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); 下面看一下setSelector是如何实现的

/**

* 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Callback

updateSelectorState();

}

/**

* 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 pressed

positionSelector(mMotionPosition, child);//确定画Selector的区域

setPressed(true); //设置ListView 的状态为pressed

if (mSelector != null) {

Drawable d = mSelector.getCurrent();

if (d != null && d instanceof TransitionDrawable) {

((TransitionDrawable) d).resetTransition();

}

}

if (mTouchModeReset != null) {

removeCallbacks(mTouchModeReset);

}

mTouchModeReset = new Runnable() {

@Override

public 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。

@Override

protected 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ListView

if (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。