Android的启动到View的绘制

初始化WindowManagerService

• ActivityThread

    private void handleLaunchActivity(ActivityClientRecord r, Intent customIntent, String reason) {
        ......
        // Initialize before creating the activity
        WindowManagerGlobal.initialize();
  
        Activity a = performLaunchActivity(r, customIntent);
  
        ...
    }
  

• android.view.WindowManagerGlobal

    public static void initialize() {
        getWindowManagerService();
    }
  
    public static IWindowManager getWindowManagerService() {
        synchronized (WindowManagerGlobal.class) {
            if (sWindowManagerService == null) {
                sWindowManagerService = IWindowManager.Stub.asInterface(
                        ServiceManager.getService("window"));
                try {
                    sWindowManagerService = getWindowManagerService();
                    ValueAnimator.setDurationScale(sWindowManagerService.getCurrentAnimatorScale());
                } catch (RemoteException e) {
                    throw e.rethrowFromSystemServer();
                }
            }
            return sWindowManagerService;
        }
    }
  

获取WindowManagerService。

获取窗口并初始化

• Activity

    final void attach(...) {
        attachBaseContext(context);
    
        ......
        mWindow = new PhoneWindow(this, window);
        mUiThread = Thread.currentThread();
  
        mWindow.setWindowManager(
                (WindowManager)context.getSystemService(Context.WINDOW_SERVICE),
                mToken, mComponent.flattenToString(),
                (info.flags & ActivityInfo.FLAG_HARDWARE_ACCELERATED) != 0);
        if (mParent != null) {
            mWindow.setContainer(mParent.getWindow());
        }
        mWindowManager = mWindow.getWindowManager();
        mCurrentConfig = config;
    }
  

• android.view.Window

    public void setWindowManager(WindowManager wm, IBinder appToken, String appName,
                                 boolean hardwareAccelerated) {
        ......
        mWindowManager = ((WindowManagerImpl)wm).createLocalWindowManager(this);
    }

    public WindowManagerImpl createLocalWindowManager(Window parentWindow) {
         return new WindowManagerImpl(mContext, parentWindow);
    }

    public WindowManager getWindowManager() {
        return mWindowManager;
    }

可以看出，mWindowManager其实是一个WindowManagerImpl实例。

给Window添加Activity布局

• android.app.ActivityThread

    final void handleResumeActivity(IBinder token,
                                    boolean clearHide, boolean  isForward, boolean reallyResume, int seq, String reason) {
       ......
        // TODO Push resumeArgs into the activity for consideration
        r = performResumeActivity(token, clearHide, reason);
  
        if (r != null) {
            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;
                l.type = WindowManager.LayoutParams.TYPE_BASE_APPLICATION;
                l.softInputMode |= forwardBit;
  
                if (a.mVisibleFromClient && !a.mWindowAdded) {
                    a.mWindowAdded = true;
                    wm.addView(decor, l);
                }
  
            }
  
            // The window is now visible if it has been added, we are not
            // simply finishing, and we are not starting another activity.
            if (!r.activity.mFinished && willBeVisible
                    && r.activity.mDecor != null && !r.hideForNow) {
                
                ...
                ViewManager wm = a.getWindowManager();
                View decor = r.window.getDecorView();
                wm.updateViewLayout(decor, l);
            }
        ......
        }
    }
  

可以看出，当首次加载布局时，会走到：

    wm.addView(decor, l);

这里面，从上面可以知道，wm其实是一个WindowManagerImpl对象。看看源码：

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

mGlobal是一个WindowManagerGlobal对象。

• android.view.WindowManagerGlobal

    public void addView(View view, ViewGroup.LayoutParams params,
                        Display display, Window parentWindow) {
        ......
  
        synchronized (mLock) {
  
            root = new ViewRootImpl(view.getContext(), display);
  
            mRoots.add(root);
        }
        
        root.setView(view, wparams, panelParentView);
        
        ......
    }
  

• android.view.ViewRootImpl

    public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) {
        synchronized (this) {
            if (mView == null) {
                mView = view;
  
                .......
                
                // Schedule the first layout -before- adding to the window
                // manager, to make sure we do the relayout before receiving
                // any other events from the system.
                requestLayout();
                
                .......
            }
        }
    }
  
    @Override
    public void requestLayout() {
        if (!mHandlingLayoutInLayoutRequest) {
            checkThread();
            mLayoutRequested = true;
            scheduleTraversals();
        }
    }
  

从这两段代码可以看出：

• View的绘制是在onResume()之后调用的
• DecorView之上还有一层View-ViewRootImpl
• 真正的View的测量、摆位、绘制可以说就是在ViewRootImpl的方法setView()中开始的
• layout的刷新方法有requestLayout()

子线程不能更新UI

每一次导致布局刷新的操作都会操作到方法requestLayout()，这里开始了线程的检查；具体看源码：

    void checkThread() {
        if (mThread != Thread.currentThread()) {
            throw new CalledFromWrongThreadException(
                    "Only the original thread that created a view hierarchy can touch its views.");
        }
    }

可以看出，当前操作UI的线程不是mThread的话，就会报子线程不能更新UI的操作。mThread是在ViewRootImpl的构造方法调用，该方法的调用链是：

ActivityThread[handleResumeActivity()]-->WindowManagerImpl[addView()]
-->WindowManagerGlobal[addView()]-->ViewRootImpl[构造方法]

即，mThread就是主线程（UI线程）。

View的绘制流程真正开始的地方

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

final TraversalRunnable mTraversalRunnable = new TraversalRunnable();

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;
        }
    }
}

真正的View的绘制流程就从这里performTraversals开始了。