Pressed狀態和clickable,duplicateParentState的關系

??? 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):

/**?
?*?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?||?//這里是為什么設置A的clickable為true的原因，否則，press?A的時候，沒有界面元素處理touch?event，最終會由Activity的onTouchEvent函數處理??
????????????(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已經是頂層元素了，沒有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：

/**?
?*?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函數內部調用了dispatchSetPressed函數，這個讓人很在意（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，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：

1. 如果不希望A的pressed=true時，B的pressed = true，該如何修改？
   1. 設置B的clickable=true
2. 如果希望A的pressed = true時，C的pressed = true，那又該如何修改？
   1. 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:

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):

@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:

/**?
?*?{@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，ListView才會執行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會調用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。