Android高级-View是如何被添加到屏幕窗口上

 View被添加到屏幕上分为以下几个步骤:

                                          

1  创建 顶层布局容器DecorView

新建一个extends的Activty 类，从onCreate方法的setContentView方法进入源码开始分析

setContentView(R.layout.activity_s_econd);

/**
 * Set the activity content from a layout resource.  The resource will be
 * inflated, adding all top-level views to the activity.
 *
 * @param layoutResID Resource ID to be inflated.
 *
 * @see #setContentView(android.view.View)
 * @see #setContentView(android.view.View, android.view.ViewGroup.LayoutParams)
 */
public void setContentView(@LayoutRes int layoutResID) {
    getWindow().setContentView(layoutResID);
    initWindowDecorActionBar();
}

其中getWindow()返回的是Window对象，

/**
 * Retrieve the current {@link android.view.Window} for the activity.
 * This can be used to directly access parts of the Window API that
 * are not available through Activity/Screen.
 *
 * @return Window The current window, or null if the activity is not
 *         visual.
 */
public Window getWindow() {
    return mWindow;
}

因 Winow是抽象类，所以我们需要找到它的实现类,红字部分提示，PhoneWindow是Window的唯一实现类，所以我们继续进入PhoneWindow的源码查看setContentView()方法

/**
 * Abstract base class for a top-level window look and behavior policy.  An
 * instance of this class should be used as the top-level view added to the
 * window manager. It provides standard UI policies such as a background, title
 * area, default key processing, etc.
 *
 * 
The only existing implementation of this abstract class is
 * android.view.PhoneWindow, which you should instantiate when needing a
 * Window.
 */
public abstract class Window {

PhoneWindow的setContentView()

@Override
public void setContentView(int layoutResID) {
    // Note: FEATURE_CONTENT_TRANSITIONS may be set in the process of installing the window
    // decor, when theme attributes and the like are crystalized. Do not check the feature
    // before this happens.
    if (mContentParent == null) {
        installDecor();  重点
    } 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);
    }
   //省略...
}

接上文的installDecor（）方法

private void installDecor() {
    mForceDecorInstall = false;
    if (mDecor == null) {
        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) {
        mContentParent = generateLayout(mDecor);
        // Set up decor part of UI to ignore fitsSystemWindows if appropriate.
        mDecor.makeOptionalFitsSystemWindows();

        final DecorContentParent decorContentParent = (DecorContentParent) mDecor.findViewById(
                R.id.decor_content_parent);
    //省略....
    }
}

从上面可以看到当mDecor  (private DecorView mDecor)  为空时候，调用generateDecor方法创建了一个新的，

protected DecorView generateDecor(int featureId) {
    // System process doesn't have application context and in that case we need to directly use
    // the context we have. Otherwise we want the application context, so we don't cling to the
    // activity.
    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());
}

中间一大段都是在初始化不同的上下文对象，而方法最终返回的是 new DecorView();

2  在顶层布局中加载基础布局ViewGroup

拿到DecorView 以后，，，继续看的installDecor（）方法中的 mContentParent = generateLayout(mDecor); 方法  其中这个

很长的一段

protected ViewGroup generateLayout(DecorView decor) {
    // Apply data from current theme.
    TypedArray a = getWindowStyle();
   //省略....

    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);
    }
     //省略很多类似于上文的requestFeature

    // Inflate the window decor.  加载dector的基础布局
    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;//这是一个优化退出的布局
        setCloseOnSwipeEnabled(true);
    }//省略很多.....
        layoutResource = 
    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");
    }
     // Remaining setup -- of background and title -- that only applies
    // to top-level windows.
    if (getContainer() == null) {
        final Drawable background;
        if (mBackgroundResource != 0) {
            background = getContext().getDrawable(mBackgroundResource);
        } else {
            background = mBackgroundDrawable;
        }
        mDecor.setWindowBackground(background);

        final Drawable frame;
        if (mFrameResource != 0) {
            frame = getContext().getDrawable(mFrameResource);
        } else {
            frame = null;
        }
        mDecor.setWindowFrame(frame);

        mDecor.setElevation(mElevation);
        mDecor.setClipToOutline(mClipToOutline);

        if (mTitle != null) {
            setTitle(mTitle);
        }
        if (mTitleColor == 0) {
            mTitleColor = mTextColor;
        }
        setTitleColor(mTitleColor);
    }

    mDecor.finishChanging();

    return contentParent;
}

通过解析基础布局，将View添加到DecorView上，这个onResourcesLoaded()方法是DecorView的

void onResourcesLoaded(LayoutInflater inflater, int layoutResource) {
    mStackId = getStackId();

//省略....
    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();
}

解析完基础布局以后，我们接着看generateLayout()方法的

ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);

通过findViewById获取一个容器，这个容器的id是固定的值，通过注释可以看到，这个是主容器的id，而且是必须要存在的

/**
 * 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;

然后将这个容器返回。

到此为止，我们的创建DecorView的操作就完成了。

                                               、

总结关系图

             

1,Window是抽象类，提供了绘制窗口的一些列API

2,PhoneWindow是Window的具体继承实现类，而且该类包含了一个DecorView对象，这个对象是所有应用窗口（Activity界面）的根View

3，DecorView是PhoneWindow的内部类，是FrameLayout的子类，是对Framelayout功能的修饰。
补充：

DecorView：顶层视图，将要显示的具体内容呈现在PhoneWindow上. DecorView

是当前Activity所有View的祖先，它并不会向用户呈现任何东西，它主要有如下几个功能，可能不全 

·  A. Dispatch ViewRoot分发来的key、touch、trackball等外部事件；

·  B. DecorView有一个直接的子View，我们称之为System Layout,这个View是从系统的Layout.xml中解析出的，它包含当前UI的风格，如是否带title、是否带process bar等。可以称这些属性为Window decorations。

·  C. 作为PhoneWindow与ViewRoot之间的桥梁，ViewRoot通过DecorView设置窗口属性。//可以这样获取 View view = getWindow().getDecorView();·  DecorView只有一个子元素为LinearLayout。代表整个Window界面，包含通知栏，标题栏，内容显示栏三块区域。DecorView里面TitleView：标题，可以设置requestWindowFeature(Window.FEATURE_NO_TITLE)取消掉ContentView：是一个id为content的FrameLayout。我们平常在Activity使用的setContentView就是设置在这里，也就是在FrameLayout上

3: 将ContentView添加到基础布局中去

  分析完上面的，我们接着回到PhoneWindow的setContentView方法中

@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);
    }
 //省略...
}

 分析完installDecor方法，创建了DecorView，根据不同的主题或者特性解析了不同的基础布局，并将基础布局加载到DecorView上，创建完成后，我们将主activity上

setContentView(R.layout.activity_s_econd);

set过来的R.layout.activity_s_econd 这个布局文件，添加到mContentParent中去,而mContentParent就是解析过的基础布局，

到此，只是完成了创建DecorView,以及将需要展示的view添加到基础布局上，真正的view并没有展示---接着看下面的文章