是很magic~。既然能find到，说明肯定有把layoutResource inflate出来再add。

  我们再进入mDecor.onResourcesLoaded(mLayoutInflater, layoutResource);看一看。注意这里是调用了DecorView里的方法，所以接下来的addView都是add到DecorView里

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();    }

  可以看到，会生成mDecorCaptionView和root。如果mDecorCaptionView不为空，则把它加入到DecorView里，然后再把root加入到它里面；否则，直接把root加入到DecorView里。

总结

Activity里有phoneWindow，phoneWindow内有decorView，decorView里子View有两种情况：
  1. 先是mDecorCaptionView，往里一级才是layoutResource inflate出来的root布局
  2. 直接是root布局
再然后，root里一定包含id为”@android:id/content”的FrameLayout，FrameLayout里才是我们通过setContentView传进去的布局