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??????? 在定義ListView的Selector時候，有個drawSelectorOnTop的屬性，如果drawSelectorOnTop為true的話，Selector的效果是畫在List Item的上面（Selector是蓋住了ListView的文字或者圖片），即Foreground前景。如果drawSelectorOnTop為false的話，Selector的效果是畫在List Item的下面，即Background背景。由于項目中恰好需要自定義View，需要實現此效果。

?????? 本文借ListView的代碼來剖析一下，

?????? ListView完成此部分功能在frameworks\base\core\java\android\widget\AbsListView.java文件中。

用mSelector即ListView要畫的Selector(資源文件)，而mSelectorRect則是想要畫的區域。

?/*** Indicates whether the list selector should be drawn on top of the children or behind*/boolean mDrawSelectorOnTop = false; 決定畫前景還是背景/*** The drawable used to draw the selector*/Drawable mSelector; ListView用中來顯示Selector的Drawable，即ListSelector對應的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(); 用來畫Selector的區域，即Selector畫的位置

AbsListView中構造方法中有獲取selector

Drawable d = a.getDrawable(com.android.internal.R.styleable.AbsListView_listSelector);if (d != null) {setSelector(d);}//默認為false,畫的是背景mDrawSelectorOnTop = a.getBoolean(com.android.internal.R.styleable.AbsListView_drawSelectorOnTop, false); 下面看一下setSelector是如何實現的 /*** 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) { //提供是否畫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)); 設置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); //需要給Selector設置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的狀態} else {mSelector.setState(StateSet.NOTHING);}}}

這樣就將Selector設置給ListView了，并且更新了drawable的狀態。

接下來我們再看一下Android是如何將drawable畫到ListView的Item上的。

在AbsListView中有個onTouchEvent的方法用來處理Touch事件，其中有一段代碼就是確定Selector要畫的區域。

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);//設置List Item狀態為 pressedpositionSelector(mMotionPosition, child);//確定畫Selector的區域setPressed(true); //設置ListView 的狀態為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();}}

接下來看看positionSelector的實現，

void positionSelector(int position, View sel) {if (position != INVALID_POSITION) {mSelectorPosition = position;}//設置Selector的區域為List Item View的邊界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();//根據View狀態更新drawable的狀態}}}private void positionSelector(int l, int t, int r, int b) {mSelectorRect.set(l - mSelectionLeftPadding, t - mSelectionTopPadding, r+ mSelectionRightPadding, b + mSelectionBottomPadding);}

好了現在已經決定了將selector畫在哪里，Selector的狀態也已經更新OK。

還差一步沒有做，那就是到底是將其怎么畫上面的呢？

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) { //將Selector畫為背景drawSelector(canvas);}super.dispatchDraw(canvas);// 用Canvas畫ListViewif (drawSelectorOnTop) { //將Selector畫為前景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);//設置drawable畫的區域selector.draw(canvas); //使用canvas將drawable畫上去}}

看到這里，想必大家都已經明白如何畫前景和背景了吧。在dispatchDraw之前調用就是畫前景，在dispatchDraw之后調用就是畫背景。

另外補充一下，本文并沒有介紹動畫部分，有興趣的可以自己研究下。

總結一下，實現這個功能需要有三個步驟：

1.設置Selector，并更新狀態（初始化時候）

2.確定Selector畫的區域，設置View的狀態，根據View狀態，更新Selector的狀態(一般是對Event的處理方法中)

3.使用Canvas在dispatchDraw中，將Selector畫上去，畫Drawable的時候需要先設置區域，再調用drawable的draw方法。

后面我再將View如何畫背景和前景補上，今天就先到這里吧。

??????

轉載于:https://my.oschina.net/shaorongjie/blog/202291

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