View的加载

Activity关联window

ActivityThread#performLaunchActivity

private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
    ...
    Activity activity = null;
    try {
        java.lang.ClassLoader cl = appContext.getClassLoader();
        // 通过反射创建出activity对象
        activity = mInstrumentation.newActivity(
                cl, component.getClassName(), r.intent);
        StrictMode.incrementExpectedActivityCount(activity.getClass());
        r.intent.setExtrasClassLoader(cl);
        r.intent.prepareToEnterProcess();
        if (r.state != null) {
            r.state.setClassLoader(cl);
        }
    } catch (Exception e) {
        ...
    }

    try {
        ...
        if (activity != null) {
            ...
            // activity内部初始化window实例PhoneWindow
            activity.attach(appContext, this, getInstrumentation(), r.token,
                    r.ident, app, r.intent, r.activityInfo, title, r.parent,
                    r.embeddedID, r.lastNonConfigurationInstances, config,
                    r.referrer, r.voiceInteractor, window, r.configCallback);
            ...
            // 调用Activity的onCreate方法
            if (r.isPersistable()) {
                mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
            } else {
                mInstrumentation.callActivityOnCreate(activity, r.state);
            }
           ...
        }
        r.setState(ON_CREATE);

        mActivities.put(r.token, r);

    } catch (SuperNotCalledException e) {
        throw e;
    } catch (Exception e) {
        ...
    }

    return activity;
}

创建和初始化DecorView

Activity#setContentView

public void setContentView(View view) {
    getWindow().setContentView(view);
    initWindowDecorActionBar();
}

PhoneWindow#setContentView

public void setContentView(int layoutResID) {
    if (mContentParent == null) {
        installDecor();   --关键代码
    } else if (!hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
        mContentParent.removeAllViews();
    }

    if (hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
        final Scene newScene = Scene.getSceneForLayout(mContentParent, layoutResID,
                getContext());
        transitionTo(newScene);
    } else {
        // 将布局加载进R.id.content的frameLayout中
        mLayoutInflater.inflate(layoutResID, mContentParent);
    }
    
    // 通知Activity视图发生改变
    final Callback cb = getCallback();
    if (cb != null && !isDestroyed()) {
        cb.onContentChanged();
    }
   ...
}

PhoneWindow#installDecor

private void installDecor() {
    mForceDecorInstall = false;
    if (mDecor == null) {
        // 创建DecorView对象,这个就是根View，是一个FrameLayout
        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) {
        // 通过窗口类型获取到布局id，创建出对应的View，加载进DecorView中(mDecor.onResourcesLoaded(mLayoutInflater, layoutResource);)
        // 因为这个条件也是为什么代码中写隐藏标题栏要写在setContentView上面的原因
        // 常规加载布局是screen_title.xml，上面ViewStub用来显示ActionBar的，下面一个FrameLayout是title显示标题的，另一个FrameLayout为主要填充区域
        // mContentParent就是这个id是R.id.content
        mContentParent = generateLayout(mDecor);
        ...
    }
}

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>

由此可知，调用setContentView会创建和初始化DecorView。但是这个时候DecorView还没有被WindowManager正式添加到Window中。

DecorView添加和显示

public void handleResumeActivity(IBinder token, boolean finalStateRequest, boolean isForward,
        String reason) {
    ...
    // 执行Activity的onResume方法
    final ActivityClientRecord r = performResumeActivity(token, finalStateRequest, reason);
    ...
    if (r.window == null && !a.mFinished && willBeVisible) {
        r.window = r.activity.getWindow();
        View decor = r.window.getDecorView();
        // decorView隐藏
        decor.setVisibility(View.INVISIBLE);
        ViewManager wm = a.getWindowManager();
        WindowManager.LayoutParams l = r.window.getAttributes();
        a.mDecor = decor;
        ...
        if (a.mVisibleFromClient) {
            if (!a.mWindowAdded) {
                a.mWindowAdded = true;
                // 后面会着重讲，里面初始化ViewRootImpl，通过WindowSession最终来完成Window的添加过程。
                // 这个步骤完成DecorView添加进window中
                wm.addView(decor, l);
            } else {
                a.onWindowAttributesChanged(l);
            }
        }
    } else if (!willBeVisible) {
        if (localLOGV) Slog.v(TAG, "Launch " + r + " mStartedActivity set");
        r.hideForNow = true;
    }
    cleanUpPendingRemoveWindows(r, false /* force */);

    if (!r.activity.mFinished && willBeVisible && r.activity.mDecor != null && !r.hideForNow) {
        ...
        // DecorView显示，到这里Activity的视图才能被用户看到
        if (r.activity.mVisibleFromClient) {
            r.activity.makeVisible();
        }
    }
   ...
}

Activity#makeVisible

void makeVisible() {
    if (!mWindowAdded) {
        ViewManager wm = getWindowManager();
        wm.addView(mDecor, getWindow().getAttributes());
        mWindowAdded = true;
    }
    mDecor.setVisibility(View.VISIBLE);
}

Activity，Window，View关系

在onCreate方法之前，Activity调用attach方法创建Window。
Window通过setContentView创建DecorView，并将布局加载进DecorView的R.id.content中，有标题栏处理标题栏。
在onResume方法之后，调用windowManager的addView方法，将DecorView添加进window中。调用makeVisible方法将DecorView显示出来。

ViewRootImpl

布局检查机制：ViewRootImpl
View的绘制是由ViewRootImpl来负责的。每个应用程序窗口的DecorView都有一个与之关联的ViewRootImpl对象，这种关联关系是由WindowManager来维护的。

ViewRootImpl初始化

ActivityThread#handleResumeActivity
调用wm.addView(decor, l);
参数是DecorView，窗口布局参数。
这个addView方法是在onResume方法之后调用。
WindowManager是对外提供的接口，实例类是WindowManagerImpl。

WindowManagerImpl#addView

public void addView(@NonNull View view, @NonNull ViewGroup.LayoutParams params) {
    applyDefaultToken(params);
    mGlobal.addView(view, params, mContext.getDisplayNoVerify(), mParentWindow,
            mContext.getUserId());
}

WindowManagerGlobal#addView

public void addView(View view, ViewGroup.LayoutParams params,
        Display display, Window parentWindow, int userId) {
    ...
    ViewRootImpl root;
    View panelParentView = null;

    synchronized (mLock) {
        ...
        // 创建ViewRootImpl实例
        root = new ViewRootImpl(view.getContext(), display);
        view.setLayoutParams(wparams);
        mViews.add(view);
        mRoots.add(root);
        mParams.add(wparams);

        try {
            // view是DecorView，这行代码将开始绘制view
            root.setView(view, wparams, panelParentView, userId);
        } catch (RuntimeException e) {
            // BadTokenException or InvalidDisplayException, clean up.
            if (index >= 0) {
                removeViewLocked(index, true);
            }
            throw e;
        }
    }
}

View绘制

public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView,
        int userId) {
    synchronized (this) {
        if (mView == null) {
            ...
            //进行View布局绘制
            requestLayout();
            ...
            try {
                mOrigWindowType = mWindowAttributes.type;
                mAttachInfo.mRecomputeGlobalAttributes = true;
                collectViewAttributes();
                adjustLayoutParamsForCompatibility(mWindowAttributes);
                // 通过WindowSession最终来完成Window的添加过程。
                // mWindowSession的类型是IWindowSession，它是一个Binder对象，真正的实现类是Session，也就是Window的添加过程是一次IPC调用。
                // 在Session内部会通过WMS来实现Window的添加
                res = mWindowSession.addToDisplayAsUser(mWindow, mSeq, mWindowAttributes,
                        getHostVisibility(), mDisplay.getDisplayId(), userId, mTmpFrame,
                        mAttachInfo.mContentInsets, mAttachInfo.mStableInsets,
                        mAttachInfo.mDisplayCutout, inputChannel,
                        mTempInsets, mTempControls);
                setFrame(mTmpFrame);
            } catch (RemoteException e) {
                mAdded = false;
                mView = null;
                mAttachInfo.mRootView = null;
                inputChannel = null;
                mFallbackEventHandler.setView(null);
                unscheduleTraversals();
                setAccessibilityFocus(null, null);
                throw new RuntimeException("Adding window failed", e);
            } finally {
                if (restore) {
                    attrs.restore();
                }
            }
            ...
            // 注意这里，ViewRootImpl实现ViewParent接口，这里将ViewRootImpl抽象为DecorView的父布局，注意只是抽象，ViewRootImpl并不是View
            view.assignParent(this);
            ...
        }
    }
}

ViewRootImpl#requestLayout

public void requestLayout() {
    if (!mHandlingLayoutInLayoutRequest) {
        // 线程检测
        checkThread();
        mLayoutRequested = true;
        // 绘制方法doTraversal是在下次loop循环才去调用而不是立即调用
        // doTraversal方法中调用performTraversals使ViewTree开始View绘制
        // 分别调用performMeasure，performLayout，performDraw，在其内部又会分别调用View的measure，layout，draw方法
        scheduleTraversals();
    }
}