??? 做Android開發的人都用過Selector,可以方便的實現View在不同狀態下的背景。不過，相信大部分開發者遇到過和我一樣的問題，本文會從源碼角度，解釋這些問題。
??????? 首先，這里簡單描述一下，我遇到的問題：
界面上有個全屏的LinearLayout A，A中有一個TextView B和Button C，其中，A的clickable=true，并設置了pressed時，背景色為灰色，B設置了pressed時，背景色為藍色
當手指觸摸C下方的空白區域時，看到了這樣的效果：
??????? 在這里看到，在沒有觸摸B的情況下，B的pressed = true，而C的pressed = false。 C的狀態暫且不討論，按照Android消息傳遞的原則，因為touch的point不在B內部，所以，touch消息應該不會交給B處理，那為什么B的pressed = true？
???????? 下面開始一步一步分析（本文分析的Android源碼為4.2.2）。
Pressed狀態的設定
??????? 從View.onTouchEvent函數看起（frameworks/base/core/java/android/view/View.java):
[java] view plain
copy public?boolean?onTouchEvent(MotionEvent?event)?{??????......??????if?(((viewFlags?&?CLICKABLE)?==?CLICKABLE?||?????????????(viewFlags?&?LONG_CLICKABLE)?==?LONG_CLICKABLE))?{??????????switch?(event.getAction())?{??????????????case?MotionEvent.ACTION_UP:??????????????????......??????????????????break;????????????????case?MotionEvent.ACTION_DOWN:??????????????????mHasPerformedLongPress?=?false;??????????????????......??????????????????????????????????boolean?isInScrollingContainer?=?isInScrollingContainer();??????????????????????????????????????????????????if?(isInScrollingContainer)?{??????????????????????mPrivateFlags?|=?PFLAG_PREPRESSED;??????????????????????if?(mPendingCheckForTap?==?null)?{??????????????????????????mPendingCheckForTap?=?new?CheckForTap();??????????????????????}??????????????????????postDelayed(mPendingCheckForTap,?ViewConfiguration.getTapTimeout());??????????????????}?else?{??????????????????????????????????????????setPressed(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，當手指觸摸A的時候，A的pressed被設置為true。
Pressed狀態的傳遞
??????? 接著，我們看看setPressed函數的實現：
[java] view plain
copy 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();????}??????dispatchSetPressed(pressed);??}?? ??????? setPressed函數內部調用了dispatchSetPressed函數，這個讓人很在意（frameworks/base/core/java/android/view/ViewGroup.java）：
[java] view plain
copy @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];??????????????????????????????????if?(!pressed?||?(!child.isClickable()?&&?!child.isLongClickable()))?{??????????????child.setPressed(pressed);??????????}??????}??}?? ??????? 原來，dispatchSetPressed函數會把pressed狀態傳遞給所有clickable=false并且longclickable=false的子元素。
??????? 到這里，前面的現象就可以解釋了，因為C是button類，clickable=true，而B的clickable=false。所以，當A被觸摸時，B的pressed=true，而C的pressed=false。那么，可以回答下面幾個小問題了：
如果不希望A的pressed=true時，B的pressed = true，該如何修改？設置B的clickable=true如果希望A的pressed = true時，C的pressed = true，那又該如何修改？設置C的clickable = false. 但是，這里可能又存在問題了，設置C的clickable=false，會導致button按鈕的onclicklistener無法工作，這是個嚴重的副作用。那么可以不修改clickable，而設置android:duplicateParentState為true。
?????? 那么duplicateParentState做了些什么呢？View.setDuplicateParentStateEnabled:
[java] view plain
copy public?void?setDuplicateParentStateEnabled(boolean?enabled)?{??????setFlags(enabled???DUPLICATE_PARENT_STATE?:?0,?DUPLICATE_PARENT_STATE);??}?? ????? 再看看View.onCreateDrawableState()
[java] view plain
copy ???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，當設置duplicateParentState為true時，View的DrawableState直接使用了其parent的。所以，他的drawable狀態會一直保持與其parent同步。
題外，為什么當ListView中包含focusable為true的控件時，OnItemClickListener不會觸發
??????? 因為ListView未重載onTouchEvent事件，所以，需要看其父類的AbsListView.onTouchEvent(frameworks/base/core/java/android/widget/AbsListView):
[java] view plain
copy @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， PerformClick對象才會執行ViewGroup.hasFocusable:
[java] view plain
copy @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，ListView才會執行performClick（AbsListView.PerformClick）：
[java] view plain
copy private?class?PerformClick?extends?WindowRunnnable?implements?Runnable?{??????int?mClickMotionPosition;????????public?void?run()?{??????????????????????????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?(view?!=?null)?{??????????????????performItemClick(view,?motionPosition,?adapter.getItemId(motionPosition));??????????????}??????????}??????}??}?? ??????? 而PerformClick會調用performItemClick(AdsListView.performItemClick)：
[java] view plain
copy @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:
[java] view plain
copy 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），會導致ItemClickListener失效。解決方案兩個：
設置特定的控件focusable = false
不使用onItemClickListener，而直接在Item上設置onClickListener監聽點擊事件。
總結
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