invalidate()方法的使用，就是重新觸發一次View的繪制流程。

入口在View類中，

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

在 invalidateInternal(...) 中 ，調用

p.invalidateChild(this, damage); //p為ViewParent

ViewGroup繼承View，實現了 ViewParent，在ViewGroup的invalidateChild()方法中有個do while 循環，循環中關鍵代碼：

parent = parent.invalidateChildInParent(location, dirty);

ViewRootImpl實現了ViewParent,在ViewRootImpl中，

@Override
public void invalidateChild(View child, Rect dirty) {invalidateChildInParent(null, dirty);
}@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;
}private void invalidateRectOnScreen(Rect dirty) {final Rect localDirty = mDirty;if (!localDirty.isEmpty() && !localDirty.contains(dirty)) {mAttachInfo.mSetIgnoreDirtyState = true;mAttachInfo.mIgnoreDirtyState = true;}// Add the new dirty rect to the current onelocalDirty.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 regionfinal 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();}
}

invalidateChild(..)方法調用invalidateChildInParent(..),

invalidateChildInParent(..)方法調用invalidateRectOnScreen(dirty)，

invalidateRectOnScreen(dirty)方法中最終會調用scheduleTraversals()

scheduleTraversals()這個方法很關鍵，會通過一系列的調用最終調用TraversalRunnable.run()方法，然后調用doTraversal()方法，doTraversal()中調用performTraversals()。

void doTraversal() {if (mTraversalScheduled) {mTraversalScheduled = false;mHandler.getLooper().getQueue().removeSyncBarrier(mTraversalBarrier);if (mProfile) {Debug.startMethodTracing("ViewAncestor");}performTraversals();if (mProfile) {Debug.stopMethodTracing();mProfile = false;}}
}

performTraversals()主要做三件事：

performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);

performLayout(lp, mWidth, mHeight);

performDraw();

最后會調用View的繪制方法：

measure(childWidthMeasureSpec, childHeightMeasureSpec);

layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());

draw(fullRedrawNeeded);

至此，整個invalidate()流程結束，貫穿著一個布局或View的所有子view，以下是流程圖：

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