Activity源码浅析－DecorView

使用Activity，我们会先在onCreate方法中，设置setContentView

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

getWindow().setContentView(layoutResId)这句话是把我们的布局放到哪里了呢？

    public Window getWindow() {        return mWindow;    }

Window是什么呢？

public abstract class Window｛......｝

Window：规定了顶层窗口的视觉和行为规范的抽象类。它的实例用于将最顶层的View添加到WindowManager。它提供了基本的UI规范比如背景，标题区域，默认的关键处理。
Window只有一个实例PhoneWindow

现在我们知道了Window是什么，接下来继续分析它的实例PhoneWidow：getWindow().setContentView(layoutResID);

public class PhoneWindow extends Window implements MenuBuilder.Callback {    //mDecor本身或者布局添加的子View    ViewGroup mContentParent;    @Override    public void setContentView(int layoutResID) {        //如果顶层容器为null，则创建Decor        if (mContentParent == null) {        //接下来分析这里,创建Decor            installDecor();        }        .........        if (mContentParent == null) {        //这里证明mContentParent是mDecor或mDecor的子View            mContentParent = generateLayout(mDecor);            ......         ｝        if (hasFeature(FEATURE_CONTENT_TRANSITIONS)) {            final Scene newScene = Scene.getSceneForLayout(mContentParent, layoutResID,                    getContext());            transitionTo(newScene);        } else {        //将设置的布局添加到mContentView            mLayoutInflater.inflate(layoutResID, mContentParent);        }        mContentParent.requestApplyInsets();        final Callback cb = getCallback();        if (cb != null && !isDestroyed()) {            cb.onContentChanged();        }        mContentParentExplicitlySet = true;｝

接下来我们分析一下installDecor做了什么呢？

    private void installDecor() {        mForceDecorInstall = false;        if (mDecor == null) {        ／/创建Decor，接下来看generateDecor            mDecor = generateDecor(-1);            ..........            }        } else {            mDecor.setWindow(this);        } }

generateDecor(int featureId)创建Decor

    protected DecorView generateDecor(int featureId) {    ....        return new DecorView(context, featureId, this, getAttributes());    }

DecorView是什么呢？

public class DecorView extends FrameLayout implements RootViewSurfaceTaker, WindowCallbacks {    ....}

DecorView实际上是一个FrameLayout

我们回到PhoneWindow的setContentView继续分析

protected ViewGroup generateLayout(DecorView decor) {        TypedArray a = getWindowStyle();        ......        //是否悬浮        mIsFloating = a.getBoolean(R.styleable.Window_windowIsFloating, false);        int flagsToUpdate = (FLAG_LAYOUT_IN_SCREEN|FLAG_LAYOUT_INSET_DECOR)                & (~getForcedWindowFlags());        if (mIsFloating) {            setLayout(WRAP_CONTENT, WRAP_CONTENT);            setFlags(0, flagsToUpdate);        } else {            setFlags(FLAG_LAYOUT_IN_SCREEN|FLAG_LAYOUT_INSET_DECOR, flagsToUpdate);        }//是否有title        if (a.getBoolean(R.styleable.Window_windowNoTitle, false)) {            requestFeature(FEATURE_NO_TITLE);        } else if (a.getBoolean(R.styleable.Window_windowActionBar, false)) {            // Don't allow an action bar if there is no title.            requestFeature(FEATURE_ACTION_BAR);        }...//是否全屏        if (a.getBoolean(R.styleable.Window_windowFullscreen, false)) {            setFlags(FLAG_FULLSCREEN, FLAG_FULLSCREEN & (~getForcedWindowFlags()));        }  ......        // 接下来Decoview inflate 子View        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添加对应的布局        mDecor.onResourcesLoaded(mLayoutInflater, layoutResource);        ......  //证明是Decor的子View          ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);        if (contentParent == null) {            throw new RuntimeException("Window couldn't find content container view");        }        ......return contentParent;｝

通过上面代码可以知道generateLayout获取了Window的属性，比如是否全屏，是否悬浮，是否有title等。然后根据这些属性来选择默认的布局文件进行映射，比如screen_title.xml

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"    android:orientation="vertical"    android:fitsSystemWindows="true">    <!-- Popout bar for action modes -->    <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:layout_width="match_parent"         android:layout_height="?android:attr/windowTitleSize"        style="?android:attr/windowTitleBackgroundStyle">        <TextView android:id="@android:id/title"             style="?android:attr/windowTitleStyle"            android:background="@null"            android:fadingEdge="horizontal"            android:gravity="center_vertical"            android:layout_width="match_parent"            android:layout_height="match_parent" />    </FrameLayout>    <FrameLayout android:id="@android:id/content"        android:layout_width="match_parent"         android:layout_height="0dip"        android:layout_weight="1"        android:foregroundGravity="fill_horizontal|top"        android:foreground="?android:attr/windowContentOverlay" /></LinearLayout>

然后通过

mDecor.onResourcesLoaded(mLayoutInflater, layoutResource);        ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);

实现mDecorView添加默认布局，和通过id获取子view：contentParent

通过上面的源码，我们就对Activity的布局层级有了一个清晰的认识，结构如图：