自定义View学习笔记（3）->invalidate

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Invalidate 源码

/** * Invalidate the whole view. If the view is visible, {@link #onDraw} will  * be called at some point in the future. This must be called from a  * UI thread. To call from a non-UI thread, call {@link #postInvalidate()}.  */public void invalidate() {        if (ViewDebug.TRACE_HIERARCHY) {        ViewDebug.trace(this, ViewDebug.HierarchyTraceType.INVALIDATE);        }        if ((mPrivateFlags & (DRAWN | HAS_BOUNDS)) == (DRAWN | HAS_BOUNDS)) {        mPrivateFlags &= ~DRAWN & ~DRAWING_CACHE_VALID;final ViewParent p = mParent;final AttachInfo ai = mAttachInfo;        if (p != null && ai != null) {final Rect r = ai.mTmpInvalRect;        r.set(0, 0, mRight - mLeft, mBottom - mTop);        // Don't call invalidate -- we don't want to internally scroll          // our own bounds          p.invalidateChild(this, r);        }        }        }

这里可以看到p.invalidateChild(this, r)（看源码只看关键部分，不然你会很晕！），其中p是ViewParent实例对象。ViewParent是一个接口，现在我们关心谁实现了这个接口？

通过千辛万苦的search，终于找到ViewParen的实现类ViewRoot：

/** * The top of a view hierarchy, implementing the needed protocol between View  * and the WindowManager.  This is for the most part an internal implementation  * detail of {@link WindowManagerImpl}.  * * {@hide} */@SuppressWarnings({"EmptyCatchBlock"})public final class ViewRoot extends Handler implements ViewParent, View.AttachInfo.Callbacks { }

那么，看看该类实现的invalidateChild()方法：public void invalidateChild(View child, Rect dirty) {        checkThread();        if (DEBUG_DRAW) Log.v(TAG, "Invalidate child: " + dirty);        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);        }        }        mDirty.union(dirty);        if (!mWillDrawSoon) {        scheduleTraversals();        }        }        //关键代码在这儿：    if (!mWillDrawSoon) {     scheduleTraversals();}// 这个方法是向Handler发送消息：public void scheduleTraversals() {        if (!mTraversalScheduled) {        mTraversalScheduled = true;        sendEmptyMessage(DO_TRAVERSAL);        }        }//接下来，看看ViewRoot的Handler的handleMessage的实现：public void handleMessage(Message msg) {        switch (msg.what) {        // 、、、          case DO_TRAVERSAL:        // 、、、          performTraversals();        }        }        performTraversals()方法，调用ViewRoot的私有方法private void draw(boolean fullRedrawNeeded)，在该方法中有句代码很关键：        [java] view plain copy        print?        mView.draw(canvas);        其实这句代码，就是调用View的draw()方法 ，关键代码：        if (!dirtyOpaque) onDraw(canvas);

也就是说，满足这个方法，就会回调onDraw()方法。到此为止，您应该明白，当我们自己调用invalidate()方法时，想使onDraw()方法回调，必须满足条件。
调用关系，请看草图！

学习总结：invalidate()方法使onDraw()被调用，实现重绘的效果

转载：http://blog.csdn.net/veryitman/article/details/6692950

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