一、postInvalidate和invalidate

其实postInvalidate最终也是调用的invalidate来实现。

    public void postInvalidate() {
        postInvalidateDelayed(0);
    }

    public void postInvalidateDelayed(long delayMilliseconds) {
        // We try only with the AttachInfo because there's no point in invalidating
        // if we are not attached to our window
        final AttachInfo attachInfo = mAttachInfo;
        if (attachInfo != null) {
            attachInfo.mViewRootImpl.dispatchInvalidateDelayed(this, delayMilliseconds);
        }
    }

AttachInfo.mViewRootImpl其实就是ViewRootImpl对象

    public void dispatchInvalidateDelayed(View view, long delayMilliseconds) {
        Message msg = mHandler.obtainMessage(MSG_INVALIDATE, view);
        mHandler.sendMessageDelayed(msg, delayMilliseconds);
    }

这里的mHandler对象，其实是ViewRootImpl内部类ViewRootHandler对象，

        @Override
        public void handleMessage(Message msg) {
            switch (msg.what) {
                case MSG_INVALIDATE:
                    ((View) msg.obj).invalidate();
                    break;
        ....
        }

而这里的msg.obj其实就是View对象

二、invalidate源码分析

image.png

从网上拿来用的一张图，自己就不画了。

View.invalidate

    public void invalidate() {
        invalidate(true);
    }

    @UnsupportedAppUsage
    public void invalidate(boolean invalidateCache) {
        invalidateInternal(0, 0, mRight - mLeft, mBottom - mTop, invalidateCache, true);
    }

View.invalidateInternal

    void invalidateInternal(int l, int t, int r, int b, boolean invalidateCache,
            boolean fullInvalidate) {
        if (mGhostView != null) {
            mGhostView.invalidate(true);
            return;
        }

        if (skipInvalidate()) {
            return;
        }

        // Reset content capture caches
        mCachedContentCaptureSession = null;

        if ((mPrivateFlags & (PFLAG_DRAWN | PFLAG_HAS_BOUNDS)) == (PFLAG_DRAWN | PFLAG_HAS_BOUNDS)
                || (invalidateCache && (mPrivateFlags & PFLAG_DRAWING_CACHE_VALID) == PFLAG_DRAWING_CACHE_VALID)
                || (mPrivateFlags & PFLAG_INVALIDATED) != PFLAG_INVALIDATED
                || (fullInvalidate && isOpaque() != mLastIsOpaque)) {
            if (fullInvalidate) {
                mLastIsOpaque = isOpaque();
                mPrivateFlags &= ~PFLAG_DRAWN;
            }

            mPrivateFlags |= PFLAG_DIRTY;

            if (invalidateCache) {
                mPrivateFlags |= PFLAG_INVALIDATED;
                mPrivateFlags &= ~PFLAG_DRAWING_CACHE_VALID;
            }

            // Propagate the damage rectangle to the parent view.
            final AttachInfo ai = mAttachInfo;
            final ViewParent p = mParent;
            if (p != null && ai != null && l < r && t < b) {
                final Rect damage = ai.mTmpInvalRect;
                damage.set(l, t, r, b);
                // 调用了ViewParent的invalidateChild方法，而ViewGroup实现了ViewParent接口
                p.invalidateChild(this, damage);
            }

            // Damage the entire projection receiver, if necessary.
            if (mBackground != null && mBackground.isProjected()) {
                final View receiver = getProjectionReceiver();
                if (receiver != null) {
                    receiver.damageInParent();
                }
            }
        }
    }

根据查找ViewParent接口实现我们可以知道，ViewGroup是ViewParent的实现类，所以这里是调用了ViewGroup.invalidateChild()

ViewGroup.invalidateChild()

    @Override
    public final void invalidateChild(View child, final Rect dirty) {
        final AttachInfo attachInfo = mAttachInfo;
        if (attachInfo != null && attachInfo.mHardwareAccelerated) {
            // HW accelerated fast path
            onDescendantInvalidated(child, child);
            return;
        }

        ViewParent parent = this;
        if (attachInfo != null) {
            
            ...

            do {
                ...
                parent = parent.invalidateChildInParent(location, dirty);
                ...
            } while (parent != null);
        }
    }

因为parent是在invalidateChild中赋值的，其实就是ViewGroup本身，所以是调用的ViewGroup的invalidateChildInParent方法

ViewGroup.invalidateChildInParent()

    @Deprecated
    @Override
    public ViewParent invalidateChildInParent(final int[] location, final Rect dirty) {
        if ((mPrivateFlags & (PFLAG_DRAWN | PFLAG_DRAWING_CACHE_VALID)) != 0) {
            // either DRAWN, or DRAWING_CACHE_VALID
            if ((mGroupFlags & (FLAG_OPTIMIZE_INVALIDATE | FLAG_ANIMATION_DONE))
                    != FLAG_OPTIMIZE_INVALIDATE) {
                dirty.offset(location[CHILD_LEFT_INDEX] - mScrollX,
                        location[CHILD_TOP_INDEX] - mScrollY);
                if ((mGroupFlags & FLAG_CLIP_CHILDREN) == 0) {
                    dirty.union(0, 0, mRight - mLeft, mBottom - mTop);
                }

                final int left = mLeft;
                final int top = mTop;

                if ((mGroupFlags & FLAG_CLIP_CHILDREN) == FLAG_CLIP_CHILDREN) {
                    if (!dirty.intersect(0, 0, mRight - left, mBottom - top)) {
                        dirty.setEmpty();
                    }
                }

                location[CHILD_LEFT_INDEX] = left;
                location[CHILD_TOP_INDEX] = top;
            } else {

                if ((mGroupFlags & FLAG_CLIP_CHILDREN) == FLAG_CLIP_CHILDREN) {
                    dirty.set(0, 0, mRight - mLeft, mBottom - mTop);
                } else {
                    // in case the dirty rect extends outside the bounds of this container
                    dirty.union(0, 0, mRight - mLeft, mBottom - mTop);
                }
                location[CHILD_LEFT_INDEX] = mLeft;
                location[CHILD_TOP_INDEX] = mTop;

                mPrivateFlags &= ~PFLAG_DRAWN;
            }
            mPrivateFlags &= ~PFLAG_DRAWING_CACHE_VALID;
            if (mLayerType != LAYER_TYPE_NONE) {
                mPrivateFlags |= PFLAG_INVALIDATED;
            }

            return mParent;
        }

        return null;
    }

但是因为ViewGroup.invalidateChild方法中执行的是一个do-while循环，不断的循环返回View的mParent，而ViewGroup最终的mParent其实是DecorView，而DecorView.mParent其实就是ViewRootImpl，所以这里最终会调用到ViewRootImpl.invalidateChildInParent

ViewRootImpl.invalidateChildInParent()

    @Override
    public ViewParent invalidateChildInParent(int[] location, Rect dirty) {
        checkThread();
        if (DEBUG_DRAW) Log.v(mTag, "Invalidate child: " + dirty);

        if (dirty == null) {
            invalidate();
            return null;
        } else if (dirty.isEmpty() && !mIsAnimating) {
            return null;
        }

        if (mCurScrollY != 0 || mTranslator != null) {
            mTempRect.set(dirty);
            dirty = mTempRect;
            if (mCurScrollY != 0) {
                dirty.offset(0, -mCurScrollY);
            }
            if (mTranslator != null) {
                mTranslator.translateRectInAppWindowToScreen(dirty);
            }
            if (mAttachInfo.mScalingRequired) {
                dirty.inset(-1, -1);
            }
        }

        invalidateRectOnScreen(dirty);

        return null;
    }

ViewRootImpl.invalidateRectOnScreen()

    private void invalidateRectOnScreen(Rect dirty) {
        final Rect localDirty = mDirty;

        // Add the new dirty rect to the current one
        localDirty.union(dirty.left, dirty.top, dirty.right, dirty.bottom);
        // Intersect with the bounds of the window to skip
        // updates that lie outside of the visible region
        final float appScale = mAttachInfo.mApplicationScale;
        final boolean intersected = localDirty.intersect(0, 0,
                (int) (mWidth * appScale + 0.5f), (int) (mHeight * appScale + 0.5f));
        if (!intersected) {
            localDirty.setEmpty();
        }
        if (!mWillDrawSoon && (intersected || mIsAnimating)) {
            scheduleTraversals();
        }
    }

ViewGroup.scheduleTraversals()

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

scheduleTraversals()方法内部通过调用Choreographer.postCallback方法，其内部就是通过Handler，然后在doFrame中调用doCallbacks，通过遍历CallbackRecord这个链表，执行保持在CallbackRecord中的action（即Runnable）的run方法。就是TraversalRunnable对象mTraversalRunnable

    void doCallbacks(int callbackType, long frameTimeNanos) {
        CallbackRecord callbacks;
        synchronized (mLock) {
            // We use "now" to determine when callbacks become due because it's possible
            // for earlier processing phases in a frame to post callbacks that should run
            // in a following phase, such as an input event that causes an animation to start.
            // 确认回调的时机
            final long now = System.nanoTime();
            callbacks = mCallbackQueues[callbackType].extractDueCallbacksLocked(
                    now / TimeUtils.NANOS_PER_MS);
            if (callbacks == null) {
                return;
            }
            ...
        try {
            Trace.traceBegin(Trace.TRACE_TAG_VIEW, CALLBACK_TRACE_TITLES[callbackType]);
            for (CallbackRecord c = callbacks; c != null; c = c.next) {
                if (DEBUG_FRAMES) {
                    Log.d(TAG, "RunCallback: type=" + callbackType
                            + ", action=" + c.action + ", token=" + c.token
                            + ", latencyMillis=" + (SystemClock.uptimeMillis() - c.dueTime));
                }
                c.run(frameTimeNanos);
            }
        } finally {
            synchronized (mLock) {
                mCallbacksRunning = false;
                do {
                    final CallbackRecord next = callbacks.next;
                    recycleCallbackLocked(callbacks);
                    callbacks = next;
                } while (callbacks != null);
            }
            Trace.traceEnd(Trace.TRACE_TAG_VIEW);
        }
    }

CallbackRecord是一个链表结构，在这里最终就会判断是否是帧callback token，如果不是，则调用action.run()，这里的action其实就是TraversalRunnable

    private static final class CallbackRecord {
        public CallbackRecord next;
        public long dueTime;
        public Object action; // Runnable or FrameCallback
        public Object token;

        @UnsupportedAppUsage
        public void run(long frameTimeNanos) {
            if (token == FRAME_CALLBACK_TOKEN) {
                ((FrameCallback)action).doFrame(frameTimeNanos);
            } else {
                ((Runnable)action).run();
            }
        }
    }

执行到TraversalRunnable的run()

ViewRootImpl.TraversalRunnable内部类

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

调用了ViewRootImpl.doTraversal()

ViewRootImpl.doTraversal()

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

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

            performTraversals();

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

在这里会调用performTraversals()

ViewRootImpl.performTraversals()

performTraversals方法比较长，所以看一部分

    private void performTraversals() {
        ...
        boolean cancelDraw = mAttachInfo.mTreeObserver.dispatchOnPreDraw() || !isViewVisible;

        if (!cancelDraw) {
            if (mPendingTransitions != null && mPendingTransitions.size() > 0) {
                for (int i = 0; i < mPendingTransitions.size(); ++i) {
                    mPendingTransitions.get(i).startChangingAnimations();
                }
                mPendingTransitions.clear();
            }

            performDraw();
        } else {
            if (isViewVisible) {
                // Try again
                scheduleTraversals();
            } else if (mPendingTransitions != null && mPendingTransitions.size() > 0) {
                for (int i = 0; i < mPendingTransitions.size(); ++i) {
                    mPendingTransitions.get(i).endChangingAnimations();
                }
                mPendingTransitions.clear();
            }
        }
    }

这个方法主要是做了三个操作，即调用performMeasure、performLayout、performDraw，而invalidate流程是否会触发performMeasure和performLayout姑且不论，因为这里需要经过繁琐的标志位的判断，但是会执行performDraw。而performMeasure和performLayout分别是measure过程和layout过程，是由View.requestLayout()或者WindowManagerImpl.addView调用WindowManagerGlobal.addView这个两种情况最终调用了ViewRootImpl.requestLayout来触发调用performMeasure和performLayout。

    private void performDraw() {
        ...
        try {
            boolean canUseAsync = draw(fullRedrawNeeded);
            if (usingAsyncReport && !canUseAsync) {
                mAttachInfo.mThreadedRenderer.setFrameCompleteCallback(null);
                usingAsyncReport = false;
            }
        } finally {
            mIsDrawing = false;
            Trace.traceEnd(Trace.TRACE_TAG_VIEW);
        }
        ...
    }

在这里调用了ViewRootImpl.draw()

ViewRootImpl.draw()

    private boolean draw(boolean fullRedrawNeeded) {
        Surface surface = mSurface;
        if (!surface.isValid()) {
            return false;
        }

        if (DEBUG_FPS) {
            trackFPS();
        }

        if (!sFirstDrawComplete) {
            synchronized (sFirstDrawHandlers) {
                sFirstDrawComplete = true;
                final int count = sFirstDrawHandlers.size();
                for (int i = 0; i< count; i++) {
                    mHandler.post(sFirstDrawHandlers.get(i));
                }
            }
        }

        scrollToRectOrFocus(null, false);

        if (mAttachInfo.mViewScrollChanged) {
            mAttachInfo.mViewScrollChanged = false;
            mAttachInfo.mTreeObserver.dispatchOnScrollChanged();
        }

        boolean animating = mScroller != null && mScroller.computeScrollOffset();
        final int curScrollY;
        if (animating) {
            curScrollY = mScroller.getCurrY();
        } else {
            curScrollY = mScrollY;
        }
        if (mCurScrollY != curScrollY) {
            mCurScrollY = curScrollY;
            fullRedrawNeeded = true;
            if (mView instanceof RootViewSurfaceTaker) {
                ((RootViewSurfaceTaker) mView).onRootViewScrollYChanged(mCurScrollY);
            }
        }

        final float appScale = mAttachInfo.mApplicationScale;
        final boolean scalingRequired = mAttachInfo.mScalingRequired;

        final Rect dirty = mDirty;
        if (mSurfaceHolder != null) {
            // The app owns the surface, we won't draw.
            dirty.setEmpty();
            if (animating && mScroller != null) {
                mScroller.abortAnimation();
            }
            return false;
        }

        if (fullRedrawNeeded) {
            dirty.set(0, 0, (int) (mWidth * appScale + 0.5f), (int) (mHeight * appScale + 0.5f));
        }

        if (DEBUG_ORIENTATION || DEBUG_DRAW) {
            Log.v(mTag, "Draw " + mView + "/"
                    + mWindowAttributes.getTitle()
                    + ": dirty={" + dirty.left + "," + dirty.top
                    + "," + dirty.right + "," + dirty.bottom + "} surface="
                    + surface + " surface.isValid()=" + surface.isValid() + ", appScale:" +
                    appScale + ", width=" + mWidth + ", height=" + mHeight);
        }

        mAttachInfo.mTreeObserver.dispatchOnDraw();

        int xOffset = -mCanvasOffsetX;
        int yOffset = -mCanvasOffsetY + curScrollY;
        final WindowManager.LayoutParams params = mWindowAttributes;
        final Rect surfaceInsets = params != null ? params.surfaceInsets : null;
        if (surfaceInsets != null) {
            xOffset -= surfaceInsets.left;
            yOffset -= surfaceInsets.top;

            // Offset dirty rect for surface insets.
            dirty.offset(surfaceInsets.left, surfaceInsets.right);
        }

        boolean accessibilityFocusDirty = false;
        final Drawable drawable = mAttachInfo.mAccessibilityFocusDrawable;
        if (drawable != null) {
            final Rect bounds = mAttachInfo.mTmpInvalRect;
            final boolean hasFocus = getAccessibilityFocusedRect(bounds);
            if (!hasFocus) {
                bounds.setEmpty();
            }
            if (!bounds.equals(drawable.getBounds())) {
                accessibilityFocusDirty = true;
            }
        }

        mAttachInfo.mDrawingTime =
                mChoreographer.getFrameTimeNanos() / TimeUtils.NANOS_PER_MS;

        boolean useAsyncReport = false;
        if (!dirty.isEmpty() || mIsAnimating || accessibilityFocusDirty) {
            if (mAttachInfo.mThreadedRenderer != null && mAttachInfo.mThreadedRenderer.isEnabled()) {
                ...
            } else {
                ...

                if (!drawSoftware(surface, mAttachInfo, xOffset, yOffset,
                        scalingRequired, dirty, surfaceInsets)) {
                    return false;
                }
            }
        }

        if (animating) {
            mFullRedrawNeeded = true;
            scheduleTraversals();
        }
        return useAsyncReport;
    }

ViewRootImpl.drawSoftWare()

    private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff,
            boolean scalingRequired, Rect dirty, Rect surfaceInsets) {

        ...

        try {
            ...

            mView.draw(canvas);

            drawAccessibilityFocusedDrawableIfNeeded(canvas);
        } finally {
            ...
        }
        return true;
    }

从这里可以看出，最终是调用了View.draw()方法。
其实invalidate()方法的使用，就是重新触发一次draw流程，进行绘制。通过View.invalidate()方法在ViewGroup的invalidateChild()方法中执行一个do-while循环，最终调用了ViewRootImpl.invalidateChildInParent()，而在ViewRootImpl.invalidateChildInParent()方法中最终触发调用了ViewRootImpl.scheduleTraversals()，又一次执行了与measure过程和layout过程一致的流程，因为scheduleTraversals()方法会通过一系列的调用最终调用TraversalRunnable.run()方法，在TraversalRunnable.run()方法中会调用performTraversals()，这个方法其实就是做了三件事：measure、layout和draw

参考下
https://www.jianshu.com/p/33b855dcde5a