View的

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

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

void invalidateInternal(int l, int t, int r, int b, boolean invalidateCache,
            boolean fullInvalidate) {
            final ViewParent p = mParent;
            if (p != null && ai != null && l < r && t < b) {
                // Parent 父类 调用
                p.invalidateChild(this, damage);
            }      
    }

p.invalidateChild(this, damage);父类调用,所以找到ViewGroup

public final void invalidateChild(View child, final Rect dirty) {
    ViewParent parent = this;
    ......
    do {
    ......
        parent = parent.invalidateChildInParent(location, dirty);
    ......    
    } while (parent != null);
}

invalidateChildInParent这个方法要么返回Parent父类,要么返回空

    public ViewParent invalidateChildInParent(final int[] location, final Rect dirty) {
        if ((mPrivateFlags & (PFLAG_DRAWN | PFLAG_DRAWING_CACHE_VALID)) != 0) {
            ....
            return mParent;
        }

        return null;
    }

而do while 就相当于一直往上找父类.找到没有父类为止.如下图

image.png

而在最外面的view是ViewRootImpl.
找到ViewRootImpl的invalidateChildInParent方法.

 public ViewParent invalidateChildInParent(int[] location, Rect dirty) {
        checkThread();
        ......
        invalidateRectOnScreen(dirty);

        return null;
    }

 private void invalidateRectOnScreen(Rect dirty) {
        ......
        if (!mWillDrawSoon && (intersected || mIsAnimating)) {
            scheduleTraversals();
        }
    }

    void scheduleTraversals() {
        if (!mTraversalScheduled) {
            ......
            mChoreographer.postCallback(
                    Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
           ......
        }
    }

ViewRootImpl的invalidateChildInParent方法里面有一个checkThread.有意思的方法.
这样可以引出另外一个问题.为什么不能在子线程更新UI.后面讲到.

    //看到mTraversalRunnable 这样Runnable
    final TraversalRunnable mTraversalRunnable = new TraversalRunnable();

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

    void doTraversal() {
        if (mTraversalScheduled) 
            //在UI的绘制流程中,这个方法是非常重要的
            performTraversals();
        }
    }

    private void performTraversals() {
         // Ask host how big it wants to be
         //测量,但是调用invalidate会导致不会走这个方法
         performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);   

        //摆放,但是调用invalidate会导致不会走这个方法
        performLayout(lp, mWidth, mHeight);     

        //绘制.调用invalidate()只会调用performDraw()方法.所以重点在这里了.
        performDraw(); 
    }

    private void draw(boolean fullRedrawNeeded) {
        if (!drawSoftware(surface, mAttachInfo, xOffset, yOffset, scalingRequired, dirty)) {
           return;
       }
    private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff,
            boolean scalingRequired, Rect dirty) 
        //这里的mView不是当前的view.是最外层的View
        mView.draw(canvas);
    }

最终调用View的draw方法.

 public void draw(Canvas canvas) {
        if (!dirtyOpaque) {
            drawBackground(canvas);
        }
        //最终调用onDraw方法.
        if (!dirtyOpaque) onDraw(canvas);
        
        dispatchDraw(canvas);
        
        // Step 6, draw decorations (foreground, scrollbars)
        onDrawForeground(canvas);
  }

总结:invalidate()流程.

一路往上跑,跑到最外层,调用draw(canvas) -> dispatchDraw(canvas) 画子孩子,一路往下画.最终画到当前调用invalidate()的View的onDrawa()方法.
invalidate()牵连着整个layout布局的View.

题外话:为什么不能在子线程更新UI?

开线程,更新UI 一般都会调用setText(),setImageView ,而setText,等都会调用到ViewRootImpl.checkThread()的方法.

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

这样方法用来检测线程的.如果不是在原始线程就会抛出异常.