Activityの生成

ActivityThread类中有个handleLaunchActivity方法。

public Activity handleLaunchActivity(ActivityClientRecord r,
        PendingTransactionActions pendingActions, Intent customIntent){
    ...
    final Activity a = performLaunchActivity(r, customIntent);
    return a;
} 

Activity对象便是在这个方法中通过反射创建了。该方法内部紧接着又调用了activity.attach方法，以及callActivityOnCreate方法

private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
    ...
    Activity activity = null;
    try {
        java.lang.ClassLoader cl = appContext.getClassLoader();
        // 内部实现
        // Class.forName(className, false, cl).asSubclass(Activity.class)
        //       .getDeclaredConstructor().newInstance();
        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) {
        //
    }
    ...
    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,
                        r.assistToken);
    ...
    //显然下面这个方法是奔着activity.onCreate()去的。
    if (r.isPersistable()) {
        mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
    } else {
        mInstrumentation.callActivityOnCreate(activity, r.state);
    }
    return activity;
}

Activity.attach()中创建了mWindow和mWindowManager两个对象。其中mWindow是PhoneWindow()。

final void attach(Context context, ...){
    mWindow = new PhoneWindow(this, window, activityConfigCallback);
    mWindow.setCallback(this);
    ...
    mWindow.setWindowManager(
                (WindowManager)context.getSystemService(Context.WINDOW_SERVICE),
                mToken, mComponent.flattenToString(),
                (info.flags & ActivityInfo.FLAG_HARDWARE_ACCELERATED) != 0);
    mWindowManager = mWindow.getWindowManager();
    ...
}

SetWindowManager的中告诉我们mWindowManager实现类是WindowManagerImpl。

public void setWindowManager(WindowManager wm, IBinder appToken, String appName,
        boolean hardwareAccelerated) {
    mAppToken = appToken;
    mAppName = appName;
    mHardwareAccelerated = hardwareAccelerated;
    if (wm == null) {
        wm = (WindowManager)mContext.getSystemService(Context.WINDOW_SERVICE);
    }
    mWindowManager = ((WindowManagerImpl)wm).createLocalWindowManager(this);
}

Activity关联布局

在onCreate的方法中，我们一般会setContentView。而setContentView内部是用的window.setContentView()。所以可以直接看PhoneWindow的setContentView方法；

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

@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) {
        //内部做了3件事：
        //new DecorView()并添加布局
        //mContentParent = findViewById(R.id.content)
        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 {
        //把layoutResId inflate到mContentParent上去
        mLayoutInflater.inflate(layoutResID, mContentParent);
    }
    mContentParent.requestApplyInsets();
    final Callback cb = getCallback();
    if (cb != null && !isDestroyed()) {
        cb.onContentChanged();
    }
    mContentParentExplicitlySet = true;
}

installDecor中为我们生成了一个DecorView，也就是所有Activity的顶层Veiw。【DecorView继承自FramLayout】

private void installDecor() {
    if (mDecor == null) {
        mDecor = generateDecor(-1);
    } else {
        mDecor.setWindow(this);
    }
    if (mContentParent == null) {
        //generateLayout中为mDecor选定一个布局（根据feature）并addView上去
        //并返回这个布局的ID为R.id.content的ViewGroup--findViewById(ID_ANDROID_CONTENT)
        mContentParent = generateLayout(mDecor);
    }
    ...
}

为顶层Decor添加parent

然后跟着生命周期走到了ActivityThread.handleResumeActivity()方法：

public void handleResumeActivity(...){
    //间接调用Activity.onResume
    final ActivityClientRecord r = performResumeActivity(token, finalStateRequest, reason);
    ...
    final Activity a = r.activity;
    if (r.window == null && !a.mFinished && willBeVisible) {
        r.window = r.activity.getWindow();
        View decor = r.window.getDecorView();
        decor.setVisibility(View.INVISIBLE);
        ViewManager wm = a.getWindowManager();
        WindowManager.LayoutParams l = r.window.getAttributes();
        a.mDecor = decor;
        ...
        //重点在这
        wm.addView(decor, l);
    }
    ...
}

windowManager就是第一步里面我加粗了的那个对象。windowManagerImpl

private final WindowManagerGlobal mGlobal = WindowManagerGlobal.getInstance();
@Override
public void addView(@NonNull View view, @NonNull ViewGroup.LayoutParams params) {
    applyDefaultToken(params);
    mGlobal.addView(view, params, mContext.getDisplayNoVerify(), mParentWindow,
            mContext.getUserId());
}

所以直接开WindowManagerGlobal.addView():这里传入的view就是decorView，也就是Activity顶层View。

public void addView(View view, ViewGroup.LayoutParams params,
        Display display, Window parentWindow, int userId) {
    ViewRootImpl root =  new ViewRootImpl(view.getContext(), display);
    ...
    //decor成了root(ViewRootImpl)的子view;
    //想点进去看的话可以进去后找 view.assignParent(this);方法
    root.setView(view, wparams, panelParentView, userId);
}

至此，ViewRootImpl就成了所有view的祖宗view了。

view绘制三部曲（之旅）

如果读者还有兴致，可以点开root.setView方法，方法中在assignParent之前有一行：

public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView,
            int userId) {
    synchronized (this) {
        ...
        requestLayout();
        ...
        view.assignParent(this);
        ...  
    } 
}

@Override
public void requestLayout() {
    if (!mHandlingLayoutInLayoutRequest) {
        checkThread();
        mLayoutRequested = true;
        scheduleTraversals();
    }
}

void checkThread() {
    //著名的线程检查的地方，流传多年的名句“子线程不能更新UI”的判断就在这里。
    //其实按上面的逻辑，root(也就是ViewRootImpl)在resume之前还未成为decorView的parent，所以可以推理出：
    //在onResume之前，其实是可以子线程更新UI的，感兴趣的可以试一试。
    if (mThread != Thread.currentThread()) {
        throw new CalledFromWrongThreadException(
            "Only the original thread that created a view hierarchy can touch its views.");
    }
}

这里的requestLayout就是整个View Tree第一次开始绘制的地方了。scheduleTraversals方法也在ViewRootImpl类中：

final TraversalRunnable mTraversalRunnable = new TraversalRunnable();
void scheduleTraversals() {
    if (!mTraversalScheduled) {
        mTraversalScheduled = true;
        mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
        mChoreographer.postCallback(
            Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
        notifyRendererOfFramePending();
        pokeDrawLockIfNeeded();
    }
}

final class TraversalRunnable implements Runnable {
    @Override
    public void run() {
        doTraversal();
    }
}

void doTraversal() {
    if (mTraversalScheduled) {
        mTraversalScheduled = false;
        mHandler.getLooper().getQueue().removeSyncBarrier(mTraversalBarrier);

        if (mProfile) {
            Debug.startMethodTracing("ViewAncestor");
        }

        performTraversals();

        if (mProfile) {
            Debug.stopMethodTracing();
            mProfile = false;
        }
    }
}

//默认的super(LayoutParams.MATCH_PARENT, LayoutParams.MATCH_PARENT);
public final WindowManager.LayoutParams mWindowAttributes = new WindowManager.LayoutParams();
private void performTraversals() {
    ...
    WindowManager.LayoutParams lp = mWindowAttributes;
    int desiredWindowWidth;
    int desiredWindowHeight;
    ...
    Rect frame = mWinFrame;//屏幕大小
    if (mFirst) {
        ...
        if (...) {
            ...
        } else {
            //宽高默认为屏幕大小
            desiredWindowWidth = frame.width();
            desiredWindowHeight = frame.height();
        }
        ...
        //把mAttachInfo向下传个子view，所以子view中的mAttachInfo，都是ViewRootImpl中的mAttachInfo。
        //view.post（）中其实就是用到的mAttachInfo.mHandler,这个mHandler就是ViewRootImpl构造时new的。
        host.dispatchAttachedToWindow(mAttachInfo, 0);
        mAttachInfo.mTreeObserver.dispatchOnWindowAttachedChange(true);
        //状态栏和导航栏在下面这个方法设置
        dispatchApplyInsets(host);
    }
    ...
    //view.post如果在resume之前执行，由于viewRootImpl还未成为decor的parent，所以拿不到mHandler，所以就会存到队列中，此时再执行
    getRunQueue().executeActions(mAttachInfo.mHandler);
    if (layoutRequested) {
        ...
        // Ask host how big it wants to be
        windowSizeMayChange |= measureHierarchy(host, lp, res,
              desiredWindowWidth, desiredWindowHeight);
    }
    ...
    performLayout(lp, mWidth, mHeight);
    mAttachInfo.mTreeObserver.dispatchOnGlobalLayout();
    ...
    boolean cancelDraw = mAttachInfo.mTreeObserver.dispatchOnPreDraw() || !isViewVisible;
    performDraw()
    
}

measureHierarchy也在该类下：

private boolean measureHierarchy(final View host, final WindowManager.LayoutParams lp,
            final Resources res, final int desiredWindowWidth, final int desiredWindowHeight){
    boolean goodMeasure = false;
    if (!goodMeasure) {
        childWidthMeasureSpec = getRootMeasureSpec(desiredWindowWidth, lp.width);
        childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height);
        performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
        if (mWidth != host.getMeasuredWidth() || mHeight != host.getMeasuredHeight()) {
            windowSizeMayChange = true;
        }
    }    
    ...
}

private static int getRootMeasureSpec(int windowSize, int rootDimension) {
    int measureSpec;
    switch (rootDimension) {

        case ViewGroup.LayoutParams.MATCH_PARENT:
            // Window can't resize. Force root view to be windowSize.
            measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);
            break;
        case ViewGroup.LayoutParams.WRAP_CONTENT:
            // Window can resize. Set max size for root view.
            measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);
            break;
        default:
            // Window wants to be an exact size. Force root view to be that size.
            measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);
            break;
    }
    return measureSpec;
}


private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {
    if (mView == null) {
        return;
    }
    //mview也就是viewRootImpl
    mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}

到这里，decor开始它的测量，由此遍历整颗view Tree。