Activity启动时View首次绘制的源码深度解析：从handleResumeActivity()到performTraversals()

public void handleResumeActivity(IBinder token, boolean finalStateRequest, 
        boolean isForward, String reason) {
    
    // 1. 执行Activity的onResume生命周期
    final ActivityClientRecord r = performResumeActivity(token, finalStateRequest, reason);
    
    if (r != null) {
        final Activity a = r.activity;
        //...
        if (r.window == null && !a.mFinished && willBeVisible) {
            // 2. 获取DecorView
            View decor = r.window.getDecorView();
            decor.setVisibility(View.INVISIBLE);
            
            // 3. 获取WindowManager
            ViewManager wm = a.getWindowManager();
            WindowManager.LayoutParams l = r.window.getAttributes();
            
            // 4. 关键调用：添加DecorView到窗口系统
            wm.addView(decor, l);
        }
    }
}

public void addView(View view, ViewGroup.LayoutParams params,
        Display display, Window parentWindow) {
    
    synchronized (mLock) {
        // 1. 创建ViewRootImpl实例
        ViewRootImpl root = new ViewRootImpl(view.getContext(), display);
        
        // 2. 保存视图引用
        mViews.add(view);
        mRoots.add(root);
        mParams.add(wparams);
        
        try {
            // 3. 核心调用：关联视图与ViewRootImpl
            root.setView(view, wparams, panelParentView);
        } catch (RuntimeException e) {
            // 异常处理
        }
    }
}

public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) {
    synchronized (this) {
        if (mView == null) {
            mView = view; // 保存DecorView引用
            
            // 1. 请求首次布局
            requestLayout();
            
            // 2. 通过Binder与WMS通信
            try {
                mWindowSession.addToDisplayAsUser(...);
            } catch (RemoteException e) {
                // 异常处理
            }
            
            // 3. 设置输入事件接收器
            mInputEventReceiver = new WindowInputEventReceiver(...);
        }
    }
}

@Override
public void requestLayout() {
    if (!mHandlingLayoutInLayoutRequest) {
        // 1. 检查是否在主线程
        checkThread();
        
        // 2. 标记布局请求
        mLayoutRequested = true;
        
        // 3. 调度遍历任务
        scheduleTraversals();
    }
}

void scheduleTraversals() {
    if (!mTraversalScheduled) {
        mTraversalScheduled = true;
        
        // 1. 插入同步屏障（确保UI绘制优先）
        mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
        
        // 2. 通过Choreographer注册VSync回调
        mChoreographer.postCallback(
            Choreographer.CALLBACK_TRAVERSAL, 
            mTraversalRunnable, // 关键回调
            null
        );
        
        // 3. 通知渲染管道
        if (!mUnbufferedInputDispatch) {
            scheduleConsumeBatchedInput();
        }
    }
}

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

void doTraversal() {
    if (mTraversalScheduled) {
        mTraversalScheduled = false;
        
        // 1. 移除同步屏障
        mHandler.getLooper().getQueue().removeSyncBarrier(mTraversalBarrier);
        
        // 2. 执行核心遍历
        performTraversals();
    }
}

private void performTraversals() {
    final View host = mView; // 即DecorView
    
    // 第一阶段：预测量
    if (mFirst) {
        // 1. 与WMS通信获取窗口尺寸
        relayoutResult = relayoutWindow(params, ...);
        
        // 2. 收集View树所需系统配置
        collectViewAttributes();
    }
    
    // 第二阶段：测量（Measure）
    if (layoutRequested) {
        // 1. 创建MeasureSpec
        int childWidthMeasureSpec = getRootMeasureSpec(...);
        int childHeightMeasureSpec = getRootMeasureSpec(...);
        
        // 2. 执行DecorView的测量
        performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
    }
    
    // 第三阶段：布局（Layout）
    if (didLayout) {
        // 1. 执行DecorView的布局
        performLayout(lp, mWidth, mHeight);
        
        // 2. 处理键盘导航变化
        if ((host.mPrivateFlags & PFLAG_FOCUSED) != 0) {
            host.restoreDefaultFocus();
        }
    }
    
    // 第四阶段：绘制（Draw）
    if (!cancelDraw && !newSurface) {
        // 1. 判断是否需要完整重绘
        boolean fullRedrawNeeded = mFullRedrawNeeded;
        
        // 2. 执行绘制
        performDraw();
    }
    
    // 第五阶段：清理
    if (mFirst) {
        mFirst = false; // 标记首次遍历完成
    }
}

private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {
    try {
        // 调用DecorView的measure()方法
        mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);
    } finally {
        Trace.traceEnd(Trace.TRACE_TAG_VIEW);
    }
}

private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth,
        int desiredWindowHeight) {
    
    // 设置宿主窗口尺寸
    host.setFrame(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
    
    // 执行DecorView的layout()
    host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
}

private void performDraw() {
    try {
        boolean canUseAsync = draw(fullRedrawNeeded);
        //...
    } finally {
        mIsDrawing = false;
    }
}

private boolean draw(boolean fullRedrawNeeded) {
    if (!drawSoftware(surface, mAttachInfo, xOffset, yOffset,
            scalingRequired, dirty, surfaceInsets)) {
        return false;
    }
    return true;
}

private boolean drawSoftware(Surface surface, AttachInfo attachInfo,
        int xoff, int yoff, boolean scalingRequired, Rect dirty, Rect surfaceInsets) {
    
    // 1. 锁定Canvas
    Canvas canvas = surface.lockCanvas(dirty);
    
    try {
        // 2. 执行DecorView的draw()
        mView.draw(canvas);
    } finally {
        // 3. 解锁并提交渲染
        surface.unlockCanvasAndPost(canvas);
    }
    return true;
}

// 错误示例：在onResume中直接获取宽高
protected void onResume() {
    super.onResume();
    int width = myView.getWidth(); // 返回0
}

// 正确做法
protected void onResume() {
    super.onResume();
    myView.post(() -> {
        int width = myView.getWidth(); // 正确值
    });
}