??????? 做Android開(kāi)發(fā)的人都用過(guò)Selector,可以方便的實(shí)現(xiàn)View在不同狀態(tài)下的背景。不過(guò)，相信大部分開(kāi)發(fā)者遇到過(guò)和我一樣的問(wèn)題，本文會(huì)從源碼角度，解釋這些問(wèn)題。

??????? 首先，這里簡(jiǎn)單描述一下，我遇到的問(wèn)題：

界面上有個(gè)全屏的LinearLayout A，A中有一個(gè)TextView B和Button C，其中，A的clickable=true，并設(shè)置了pressed時(shí)，背景色為灰色，B設(shè)置了pressed時(shí)，背景色為藍(lán)色

當(dāng)手指觸摸C下方的空白區(qū)域時(shí)，看到了這樣的效果：

??????? 在這里看到，在沒(méi)有觸摸B的情況下，B的pressed = true，而C的pressed = false。 C的狀態(tài)暫且不討論，按照Android消息傳遞的原則，因?yàn)閠ouch的point不在B內(nèi)部，所以，touch消息應(yīng)該不會(huì)交給B處理，那為什么B的pressed = true？

???????? 下面開(kāi)始一步一步分析（本文分析的Android源碼為4.2.2）。

Pressed狀態(tài)的設(shè)定

??????? 從View.onTouchEvent函數(shù)看起（frameworks/base/core/java/android/view/View.java):

????/**
?????*?Implement?this?method?to?handle?touch?screen?motion?events.
?????*
?????*?@param?event?The?motion?event.
?????*?@return?True?if?the?event?was?handled,?false?otherwise.
?????*/
????public?boolean?onTouchEvent(MotionEvent?event)?{
????????......
????????if?(((viewFlags?&?CLICKABLE)?==?CLICKABLE?||?//這里是為什么設(shè)置A的clickable為true的原因，否則，press?A的時(shí)候，沒(méi)有界面元素處理touch?event，最終會(huì)由Activity的onTouchEvent函數(shù)處理
????????????????(viewFlags?&?LONG_CLICKABLE)?==?LONG_CLICKABLE))?{
????????????switch?(event.getAction())?{
????????????????case?MotionEvent.ACTION_UP:
????????????????????......
????????????????????break;

????????????????case?MotionEvent.ACTION_DOWN:
????????????????????mHasPerformedLongPress?=?false;
????????????????????......
????????????????????//?Walk?up?the?hierarchy?to?determine?if?we're?inside?a?scrolling?container.
????????????????????boolean?isInScrollingContainer?=?isInScrollingContainer();//A已經(jīng)是頂層元素了，沒(méi)有ScrollView之類的控件存在，所以，isInScrollingContainer?=?false

????????????????????//?For?views?inside?a?scrolling?container,?delay?the?pressed?feedback?for
????????????????????//?a?short?period?in?case?this?is?a?scroll.
????????????????????if?(isInScrollingContainer)?{
????????????????????????mPrivateFlags?|=?PFLAG_PREPRESSED;
????????????????????????if?(mPendingCheckForTap?==?null)?{
????????????????????????????mPendingCheckForTap?=?new?CheckForTap();
????????????????????????}
????????????????????????postDelayed(mPendingCheckForTap,?ViewConfiguration.getTapTimeout());
????????????????????}?else?{
????????????????????????//?Not?inside?a?scrolling?container,?so?show?the?feedback?right?away
????????????????????????setPressed(true);//A設(shè)置pressed?=?true
????????????????????????checkForLongClick(0);
????????????????????}
????????????????????break;

????????????????case?MotionEvent.ACTION_CANCEL:
????????????????????......
????????????????????break;

????????????????case?MotionEvent.ACTION_MOVE:
????????????????????......
????????????????????break;
????????????}
????????????return?true;
????????}

????????return?false;
????}

???????

??????? 從上面的代碼我們知道，當(dāng)手指觸摸A的時(shí)候，A的pressed被設(shè)置為true。

Pressed狀態(tài)的傳遞

??????? 接著，我們看看setPressed函數(shù)的實(shí)現(xiàn)：

????/**
?????*?Sets?the?pressed?state?for?this?view.
?????*
?????*?@param?pressed?Pass?true?to?set?the?View's?internal?state?to?"pressed",?or?false?to?reverts
?????*????????????????the?View's?internal?state?from?a?previously?set?"pressed"?state.
?????*?@see?#isClickable()
?????*?@see?#setClickable(boolean)
?????*/
????public?void?setPressed(boolean?pressed)?{
????????final?boolean?needsRefresh?=?pressed?!=?((mPrivateFlags?&?PFLAG_PRESSED)?==?PFLAG_PRESSED);

????????if?(pressed)?{
????????????mPrivateFlags?|=?PFLAG_PRESSED;
????????}?else?{
????????????mPrivateFlags?&=?~PFLAG_PRESSED;
????????}

????????if?(needsRefresh)?{
????????????refreshDrawableState();//切換背景圖片
????????}
????????dispatchSetPressed(pressed);
????}

??????? setPressed函數(shù)內(nèi)部調(diào)用了dispatchSetPressed函數(shù)，這個(gè)讓人很在意（frameworks/base/core/java/android/view/ViewGroup.java）：

????@Override
????protected?void?dispatchSetPressed(boolean?pressed)?{
????????final?View[]?children?=?mChildren;
????????final?int?count?=?mChildrenCount;
????????for?(int?i?=?0;?i?<?count;?i++)?{
????????????final?View?child?=?children[i];
????????????//?Children?that?are?clickable?on?their?own?should?not
????????????//?show?a?pressed?state?when?their?parent?view?does.
????????????//?Clearing?a?pressed?state?always?propagates.
????????????if?(!pressed?||?(!child.isClickable()?&&?!child.isLongClickable()))?{
????????????????child.setPressed(pressed);
????????????}
????????}
????}

??????? 原來(lái)，dispatchSetPressed函數(shù)會(huì)把pressed狀態(tài)傳遞給所有clickable=false并且longclickable=false的子元素。

??????? 到這里，前面的現(xiàn)象就可以解釋了，因?yàn)镃是button類，clickable=true，而B(niǎo)的clickable=false。所以，當(dāng)A被觸摸時(shí)，B的pressed=true，而C的pressed=false。那么，可以回答下面幾個(gè)小問(wèn)題了：

如果不希望A的pressed=true時(shí)，B的pressed = true，該如何修改？ 設(shè)置B的clickable=true如果希望A的pressed = true時(shí)，C的pressed = true，那又該如何修改？ 設(shè)置C的clickable = false. 但是，這里可能又存在問(wèn)題了，設(shè)置C的clickable=false，會(huì)導(dǎo)致button按鈕的onclicklistener無(wú)法工作，這是個(gè)嚴(yán)重的副作用。那么可以不修改clickable，而設(shè)置android:duplicateParentState為true。

?????? 那么duplicateParentState做了些什么呢？View.setDuplicateParentStateEnabled:

????public?void?setDuplicateParentStateEnabled(boolean?enabled)?{
????????setFlags(enabled???DUPLICATE_PARENT_STATE?:?0,?DUPLICATE_PARENT_STATE);
????}

????? 再看看View.onCreateDrawableState()

?/**
?????*?Generate?the?new?{@link?android.graphics.drawable.Drawable}?state?for
?????*?this?view.?This?is?called?by?the?view
?????*?system?when?the?cached?Drawable?state?is?determined?to?be?invalid.??To
?????*?retrieve?the?current?state,?you?should?use?{@link?#getDrawableState}.
?????*
?????*?@param?extraSpace?if?non-zero,?this?is?the?number?of?extra?entries?you
?????*???????????????????would?like?in?the?returned?array?in?which?you?can?place?your?own
?????*???????????????????states.
?????*?@return?Returns?an?array?holding?the?current?{@link?Drawable}?state?of
?????*?the?view.
?????*?@see?#mergeDrawableStates(int[],?int[])
?????*/
????protected?int[]?onCreateDrawableState(int?extraSpace)?{
????????if?((mViewFlags?&?DUPLICATE_PARENT_STATE)?==?DUPLICATE_PARENT_STATE?&&
????????????????mParent?instanceof?View)?{
????????????return?((View)?mParent).onCreateDrawableState(extraSpace);
????????}

????????......
????}

??????? 從上面的代碼，可以看到，當(dāng)設(shè)置duplicateParentState為true時(shí)，View的DrawableState直接使用了其parent的。所以，他的drawable狀態(tài)會(huì)一直保持與其parent同步。

題外，為什么當(dāng)ListView中包含focusable為true的控件時(shí)，OnItemClickListener不會(huì)觸發(fā)

??????? 因?yàn)長(zhǎng)istView未重載onTouchEvent事件，所以，需要看其父類的AbsListView.onTouchEvent(frameworks/base/core/java/android/widget/AbsListView):

????@Override
????public?boolean?onTouchEvent(MotionEvent?ev)?{
????????......
????????switch?(action?&?MotionEvent.ACTION_MASK)?{
????????case?MotionEvent.ACTION_DOWN:?{
????????????......
????????????break;
????????}

????????case?MotionEvent.ACTION_MOVE:?{
????????????......
????????????break;
????????}

????????case?MotionEvent.ACTION_UP:?{
????????????switch?(mTouchMode)?{
????????????case?TOUCH_MODE_DOWN:
????????????case?TOUCH_MODE_TAP:
????????????case?TOUCH_MODE_DONE_WAITING:
????????????????final?int?motionPosition?=?mMotionPosition;
????????????????final?View?child?=?getChildAt(motionPosition?-?mFirstPosition);

????????????????final?float?x?=?ev.getX();
????????????????final?boolean?inList?=?x?>?mListPadding.left?&&?x?<?getWidth()?-?mListPadding.right;

????????????????if?(child?!=?null?&&?!child.hasFocusable()?&&?inList)?{
????????????????????if?(mTouchMode?!=?TOUCH_MODE_DOWN)?{
????????????????????????child.setPressed(false);
????????????????????}

????????????????????if?(mPerformClick?==?null)?{
????????????????????????mPerformClick?=?new?PerformClick();
????????????????????}

????????????????????......
????????????????????performClick.run();
????????????????????......
????????????????}
????????????????......
????????????case?TOUCH_MODE_SCROLL:
????????????????......
????????????????break;

????????????case?TOUCH_MODE_OVERSCROLL:
????????????......
????????????break;
????????}
????????......
????????case?MotionEvent.ACTION_CANCEL:?{
????????????......
????????????break;
????????}

????????case?MotionEvent.ACTION_POINTER_UP:?{
????????????......
????????????break;
????????}

????????case?MotionEvent.ACTION_POINTER_DOWN:?{
????????????......
????????????break;
????????}
????????}

????????return?true;
????}

???????? 僅在child.hasFocusable()=false時(shí)， PerformClick對(duì)象才會(huì)執(zhí)行ViewGroup.hasFocusable:

????/**
?????*?{@inheritDoc}
?????*/
????@Override
????public?boolean?hasFocusable()?{
????????if?((mViewFlags?&?VISIBILITY_MASK)?!=?VISIBLE)?{
????????????return?false;
????????}

????????if?(isFocusable())?{
????????????return?true;
????????}

????????final?int?descendantFocusability?=?getDescendantFocusability();
????????if?(descendantFocusability?!=?FOCUS_BLOCK_DESCENDANTS)?{
????????????final?int?count?=?mChildrenCount;
????????????final?View[]?children?=?mChildren;

????????????for?(int?i?=?0;?i?<?count;?i++)?{
????????????????final?View?child?=?children[i];
????????????????if?(child.hasFocusable())?{
????????????????????return?true;
????????????????}
????????????}
????????}

????????return?false;
????}

??????? 僅在所有的clild的hasFocusable為false時(shí)，ListView才會(huì)執(zhí)行performClick（AbsListView.PerformClick）：

????private?class?PerformClick?extends?WindowRunnnable?implements?Runnable?{
????????int?mClickMotionPosition;

????????public?void?run()?{
????????????//?The?data?has?changed?since?we?posted?this?action?in?the?event?queue,
????????????//?bail?out?before?bad?things?happen
????????????if?(mDataChanged)?return;

????????????final?ListAdapter?adapter?=?mAdapter;
????????????final?int?motionPosition?=?mClickMotionPosition;
????????????if?(adapter?!=?null?&&?mItemCount?>?0?&&
????????????????????motionPosition?!=?INVALID_POSITION?&&
????????????????????motionPosition?<?adapter.getCount()?&&?sameWindow())?{
????????????????final?View?view?=?getChildAt(motionPosition?-?mFirstPosition);
????????????????//?If?there?is?no?view,?something?bad?happened?(the?view?scrolled?off?the
????????????????//?screen,?etc.)?and?we?should?cancel?the?click
????????????????if?(view?!=?null)?{
????????????????????performItemClick(view,?motionPosition,?adapter.getItemId(motionPosition));
????????????????}
????????????}
????????}
????}

??????? 而PerformClick會(huì)調(diào)用performItemClick(AdsListView.performItemClick)：

?@Override
????public?boolean?performItemClick(View?view,?int?position,?long?id)?{
????????boolean?handled?=?false;
????????boolean?dispatchItemClick?=?true;

????????......

????????if?(dispatchItemClick)?{
????????????handled?|=?super.performItemClick(view,?position,?id);
????????}

????????return?handled;
????}

?????? AdapterView.preformItemClick:

????/**
?????*?Call?the?OnItemClickListener,?if?it?is?defined.
?????*
?????*?@param?view?The?view?within?the?AdapterView?that?was?clicked.
?????*?@param?position?The?position?of?the?view?in?the?adapter.
?????*?@param?id?The?row?id?of?the?item?that?was?clicked.
?????*?@return?True?if?there?was?an?assigned?OnItemClickListener?that?was
?????*?????????called,?false?otherwise?is?returned.
?????*/
????public?boolean?performItemClick(View?view,?int?position,?long?id)?{
????????if?(mOnItemClickListener?!=?null)?{
????????????playSoundEffect(SoundEffectConstants.CLICK);
????????????if?(view?!=?null)?{
????????????????view.sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_CLICKED);
????????????}
????????????mOnItemClickListener.onItemClick(this,?view,?position,?id);
????????????return?true;
????????}

????????return?false;
????}

??????? 所以，如果ListView item中包含focusable為true的控件（例如：button， radiobutton），會(huì)導(dǎo)致ItemClickListener失效。解決方案兩個(gè)：

設(shè)置特定的控件focusable = false

不使用onItemClickListener，而直接在Item上設(shè)置onClickListener監(jiān)聽(tīng)點(diǎn)擊事件。