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在定义ListView的Selector时候，有个drawSelectorOnTop的属性，如果drawSelectorOnTop为true的话，Selector的效果是画在List Item的上面（Selector是盖住了ListView的文字或者图片），即Foreground前景。如果drawSelectorOnTop为false的话，Selector的效果是画在List Item的下面，即Background背景。由于项目中恰好需要自定义View，需要实现此效果。
本文借ListView的代码来剖析一下，
ListView完成此部分功能在frameworks\base\core\java\android\widget\AbsListView.java文件中。
用mSelector即ListView要画的Selector(资源文件)，而mSelectorRect则是想要画的区域。
/*** Indicates whether the list selector should be drawn on top of the children or behind*/boolean mDrawSelectorOnTop = false; 决定画前景还是背景/*** The drawable used to draw the selector*/Drawable mSelector; ListView用中来显示Selector的Drawable，即ListSelector对应的XML文件/*** The current position of the selector in the list.*/int mSelectorPosition = INVALID_POSITION;/*** Defines the selector's location and dimension at drawing time*/Rect mSelectorRect = new Rect(); 用来画Selector的区域，即Selector画的位置AbsListView中构造方法中有获取selector
Drawable d = a.getDrawable(com.android.internal.R.styleable.AbsListView_listSelector);if (d != null) {setSelector(d);}//默认为false,画的是背景mDrawSelectorOnTop = a.getBoolean(com.android.internal.R.styleable.AbsListView_drawSelectorOnTop, false);/*** Controls whether the selection highlight drawable should be drawn on top of the item or* behind it.** @param onTop If true, the selector will be drawn on the item it is highlighting. The default* is false.** @attr ref android.R.styleable#AbsListView_drawSelectorOnTop*/public void setDrawSelectorOnTop(boolean onTop) { //提供是否画前景或者背景的接口mDrawSelectorOnTop = onTop;}/*** Set a Drawable that should be used to highlight the currently selected item.** @param resID A Drawable resource to use as the selection highlight.** @attr ref android.R.styleable#AbsListView_listSelector*/public void setSelector(int resID) {setSelector(getResources().getDrawable(resID)); 设置listSelector的XML文件}public void setSelector(Drawable sel) {if (mSelector != null) {mSelector.setCallback(null);unscheduleDrawable(mSelector);}mSelector = sel;Rect padding = new Rect();sel.getPadding(padding);mSelectionLeftPadding = padding.left;mSelectionTopPadding = padding.top;mSelectionRightPadding = padding.right;mSelectionBottomPadding = padding.bottom;sel.setCallback(this); //需要给Selector设置CallbackupdateSelectorState(); }/*** Returns the selector {@link android.graphics.drawable.Drawable} that is used to draw the* selection in the list.** @return the drawable used to display the selector*/public Drawable getSelector() {return mSelector;}void updateSelectorState() {if (mSelector != null) {if (shouldShowSelector()) {mSelector.setState(getDrawableState());//更新Selector的状态} else {mSelector.setState(StateSet.NOTHING);}}}这样就将Selector设置给ListView了，并且更新了drawable的状态。
接下来我们再看一下Android是如何将drawable画到ListView的Item上的。
在AbsListView中有个onTouchEvent的方法用来处理Touch事件，其中有一段代码就是确定Selector要画的区域。
if (mTouchMode == TOUCH_MODE_DOWN || mTouchMode == TOUCH_MODE_TAP) {final Handler handler = getHandler();if (handler != null) {handler.removeCallbacks(mTouchMode == TOUCH_MODE_DOWN ?mPendingCheckForTap : mPendingCheckForLongPress);}mLayoutMode = LAYOUT_NORMAL;if (!mDataChanged && mAdapter.isEnabled(motionPosition)) {mTouchMode = TOUCH_MODE_TAP;setSelectedPositionInt(mMotionPosition);layoutChildren();child.setPressed(true);//设置List Item状态为 pressedpositionSelector(mMotionPosition, child);//确定画Selector的区域setPressed(true); //设置ListView 的状态为pressedif (mSelector != null) {Drawable d = mSelector.getCurrent();if (d != null && d instanceof TransitionDrawable) {((TransitionDrawable) d).resetTransition();}}if (mTouchModeReset != null) {removeCallbacks(mTouchModeReset);}mTouchModeReset = new Runnable() {@Overridepublic void run() {mTouchMode = TOUCH_MODE_REST;child.setPressed(false);setPressed(false);if (!mDataChanged) {performClick.run();}}};postDelayed(mTouchModeReset,ViewConfiguration.getPressedStateDuration());} else {mTouchMode = TOUCH_MODE_REST;updateSelectorState();}return true;} else if (!mDataChanged && mAdapter.isEnabled(motionPosition)) {performClick.run();}}接下来看看positionSelector的实现，
void positionSelector(int position, View sel) {if (position != INVALID_POSITION) {mSelectorPosition = position;}//设置Selector的区域为List Item View的边界final Rect selectorRect = mSelectorRect; selectorRect.set(sel.getLeft(), sel.getTop(), sel.getRight(), sel.getBottom());if (sel instanceof SelectionBoundsAdjuster) {((SelectionBoundsAdjuster)sel).adjustListItemSelectionBounds(selectorRect);}positionSelector(selectorRect.left, selectorRect.top, selectorRect.right,selectorRect.bottom);final boolean isChildViewEnabled = mIsChildViewEnabled;if (sel.isEnabled() != isChildViewEnabled) {mIsChildViewEnabled = !isChildViewEnabled;if (getSelectedItemPosition() != INVALID_POSITION) {refreshDrawableState();//根据View状态更新drawable的状态}}}private void positionSelector(int l, int t, int r, int b) {mSelectorRect.set(l - mSelectionLeftPadding, t - mSelectionTopPadding, r+ mSelectionRightPadding, b + mSelectionBottomPadding);}好了现在已经决定了将selector画在哪里，Selector的状态也已经更新OK。
还差一步没有做，那就是到底是将其怎么画上面的呢？
答案就在AbsListView.java里的dispatchDraw方法里面。
@Overrideprotected void dispatchDraw(Canvas canvas) {int saveCount = 0;final boolean clipToPadding = (mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK;if (clipToPadding) {saveCount = canvas.save();final int scrollX = mScrollX;final int scrollY = mScrollY;canvas.clipRect(scrollX + mPaddingLeft, scrollY + mPaddingTop,scrollX + mRight - mLeft - mPaddingRight,scrollY + mBottom - mTop - mPaddingBottom);mGroupFlags &= ~CLIP_TO_PADDING_MASK;}final boolean drawSelectorOnTop = mDrawSelectorOnTop;if (!drawSelectorOnTop) { //将Selector画为背景drawSelector(canvas);}super.dispatchDraw(canvas);// 用Canvas画ListViewif (drawSelectorOnTop) { //将Selector画为前景drawSelector(canvas);}if (clipToPadding) {canvas.restoreToCount(saveCount);mGroupFlags |= CLIP_TO_PADDING_MASK;}}private void drawSelector(Canvas canvas) {if (!mSelectorRect.isEmpty()) {final Drawable selector = mSelector;selector.setBounds(mSelectorRect);//设置drawable画的区域selector.draw(canvas); //使用canvas将drawable画上去}}看到这里，想必大家都已经明白如何画前景和背景了吧。在dispatchDraw之前调用就是画前景，在dispatchDraw之后调用就是画背景。
另外补充一下，本文并没有介绍动画部分，有兴趣的可以自己研究下。
总结一下，实现这个功能需要有三个步骤：
1.设置Selector，并更新状态（初始化时候）
2.确定Selector画的区域，设置View的状态，根据View状态，更新Selector的状态(一般是对Event的处理方法中)
3.使用Canvas在dispatchDraw中，将Selector画上去，画Drawable的时候需要先设置区域，再调用drawable的draw方法。
后面我再将View如何画背景和前景补上，今天就先到这里吧。
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