Android的Touch系統簡介(一)
一、Android touch事件的相關概念
使用者的Touch事件被包裝成MotionEvent
使用者當前的touch事件主要型別有:
ACTION_DOWN: 表示使用者開始觸控.
ACTION_MOVE: 表示使用者在移動(手指或者其他)
ACTION_UP:表示使用者擡起了手指
ACTION_OUTSIDE: 表示使用者觸碰超出了正常的UI邊界.
ACTION_POINTER_DOWN:有一個非主要的手指按下了.
ACTION_POINTER_UP:一個非主要的手指擡起來了
touch事件的元資料包括:touch的位置
手指的個數
touch事件的時間
一個touch手勢被定義為以ACTION_DOWN開始和以 ACTION_UP結束。
二、Touch事件的處理流程
當用戶觸控式螢幕幕時,觸發Activity呼叫dispatchTouchEvent
事件物件會按自頂向下的順序在View
Tree中傳遞
父View(ViewGroups)會呼叫dispatchTouchEvent將Event傳遞給子View
Event在任何時候都可能被攔截
事件流會順著View鏈遞歸向下傳遞直到被消耗若某個View想處理touch事件,必須先消耗ACTION_DOWN。考慮到效率,後續的事件將不會向下傳遞。
若某個事件未被消耗,最後會被Activity的onTouchEvent()消耗
若任何View或ViewGroup設定了OnTouchListener,touch事件將被攔截。
/** * Called to process touch screen events. You can override this to * intercept all touch screen events before they are dispatched to the * window. Be sure to call this implementation for touch screen events * that should be handled normally. * * @param ev The touch screen event. * * @return boolean Return true if this event was consumed. */ public boolean dispatchTouchEvent(MotionEvent ev) { if (ev.getAction() == MotionEvent.ACTION_DOWN) { onUserInteraction(); } if (getWindow().superDispatchTouchEvent(ev)) { return true; } return onTouchEvent(ev); }
由程式碼可以看出,對於應用層,該函式在touch事件發生後首先被呼叫。onUserInteraction()是一個空函式,可被使用者過載以進行相關處理。Event隨後將被傳遞到關聯到root view的window。若子view消耗了該Event,則返回true,否則Event最後被Activity的onTouchEvent()消耗。
ViewGroup.dispatchTouchEvent()的原始碼分析如下:
public boolean dispatchTouchEvent(MotionEvent ev) {
if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onTouchEvent(ev, 1);
}
boolean handled = false;
if (onFilterTouchEventForSecurity(ev)) {
final int action = ev.getAction();
final int actionMasked = action & MotionEvent.ACTION_MASK;
// 處理初始的down事件
if (actionMasked == MotionEvent.ACTION_DOWN) {
//當新開始一個touch事件時,拋棄先前的touch狀態
//當app切換,發生ANR或一些其他的touch狀態發生時,framework會丟棄或取消先前的touch狀態
cancelAndClearTouchTargets(ev);
resetTouchState();
}
// 檢查是否進行事件攔截
final boolean intercepted;
if (actionMasked == MotionEvent.ACTION_DOWN
|| mFirstTouchTarget != null) {
final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
if (!disallowIntercept) {
//回撥onInterceptTouchEvent(),返回false表示不攔截touch,否則攔截touch事件。
intercepted = onInterceptTouchEvent(ev);
ev.setAction(action); // restore action in case it was changed
} else {
intercepted = false;
}
} else {
//沒有touch事件的傳遞物件,同時touch動作不是初始動作down,所以ViewGroup繼續攔截事件
intercepted = true;
}
// 檢查cancel事件
final boolean canceled = resetCancelNextUpFlag(this)
|| actionMasked == MotionEvent.ACTION_CANCEL;
// 如果有第二個手指touch,更新touch目標列表。touch目標列表是一個View陣列
final boolean split = (mGroupFlags & FLAG_SPLIT_MOTION_EVENTS) != 0;
TouchTarget newTouchTarget = null;
boolean alreadyDispatchedToNewTouchTarget = false;
if (!canceled && !intercepted) {
if (actionMasked == MotionEvent.ACTION_DOWN
|| (split && actionMasked == MotionEvent.ACTION_POINTER_DOWN)
|| actionMasked == MotionEvent.ACTION_HOVER_MOVE) {
final int actionIndex = ev.getActionIndex(); // always 0 for down
final int idBitsToAssign = split ? 1 << ev.getPointerId(actionIndex)
: TouchTarget.ALL_POINTER_IDS;
// Clean up earlier touch targets for this pointer id in case they
// have become out of sync.
removePointersFromTouchTargets(idBitsToAssign);
final int childrenCount = mChildrenCount;
if (newTouchTarget == null && childrenCount != 0) {
final float x = ev.getX(actionIndex);
final float y = ev.getY(actionIndex);
// 找到一個能接受Event的子View,再對子View的View樹進行遍歷
final View[] children = mChildren;
final boolean customOrder = isChildrenDrawingOrderEnabled();
//判斷每個子View是否是TouchTarget,若是則新增到TouchTarget連結串列中
for (int i = childrenCount - 1; i >= 0; i--) {
final int childIndex = customOrder ?
getChildDrawingOrder(childrenCount, i) : i;
final View child = children[childIndex];
if (!canViewReceivePointerEvents(child)
|| !isTransformedTouchPointInView(x, y, child, null)) {
continue;
}
newTouchTarget = getTouchTarget(child);
if (newTouchTarget != null) {
// 若子View處於touch目標中,同時已經接收了touch事件,則為器增加新的touch點
newTouchTarget.pointerIdBits |= idBitsToAssign;
break;
}
resetCancelNextUpFlag(child);
//把MotionEvent的點座標轉換到子View的座標系中,為ViewGroup建立一個新TouchTarget,TouchTarget包含了子View
if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {
// Child wants to receive touch within its bounds.
mLastTouchDownTime = ev.getDownTime();
mLastTouchDownIndex = childIndex;
mLastTouchDownX = ev.getX();
mLastTouchDownY = ev.getY();
newTouchTarget = addTouchTarget(child, idBitsToAssign);
alreadyDispatchedToNewTouchTarget = true;
break;
}
}
}
if (newTouchTarget == null && mFirstTouchTarget != null) {
// 沒有發現接收event的子View,把Touch點賦給最早新增到TouchTarget鏈中的物件
newTouchTarget = mFirstTouchTarget;
while (newTouchTarget.next != null) {
newTouchTarget = newTouchTarget.next;
}
newTouchTarget.pointerIdBits |= idBitsToAssign;
}
}
}
// 傳遞給touch目標
if (mFirstTouchTarget == null) {
// 若沒有Touch目標,則把自己當成一個View,呼叫
handled = dispatchTransformedTouchEvent(ev, canceled, null,
TouchTarget.ALL_POINTER_IDS);
} else {
// Dispatch to touch targets, excluding the new touch target if we already
// dispatched to it. Cancel touch targets if necessary.
TouchTarget predecessor = null;
TouchTarget target = mFirstTouchTarget;
while (target != null) {
final TouchTarget next = target.next;
//若已被處理,則忽略。
if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) {
handled = true;
} else {
final boolean cancelChild = resetCancelNextUpFlag(target.child)
|| intercepted;
//傳遞給子View處理
if (dispatchTransformedTouchEvent(ev, cancelChild,
target.child, target.pointerIdBits)) {
handled = true;
}
if (cancelChild) {
if (predecessor == null) {
mFirstTouchTarget = next;
} else {
predecessor.next = next;
}
target.recycle();
target = next;
continue;
}
}
predecessor = target;
target = next;
}
}
// 若在觸控點發生了up或cancel,則更新TouchTarget連結串列
if (canceled
|| actionMasked == MotionEvent.ACTION_UP
|| actionMasked == MotionEvent.ACTION_HOVER_MOVE) {
resetTouchState();
} else if (split && actionMasked == MotionEvent.ACTION_POINTER_UP) {
final int actionIndex = ev.getActionIndex();
final int idBitsToRemove = 1 << ev.getPointerId(actionIndex);
removePointersFromTouchTargets(idBitsToRemove);
}
}
if (!handled && mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onUnhandledEvent(ev, 1);
}
return handled;
}
ViewGroup中將TouchEvent傳遞給子View的函式為dispatchTransformedTouchEvent(),原始碼如下:
/**
* Transforms a motion event into the coordinate space of a particular child view,
* filters out irrelevant pointer ids, and overrides its action if necessary.
* If child is null, assumes the MotionEvent will be sent to this ViewGroup instead.
*/
private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,
View child, int desiredPointerIdBits) {
final boolean handled;
// Canceling motions is a special case. We don't need to perform any transformations
// or filtering. The important part is the action, not the contents.
// cancel動作是個特列,無需座標轉換或過濾。
final int oldAction = event.getAction();
if (cancel || oldAction == MotionEvent.ACTION_CANCEL) {
event.setAction(MotionEvent.ACTION_CANCEL);
if (child == null) {
handled = super.dispatchTouchEvent(event);
} else {
handled = child.dispatchTouchEvent(event);
}
event.setAction(oldAction);
return handled;
}
// 計算將被傳遞的點的數量。
final int oldPointerIdBits = event.getPointerIdBits();
final int newPointerIdBits = oldPointerIdBits & desiredPointerIdBits;
// Motion事件沒有對應點,則丟棄這個Motion
if (newPointerIdBits == 0) {
return false;
}
/*若點的數量一致則無需進行不相關的點座標轉換,呼叫子View的dispatchTouchEvent*/
// If the number of pointers is the same and we don't need to perform any fancy
// irreversible transformations, then we can reuse the motion event for this
// dispatch as long as we are careful to revert any changes we make.
// Otherwise we need to make a copy.
/*該變數用於儲存座標轉換後的MoetionEvent*/
final MotionEvent transformedEvent;
if (newPointerIdBits == oldPointerIdBits) {
if (child == null || child.hasIdentityMatrix()) {
if (child == null) {
handled = super.dispatchTouchEvent(event);
} else {
final float offsetX = mScrollX - child.mLeft;
final float offsetY = mScrollY - child.mTop;
/*直接對MotionEvent進行座標變換,將MotionEvent傳遞下去*/
event.offsetLocation(offsetX, offsetY);
handled = child.dispatchTouchEvent(event);
/*回覆MotionEvent*/
event.offsetLocation(-offsetX, -offsetY);
}
return handled;
}
transformedEvent = MotionEvent.obtain(event);
} else {
transformedEvent = event.split(newPointerIdBits);
}
// Perform any necessary transformations and dispatch.
if (child == null) {
/*呼叫父類即View的dispatchTouchEvent方法,該方法會呼叫onTouchEvent*/
handled = super.dispatchTouchEvent(transformedEvent);
} else {
final float offsetX = mScrollX - child.mLeft;
final float offsetY = mScrollY - child.mTop;
transformedEvent.offsetLocation(offsetX, offsetY);
if (! child.hasIdentityMatrix()) {
transformedEvent.transform(child.getInverseMatrix());
}
/*傳遞給子View處理*/
handled = child.dispatchTouchEvent(transformedEvent);
}
// Done.
transformedEvent.recycle();
return handled;
}
View物件的dispatchTouchEvent程式碼如下:
/**
* Pass the touch screen motion event down to the target view, or this
* view if it is the target.
*
* @param event The motion event to be dispatched.
* @return True if the event was handled by the view, false otherwise.
*/
public boolean dispatchTouchEvent(MotionEvent event) {
if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onTouchEvent(event, 0);
}
if (onFilterTouchEventForSecurity(event)) {
//noinspection SimplifiableIfStatement
ListenerInfo li = mListenerInfo;
/*先呼叫listener介面*/
if (li != null && li.mOnTouchListener != null && (mViewFlags & ENABLED_MASK) == ENABLED
&& li.mOnTouchListener.onTouch(this, event)) {
return true;
}
/*若MotionEvent未被消耗,則呼叫View的onTouchEvent *
* ViewGroup中沒有定義onTouchEvent,故做後呼叫View中的onTouchEvent*/
if (onTouchEvent(event)) {
return true;
}
}
if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onUnhandledEvent(event, 0);
}
return false;
}
小結:
onInterceptTouchEvent:
onInterceptTouchEvent是在ViewGroup裡面定義的,被ViewGroup.dispatchTouchEvent()呼叫,用於攔截所有的touch事件。預設返回false,表示不攔截touch事件,ViewGroup.dispatchTouchEvent()會呼叫子View的dispatchTouchEvent,將touch事件傳遞到子View中。若子View的dispatchTouchEvent 返回false,則ViewGroup的onTouchEvent會被呼叫;若子View的dispatchTouchEvent 返回true,表示消耗了手勢事件,ViewGroup的onTouchEvent則不會被呼叫。若ViewGroup.onInterceptTouchEvent()返回true,表示Touch事件被攔截,ViewGroup. dispatchTransformedTouchEvent()函式將被呼叫,該函式會呼叫super.dispatchTouchEvent(event),即View的dispatchEvent(),該函式首先會呼叫View.OnTouchListener.onTouch().若listener未消耗Touch事件,則會呼叫View.onTouchEvent().
onTouchEvent:
view中定義的方法onTouchEvent預設返回true,表示消耗了一個touch事件,ViewGroup中定義的onTouchEvent預設返回false,表示不處理Touch手勢事件。手勢事件型別包括ACTION_DOWN,ACTION_MOVE,ACTION_UP,ACTION_CANCEL等事件。