一、前言

今天我们来讲讲Activity，Window，DecorView之间的关系。

二、Activity

我们布局一个页面一般都是从setContentView开始，传入我们的XML布局，那么我们就从这个方法入手。
我们要知道每个Activity都是有一个自己的Window负责来显示页面的，而Window是一个抽象类，他的唯一实现类是PhoneWindow（对Window不太了解的以后我在找时间讲解一下）。我们Activity调用setContentView()方法其实调用的就是PhoneWindow的setContentView()方法，下面我们来看一下Activity中setContentView()源码：

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

可以看到这里调用的是getWindow().setContentView()，这个getWindow()是什么：

    public Window getWindow() {
        return mWindow;
    }

这个mWindow哪里创建的，就在Activity的attach()方法中：

    final void attach(Context context, ActivityThread aThread,
            Instrumentation instr, IBinder token, int ident,
            Application application, Intent intent, ActivityInfo info,
            CharSequence title, Activity parent, String id,
            NonConfigurationInstances lastNonConfigurationInstances,
            Configuration config, String referrer, IVoiceInteractor voiceInteractor,
            Window window, ActivityConfigCallback activityConfigCallback) {
        attachBaseContext(context);

        mFragments.attachHost(null /*parent*/);

        mWindow = new PhoneWindow(this, window, activityConfigCallback);
        ......
    }

下面我们来看看PhoneWindow的setContentView()方法：

    @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();
        } 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;
    }

这里先判断mContentParent是否为空，为空则执行installDecor()，这个跟初始化DecorView有关。然后判断是否需要转场动画，不需要就直接加载layout到mContentParent上。那么我们先来看一下DecorView，ContentParent都是什么：

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

    // 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;

这里根据官方注释，DecorView是Window上的顶级视图，mContentParent是放置窗口内容的视图，要么是
mDecor本身，或内容所在的mDecor的子级。我们再跟下去看一下DecorView的源码，看看他是什么View：

public class DecorView extends FrameLayout implements RootViewSurfaceTaker, WindowCallbacks {
    private static final String TAG = "DecorView";
    ......
}

可以看到DecorView继承自FrameLayout 。好了，那我们继续来看installDecor()都做了什么：

    private void installDecor() {
        mForceDecorInstall = false;
        if (mDecor == null) {
            mDecor = generateDecor(-1);
            .......
        } else {
            mDecor.setWindow(this);
        }
        if (mContentParent == null) {
            mContentParent = generateLayout(mDecor);
            ......
        }else
        ......
    }

这里我们可以看到先创建DectorView，然后再创建了mContentParent ，我们先来看generateDecor()都做了什么：

    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());
                if (mTheme != -1) {
                    context.setTheme(mTheme);
                }
            }
        } else {
            context = getContext();
        }
        return new DecorView(context, featureId, this, getAttributes());
    }

这里直接实例化了一个DectorView，我们再来看看generateLayout()：

    protected ViewGroup generateLayout(DecorView decor) {
        // Apply data from current theme.

        TypedArray a = getWindowStyle();
        ......
        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);
        }
        ......
        // Inflate the window decor.

        int layoutResource;
        int features = getLocalFeatures();
        ......
        } 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);
        ......
        return contentParent;

这里逻辑代码非常多，我就贴出一些关键的地方。这里先是通过getWindowStyle()获取当前Window的TypedArray，然后再通过TypedArray对Feature状态位进行设置，比如判断当前Window是否为悬浮状态，是否全屏，是否显示ActionBar，是否透明等等，最后就是通过设置好的Feature获取对应的layoutResource，这些layoutResource都是Android系统内提供的，这些在依赖库里都是可以找到的，这里我们以screen_simple举例：

1602601388(1).png

1602601440(1).png

下面我们来看一下这个xml布局：

<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>

可以看到根节点是LinearLayout，当然加载不同的xml布局的根节点也不同，只是这里是LinearLayout。ViewStub是用来延迟加载的一种组件，是用来动态显示actionBar的，而id为content的这个FrameLayout就是我们真正加载布局的地方了，我们传入的布局就放在这里。记住这个android:id="@android:id/content"，其他类型的布局样式不同，根节点不同，但是真正加载用户布局的id始终都为content。
然后我们看一下mDecor.onResourcesLoaded(mLayoutInflater, layoutResource)方法：

    void onResourcesLoaded(LayoutInflater inflater, int layoutResource) {
        if (mBackdropFrameRenderer != null) {
            loadBackgroundDrawablesIfNeeded();
            mBackdropFrameRenderer.onResourcesLoaded(
                    this, mResizingBackgroundDrawable, mCaptionBackgroundDrawable,
                    mUserCaptionBackgroundDrawable, getCurrentColor(mStatusColorViewState),
                    getCurrentColor(mNavigationColorViewState));
        }

        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();
    }

这里是将获取的layoutResource渲染出来，然后加入到DectorView中。
最后就是ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT)获取到contentParent 返回赋值给mContentParent。我们来看一下ID_ANDROID_CONTENT是什么：

    /**
     * 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;

到这里我们的mContentParent容器就创建完成了，我们现在来看一下Activity，Window（PhoneWindow），DectorView，ContentParent的大概关系图：

1602603473(1).png

三、AppCompatActivity

这个AppComPatActivity是我们目前常用的继承对象，为什么要拿出来说呢，因为他跟Activity中UI绘制流程不一样啊，这是谷歌后期推出为了给Activity各种问题填坑用的，我们来看看他的setContentView()：

    @Override
    public void setContentView(@LayoutRes int layoutResID) {
        getDelegate().setContentView(layoutResID);
    }

漂亮，跟Activity中完全不一样，感谢谷歌。我们来看看getDelegate()是什么：

    /**
     * @return The {@link AppCompatDelegate} being used by this Activity.
     */
    @NonNull
    public AppCompatDelegate getDelegate() {
        if (mDelegate == null) {
            mDelegate = AppCompatDelegate.create(this, this);
        }
        return mDelegate;
    }

再来看看AppCompatDelegate.create(this, this)：

    /**
     * Create an {@link androidx.appcompat.app.AppCompatDelegate} to use with {@code activity}.
     *
     * @param callback An optional callback for AppCompat specific events
     */
    @NonNull
    public static AppCompatDelegate create(@NonNull Activity activity,
            @Nullable AppCompatCallback callback) {
        return new AppCompatDelegateImpl(activity, callback);
    }

继续，AppCompatDelegateImpl(activity, callback)：

    AppCompatDelegateImpl(Activity activity, AppCompatCallback callback) {
        this(activity, null, callback, activity);
    }

    ......

    private AppCompatDelegateImpl(Context context, Window window, AppCompatCallback callback,
            Object host) {
        mContext = context;
        mAppCompatCallback = callback;
        mHost = host;

        if (mLocalNightMode == MODE_NIGHT_UNSPECIFIED && mHost instanceof Dialog) {
            final AppCompatActivity activity = tryUnwrapContext();
            if (activity != null) {
                // This code path is used to detect when this Delegate is a child Delegate from
                // an Activity, primarily for Dialogs. Dialogs use the Activity as it's Context,
                // so we want to make sure that the this 'child' delegate does not interfere
                // with the Activity config. The simplest way to do that is to match the
                // outer Activity's local night mode
                mLocalNightMode = activity.getDelegate().getLocalNightMode();
            }
        }
        if (mLocalNightMode == MODE_NIGHT_UNSPECIFIED) {
            // Try and read the current night mode from our static store
            final Integer value = sLocalNightModes.get(mHost.getClass());
            if (value != null) {
                mLocalNightMode = value;
                // Finally remove the value
                sLocalNightModes.remove(mHost.getClass());
            }
        }

        if (window != null) {
            attachToWindow(window);
        }

        // Preload appcompat-specific handling of drawables that should be handled in a special
        // way (for tinting etc). After the following line completes, calls from AppCompatResources
        // to ResourceManagerInternal (in appcompat-resources) will handle those internal drawable
        // paths correctly without having to go through AppCompatDrawableManager APIs.
        AppCompatDrawableManager.preload();
    }

看到这里我们发现getDelegate()是获取代理的方法，会通过mDelegate = AppCompatDelegate.create(this, this)来创建代理对象，最后会的的是AppCompatDelegateImpl对象。这里要说明一点，我这个是AndroidX的代码，如果你的项目没有转到AndroidX是有些不一样的，但是大方向没问题。我们来看一下AppCompatDelegateImpl中setContentView() 方法：

    @Override
    public void setContentView(int resId) {
        ensureSubDecor();
        ViewGroup contentParent = mSubDecor.findViewById(android.R.id.content);
        contentParent.removeAllViews();
        LayoutInflater.from(mContext).inflate(resId, contentParent);
        mAppCompatWindowCallback.getWrapped().onContentChanged();
    }

和Activity中差不多，也是获取到了contentParent后去渲染用户布局，但是这里是ensureSubDecor()，我们来看一下这个方法：

    private void ensureSubDecor() {
        if (!mSubDecorInstalled) {
            mSubDecor = createSubDecor();
        ......
    }

继续看createSubDecor()：

    private ViewGroup createSubDecor() {
        TypedArray a = mContext.obtainStyledAttributes(R.styleable.AppCompatTheme);

        if (!a.hasValue(R.styleable.AppCompatTheme_windowActionBar)) {
            a.recycle();
            throw new IllegalStateException(
                    "You need to use a Theme.AppCompat theme (or descendant) with this activity.");
        }
        ......
        // Now let's make sure that the Window has installed its decor by retrieving it
        ensureWindow();
        mWindow.getDecorView();

        final LayoutInflater inflater = LayoutInflater.from(mContext);
        ViewGroup subDecor = null;
        if (!mWindowNoTitle) {
            if (mIsFloating) {
                // If we're floating, inflate the dialog title decor
                subDecor = (ViewGroup) inflater.inflate(
                        R.layout.abc_dialog_title_material, null);
            ......
        } else {
            if (mOverlayActionMode) {
                subDecor = (ViewGroup) inflater.inflate(
                        R.layout.abc_screen_simple_overlay_action_mode, null);
            } else {
                subDecor = (ViewGroup) inflater.inflate(R.layout.abc_screen_simple, null);
            }
            ......
        }
        ......
        final ContentFrameLayout contentView = (ContentFrameLayout) subDecor.findViewById(
                R.id.action_bar_activity_content);

        final ViewGroup windowContentView = (ViewGroup) mWindow.findViewById(android.R.id.content);
        if (windowContentView != null) {
            // There might be Views already added to the Window's content view so we need to
            // migrate them to our content view
            while (windowContentView.getChildCount() > 0) {
                final View child = windowContentView.getChildAt(0);
                windowContentView.removeViewAt(0);
                contentView.addView(child);
            }

            // Change our content FrameLayout to use the android.R.id.content id.
            // Useful for fragments.
            windowContentView.setId(View.NO_ID);
            contentView.setId(android.R.id.content);

            // The decorContent may have a foreground drawable set (windowContentOverlay).
            // Remove this as we handle it ourselves
            if (windowContentView instanceof FrameLayout) {
                ((FrameLayout) windowContentView).setForeground(null);
            }
        }

        // Now set the Window's content view with the decor
        mWindow.setContentView(subDecor);
        ......

        return subDecor;
    }

这个方法逻辑也是非常多，我就截取一些重要的展示一下。可以看到刚开始跟Activity是一样的，获取TypedArray从而来判断加载那个xml布局，但是中间多了一个mWindow.getDecorView()方法，我们来看看这个方法，在PhoneWindow中：

    @Override
    public final View getDecorView() {
        if (mDecor == null || mForceDecorInstall) {
            installDecor();
        }
        return mDecor;
    }

熟悉的味道，不讲了，跟之前一样获取到DectorView，下面继续看，我们看到一个熟悉的布局abc_screen_simple，之前的是screen_simple，我们来找一下这个布局：

1602605344(1).png

1602605372(1).png

老规矩，看一下这个布局xml：

<androidx.appcompat.widget.FitWindowsLinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/action_bar_root"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:fitsSystemWindows="true">

    <androidx.appcompat.widget.ViewStubCompat
        android:id="@+id/action_mode_bar_stub"
        android:inflatedId="@+id/action_mode_bar"
        android:layout="@layout/abc_action_mode_bar"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" />

    <include layout="@layout/abc_screen_content_include" />

</androidx.appcompat.widget.FitWindowsLinearLayout>

继续看abc_screen_content_include：

<merge xmlns:android="http://schemas.android.com/apk/res/android">

    <androidx.appcompat.widget.ContentFrameLayout
            android:id="@id/action_bar_activity_content"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:foregroundGravity="fill_horizontal|top"
            android:foreground="?android:attr/windowContentOverlay" />

</merge>

嗯？怎么回事，这里id怎么是action_bar_activity_content，之前是content啊，我们继续看createSubDecor()后续代码：

        final ContentFrameLayout contentView = (ContentFrameLayout) subDecor.findViewById(
                R.id.action_bar_activity_content);

        final ViewGroup windowContentView = (ViewGroup) mWindow.findViewById(android.R.id.content);
        if (windowContentView != null) {
            // There might be Views already added to the Window's content view so we need to
            // migrate them to our content view
            while (windowContentView.getChildCount() > 0) {
                final View child = windowContentView.getChildAt(0);
                windowContentView.removeViewAt(0);
                contentView.addView(child);
            }

            // Change our content FrameLayout to use the android.R.id.content id.
            // Useful for fragments.
            windowContentView.setId(View.NO_ID);
            contentView.setId(android.R.id.content);

            // The decorContent may have a foreground drawable set (windowContentOverlay).
            // Remove this as we handle it ourselves
            if (windowContentView instanceof FrameLayout) {
                ((FrameLayout) windowContentView).setForeground(null);
            }
        }

        // Now set the Window's content view with the decor
        mWindow.setContentView(subDecor);
        ......

        return subDecor;

contentView就是subDecor中id为action_bar_activity_content的ContentFrameLayout，windowContentView就是PhoneWindow中DectorView中id为content的FrameLayout，然后windowContentView的id设置为了NO_ID，contentView的id设置为了android.R.id.content，再调用mWindow.setContentView把subDecor设置进去了，我们来看看PhoneWindow.setContentView(View) 方法：

    @Override
    public void setContentView(View view) {
        setContentView(view, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));
    }

继续：

    @Override
    public void setContentView(View view, ViewGroup.LayoutParams params) {
        // 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();
        } else if (!hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
            mContentParent.removeAllViews();
        }

        if (hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
            view.setLayoutParams(params);
            final Scene newScene = new Scene(mContentParent, view);
            transitionTo(newScene);
        } else {
            mContentParent.addView(view, params);
        }
        mContentParent.requestApplyInsets();
        final Callback cb = getCallback();
        if (cb != null && !isDestroyed()) {
            cb.onContentChanged();
        }
        mContentParentExplicitlySet = true;
    }

这边mContentParent 是不会为空的，因为之前mWindow.getDecorView()这个方法已经创建了DectorView，以及mContentParent，所以在没有转场动画的情况下，直接调用mContentParent.addView(view, params)将subDecor放入了mContentParent中。

最后我们来看看AppCompatActivity结构图：

1602606366(1).png

Android：Activity，Window，DecorView之间的关系

Android：Activity，Window，DecorView之间的关系

一、前言

二、Activity

三、AppCompatActivity