View绘制流程和requestLayout()原理

我知道View的绘制流程分为 measure、layout、draw，那么一个View显示在Activity上的完整流程是什么呢？

在Activity启动流程中，Activity创建会执行Activity的acttach()。

1、Activity & attach()

    // 设置WindowManager    
	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());
        }
	   // 赋值
        mWindowManager = mWindow.getWindowManager();

在Activity的attach()方法中 设置/初始化 WindowManager

1.1 Window & setWindowManager()

    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);
    }

创建WindowManager 对象 WindowManagerImpl

1.2 WindowManager 和 ViewManager

public interface WindowManager extends ViewManager {
    ...
	public static class LayoutParams extends ViewGroup.LayoutParams implements Parcelable {
        ...
    }
}

WindowManager 是一个接口，里面规定了一些Window的熟悉值和封装了LayoutParams内部类，同时又继承了ViewManager 接口

/** Interface to let you add and remove child views to an Activity. To get an instance
  * of this class, call {@link android.content.Context#getSystemService(java.lang.String) Context.getSystemService()}.
  */
public interface ViewManager
{
    /**
     * Assign the passed LayoutParams to the passed View and add the view to the window.
     * 
Throws {@link android.view.WindowManager.BadTokenException} for certain programming
     * errors, such as adding a second view to a window without removing the first view.
     * 
Throws {@link android.view.WindowManager.InvalidDisplayException} if the window is on a
     * secondary {@link Display} and the specified display can't be found
     * (see {@link android.app.Presentation}).
     * @param view The view to be added to this window.
     * @param params The LayoutParams to assign to view.
     */
    public void addView(View view, ViewGroup.LayoutParams params);
    public void updazhteViewLayout(View view, ViewGroup.LayoutParams params);
    public void removeView(View view);
}

而ViewManager 只提供了三个方法：

• addView() : 添加View到Window
• updazhteViewLayout() : 更新View
• removeView() : 从Window移除View

WindowManager 和 ViewManager 都是接口，因此，上面三个方法的具体实现，在WindowManager的实现类WindowManagerImpl 中

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

创建WindowManager 的实现类 WindowManagerImpl

1.3、WindowManagerImpl

public final class WindowManagerImpl implements WindowManager {
    @UnsupportedAppUsage
    private final WindowManagerGlobal mGlobal = WindowManagerGlobal.getInstance();
    private final Context mContext;
    private final Window mParentWindow;

    private IBinder mDefaultToken;

    public WindowManagerImpl(Context context) {
        this(context, null);
    }

    private WindowManagerImpl(Context context, Window parentWindow) {
        mContext = context;
        mParentWindow = parentWindow;
    }

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

    public WindowManagerImpl createPresentationWindowManager(Context displayContext) {
        return new WindowManagerImpl(displayContext, mParentWindow);
    }

    /**
     * Sets the window token to assign when none is specified by the client or
     * available from the parent window.
     *
     * @param token The default token to assign.
     */
    public void setDefaultToken(IBinder token) {
        mDefaultToken = token;
    }

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

    @Override
    public void updateViewLayout(@NonNull View view, @NonNull ViewGroup.LayoutParams params) {
        applyDefaultToken(params);
        mGlobal.updateViewLayout(view, params);
    }

    private void applyDefaultToken(@NonNull ViewGroup.LayoutParams params) {
        // Only use the default token if we don't have a parent window.
        if (mDefaultToken != null && mParentWindow == null) {
            if (!(params instanceof WindowManager.LayoutParams)) {
                throw new IllegalArgumentException("Params must be WindowManager.LayoutParams");
            }

            // Only use the default token if we don't already have a token.
            final WindowManager.LayoutParams wparams = (WindowManager.LayoutParams) params;
            if (wparams.token == null) {
                wparams.token = mDefaultToken;
            }
        }
    }

    @Override
    public void removeView(View view) {
        mGlobal.removeView(view, false);
    }

    @Override
    public void removeViewImmediate(View view) {
        mGlobal.removeView(view, true);
    }

    ...
}

WindowManagerImpl 是 WindowManager 的实现类，所以了解其内部构造非常重要。值得注意的有：

• WindowManagerGlobal mGlobal = WindowManagerGlobal.getInstance(); 声明了一个全局的WindowManagerGlobal对象，WindowManagerGlobal 负责处理WindowManager具体操作
• addView() 、removeView()、updateViewLayout()等具体实现都在WindowManagerGlobal中

2、ActivityThread & handleResumeActivity()

    @Override
    public void handleResumeActivity(IBinder token, boolean finalStateRequest, boolean isForward,
            String reason) {
        ...
        // 执行Activity.Resume()方法
        final ActivityClientRecord r = performResumeActivity(token, finalStateRequest, reason);
        ...
        final Activity a = r.activity;

        ...
        if (r.window == null && !a.mFinished && willBeVisible) {
        	// 在第一步中初始化的WindowManager，继承ViewManager
            ViewManager wm = a.getWindowManager();
            ...
            if (a.mVisibleFromClient) {
                if (!a.mWindowAdded) {
                    a.mWindowAdded = true;
                    // 将DecorView添加到Window上
                    wm.addView(decor, l);
                } else {
                    a.onWindowAttributesChanged(l);
                }
            }

        } else if (!willBeVisible) {
            ...
        }

      ...
    }

在Activity执行performResumeActivity以后，会执行ViewManager.addView()方法，将DecorView添加到Window上。

由此可以得到：View的绘制是在Activity onResume()之后进行的。

WindowManager的实现类是WindowManagerImpl

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

然后执行WindowManagerGlobal.addView()方法

3、WindowManagerGlobal & addView()

    public void addView(View view, ViewGroup.LayoutParams params,
            Display display, Window parentWindow) {
        ...

        final WindowManager.LayoutParams wparams = (WindowManager.LayoutParams) params;
        if (parentWindow != null) {
            parentWindow.adjustLayoutParamsForSubWindow(wparams);
        } else {
            ...
        }

        ViewRootImpl root;
        View panelParentView = null;

        synchronized (mLock) {
          
           ...

            root = new ViewRootImpl(view.getContext(), display);

            view.setLayoutParams(wparams);

            mViews.add(view);
            mRoots.add(root);
            mParams.add(wparams);

            // do this last because it fires off messages to start doing things
            try {
                root.setView(view, wparams, panelParentView);
            } catch (RuntimeException e) {
                
                throw e;
            }
        }
    }

通过调用root.setView()将DecorView加入到Window中

4、ViewRootImpl & setView()

    public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) {
        synchronized (this) {
            if (mView == null) {
                mView = view;
                ... 
                view.assignParent(this);
                ...
                requestLayout();
                ...
            }
        }
    }

该方法主要作用有：

• 将 DecorView 赋值给 mView

• 调用view.assignParent(this) 将 ViewRootImpl 赋值给View中的mParent。

    void assignParent(ViewParent parent) {
        if (mParent == null) {
            mParent = parent;
        } else if (parent == null) {
            mParent = null;
        } else {
            throw new RuntimeException("view " + this + " being added, but"
                    + " it already has a parent");
        }
    }

• 执行requestLayout() 进行绘制

继续执行requestLayout()

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

执行scheduleTraversals();

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

通过mChoreographer.postCallback()执行mTraversalRunnable，在异步中执行绘制操作。

    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;
            }
        }
    }

最后会执行performTraversals()执行View的绘制操作

5、ViewRootImpl & performTraversals()

    private void performTraversals() {
        ...
        // Ask host how big it wants to be
        performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
        ...
        if (didLayout) {
            performLayout(lp, mWidth, mHeight); 
        }
        ...
   		performDraw();
        ...
    }

该方法会依次执行 performMeasure()、performLayout()、performDraw() 等方法，分别对View进行测量、布局、绘制

二、requestLayout() 原理

在自定义View和 ViewGroup 的时候，我们可以通过调用 requestLayout() 来对View重新进行测量、布局和绘制。

requestLayout() 源码如下：

    public void requestLayout() {
        if (mMeasureCache != null) mMeasureCache.clear();

        if (mAttachInfo != null && mAttachInfo.mViewRequestingLayout == null) {
            // Only trigger request-during-layout logic if this is the view requesting it,
            // not the views in its parent hierarchy
            ViewRootImpl viewRoot = getViewRootImpl();
            if (viewRoot != null && viewRoot.isInLayout()) {
                if (!viewRoot.requestLayoutDuringLayout(this)) {
                    return;
                }
            }
            mAttachInfo.mViewRequestingLayout = this;
        }

        mPrivateFlags |= PFLAG_FORCE_LAYOUT;
        mPrivateFlags |= PFLAG_INVALIDATED;

        if (mParent != null && !mParent.isLayoutRequested()) {
            mParent.requestLayout();
        }
        if (mAttachInfo != null && mAttachInfo.mViewRequestingLayout == this) {
            mAttachInfo.mViewRequestingLayout = null;
        }
    }

会调用 mParent.requestLayout()

ViewParent 是一个接口，而在上面第4步 ViewRootImpl.setView() 方法中会调用 view.assignParent(this); 方法，将ViewRootImpl 赋值给 mParent 。

因此调用 mParent.requestLayout() 会执行 ViewRootImpl. requestLayout() 方法，继而执行performTraversals()方法，对View进行测量、布局和绘制操作