詳細分析Android中onTouch事件傳遞機制
onTach介紹
ontach是Android系統(tǒng)中整個事件機制的基礎(chǔ)。Android中的其他事件,如onClick、onLongClick等都是以onTach為基礎(chǔ)的。
onTach包括從手指按下到離開手機屏幕的整個過程,在微觀形式上,具體表現(xiàn)為action_down、action_move和action_up等過程。
onTach兩種主要定義形式如下:
1.在自定義控件中,常見的有重寫onTouchEvent(MotionEvent ev)方法。如在開發(fā)中經(jīng)??梢钥吹街貙懙膐nTouchEvent方法,
并且其中有針對不同的微觀表現(xiàn)(action_down、action_move和action_up等)做出的相應(yīng)判斷,執(zhí)行邏輯并可能返回不同的布爾值。
2.在代碼中,直接對現(xiàn)有控件設(shè)置setOnTouchListener監(jiān)聽器。并重寫監(jiān)聽器的onTouch方法。onTouch回調(diào)函數(shù)中有view和MotionEvent
onTouch事件傳遞機制
大家都知道一般我們使用的UI控件都是繼承自共同的父類——View。所以View這個類應(yīng)該掌管著onTouch事件的相關(guān)處理。那就讓我們?nèi)タ纯矗涸赩iew中尋找Touch相關(guān)的方法,其中一個很容易地引起了我們的注意: dispatchTouchEvent(MotionEvent event) 。
根據(jù)方法名的意思應(yīng)該是負責(zé)分發(fā)觸摸事件的,下面給出了源碼:
/**
* 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 the event should be handled by accessibility focus first.
if (event.isTargetAccessibilityFocus()) {
// We don't have focus or no virtual descendant has it, do not handle the event.
if (!isAccessibilityFocusedViewOrHost()) {
return false;
}
// We have focus and got the event, then use normal event dispatch.
event.setTargetAccessibilityFocus(false);
}
boolean result = false;
if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onTouchEvent(event, 0);
}
final int actionMasked = event.getActionMasked();
if (actionMasked == MotionEvent.ACTION_DOWN) {
// Defensive cleanup for new gesture
stopNestedScroll();
}
if (onFilterTouchEventForSecurity(event)) {
//noinspection SimplifiableIfStatement
ListenerInfo li = mListenerInfo;
if (li != null && li.mOnTouchListener != null
&& (mViewFlags & ENABLED_MASK) == ENABLED
&& li.mOnTouchListener.onTouch(this, event)) {
result = true;
}
if (!result && onTouchEvent(event)) {
result = true;
}
}
if (!result && mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onUnhandledEvent(event, 0);
}
// Clean up after nested scrolls if this is the end of a gesture;
// also cancel it if we tried an ACTION_DOWN but we didn't want the rest
// of the gesture.
if (actionMasked == MotionEvent.ACTION_UP ||
actionMasked == MotionEvent.ACTION_CANCEL ||
(actionMasked == MotionEvent.ACTION_DOWN && !result)) {
stopNestedScroll();
}
return result;
}
源碼有點長,但我們不必每一行都看。首先注意到dispatchTouchEvent的返回值是boolean類型的,注釋上的解釋:@return True if the event was handled by the view, false otherwise.也就是說如果該觸摸事件被這個View消費了就返回true,否則返回false。在方法中首先判斷了該event是否是否得到了焦點,如果沒有得到焦點直接返回false。然后讓我們把目光轉(zhuǎn)向if (li != null && li.mOnTouchListener != null&& (mViewFlags & ENABLED_MASK) == ENABLED&& li.mOnTouchListener.onTouch(this, event))這個片段,看到這里有一個名為li的局部變量,屬于 ListenerInfo 類,經(jīng) mListenerInfo 賦值得到。ListenerInfo只是一個包裝類,里面封裝了大量的監(jiān)聽器。
再在 View 類中去尋找 mListenerInfo ,可以看到下面的代碼:
ListenerInfo getListenerInfo() {
if (mListenerInfo != null) {
return mListenerInfo;
}
mListenerInfo = new ListenerInfo();
return mListenerInfo;
}
因此我們可以知道m(xù)ListenerInfo是不為空的,所以li也不是空,第一個判斷為true,然后看到li.mOnTouchListener,前面說過ListenerInfo是一個監(jiān)聽器的封裝類,所以我們同樣去追蹤mOnTouchListener:
/**
* Register a callback to be invoked when a touch event is sent to this view.
* @param l the touch listener to attach to this view
*/
public void setOnTouchListener(OnTouchListener l) {
getListenerInfo().mOnTouchListener = l;
}
正是通過上面的方法來設(shè)置 mOnTouchListener 的,我想上面的方法大家肯定都很熟悉吧,正是我們平時經(jīng)常用的 xxx.setOnTouchListener ,好了我們從中得知如果設(shè)置了OnTouchListener則第二個判斷也為true,第三個判斷為如果該View是否為enable,默認都是enable的,所以同樣為true。還剩最后一個:li.mOnTouchListener.onTouch(this, event) ,顯然是回調(diào)了第二個判斷中監(jiān)聽器的onTouch()方法,如果onTouch()方法返回true,則上面四個判斷全部為true,dispatchTouchEvent()方法會返回true,并且不會執(zhí)行if (!result && onTouchEvent(event))這個判斷;而在這個判斷中我們又看到了一個熟悉的方法:onTouchEvent() 。所以想要執(zhí)行onTouchEvent,則在上面的四個判斷中必須至少有一個false。
那就假定我們在onTouch()方法中返回的是false,這樣就順利地執(zhí)行了onTouchEvent,那就看看onTouchEvent的源碼吧:
/**
* Implement this method to handle touch screen motion events.
* <p>
* If this method is used to detect click actions, it is recommended that
* the actions be performed by implementing and calling
* {@link #performClick()}. This will ensure consistent system behavior,
* including:
* <ul>
* <li>obeying click sound preferences
* <li>dispatching OnClickListener calls
* <li>handling {@link AccessibilityNodeInfo#ACTION_CLICK ACTION_CLICK} when
* accessibility features are enabled
* </ul>
*
* @param event The motion event.
* @return True if the event was handled, false otherwise.
*/
public boolean onTouchEvent(MotionEvent event) {
final float x = event.getX();
final float y = event.getY();
final int viewFlags = mViewFlags;
final int action = event.getAction();
if ((viewFlags & ENABLED_MASK) == DISABLED) {
if (action == MotionEvent.ACTION_UP && (mPrivateFlags & PFLAG_PRESSED) != 0) {
setPressed(false);
}
// A disabled view that is clickable still consumes the touch
// events, it just doesn't respond to them.
return (((viewFlags & CLICKABLE) == CLICKABLE
|| (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE)
|| (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE);
}
if (mTouchDelegate != null) {
if (mTouchDelegate.onTouchEvent(event)) {
return true;
}
}
if (((viewFlags & CLICKABLE) == CLICKABLE ||
(viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE) ||
(viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE) {
switch (action) {
case MotionEvent.ACTION_UP:
boolean prepressed = (mPrivateFlags & PFLAG_PREPRESSED) != 0;
if ((mPrivateFlags & PFLAG_PRESSED) != 0 || prepressed) {
// take focus if we don't have it already and we should in
// touch mode.
boolean focusTaken = false;
if (isFocusable() && isFocusableInTouchMode() && !isFocused()) {
focusTaken = requestFocus();
}
if (prepressed) {
// The button is being released before we actually
// showed it as pressed. Make it show the pressed
// state now (before scheduling the click) to ensure
// the user sees it.
setPressed(true, x, y);
}
if (!mHasPerformedLongPress && !mIgnoreNextUpEvent) {
// This is a tap, so remove the longpress check
removeLongPressCallback();
// Only perform take click actions if we were in the pressed state
if (!focusTaken) {
// Use a Runnable and post this rather than calling
// performClick directly. This lets other visual state
// of the view update before click actions start.
if (mPerformClick == null) {
mPerformClick = new PerformClick();
}
if (!post(mPerformClick)) {
performClick();
}
}
}
if (mUnsetPressedState == null) {
mUnsetPressedState = new UnsetPressedState();
}
if (prepressed) {
postDelayed(mUnsetPressedState,
ViewConfiguration.getPressedStateDuration());
} else if (!post(mUnsetPressedState)) {
// If the post failed, unpress right now
mUnsetPressedState.run();
}
removeTapCallback();
}
mIgnoreNextUpEvent = false;
break;
case MotionEvent.ACTION_DOWN:
mHasPerformedLongPress = false;
if (performButtonActionOnTouchDown(event)) {
break;
}
// Walk up the hierarchy to determine if we're inside a scrolling container.
boolean isInScrollingContainer = isInScrollingContainer();
// 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();
}
mPendingCheckForTap.x = event.getX();
mPendingCheckForTap.y = event.getY();
postDelayed(mPendingCheckForTap, ViewConfiguration.getTapTimeout());
} else {
// Not inside a scrolling container, so show the feedback right away
setPressed(true, x, y);
checkForLongClick(0);
}
break;
case MotionEvent.ACTION_CANCEL:
setPressed(false);
removeTapCallback();
removeLongPressCallback();
mInContextButtonPress = false;
mHasPerformedLongPress = false;
mIgnoreNextUpEvent = false;
break;
case MotionEvent.ACTION_MOVE:
drawableHotspotChanged(x, y);
// Be lenient about moving outside of buttons
if (!pointInView(x, y, mTouchSlop)) {
// Outside button
removeTapCallback();
if ((mPrivateFlags & PFLAG_PRESSED) != 0) {
// Remove any future long press/tap checks
removeLongPressCallback();
setPressed(false);
}
}
break;
}
return true;
}
return false;
}
這段源碼比 dispatchTouchEvent 的還要長,不過同樣我們挑重點的看:
if (((viewFlags & CLICKABLE) == CLICKABLE || (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE) || (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE)看到這句話就大概知道了主要是判斷該view是否是可點擊的,如果可以點擊則接著執(zhí)行,否則直接返回false。可以看到if里面用switch來判斷是哪種觸摸事件,但在最后都是返回true的。
還有一點要注意:在 ACTION_UP 中會執(zhí)行 performClick() 方法:
public boolean performClick() {
final boolean result;
final ListenerInfo li = mListenerInfo;
if (li != null && li.mOnClickListener != null) {
playSoundEffect(SoundEffectConstants.CLICK);
li.mOnClickListener.onClick(this);
result = true;
} else {
result = false;
}
sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_CLICKED);
return result;
}
可以看到上面的li.mOnClickListener.onClick(this); ,沒錯,我們好像又有了新的發(fā)現(xiàn)。根據(jù)上面的經(jīng)驗,這句代碼會去回調(diào)我們設(shè)置好的點擊事件監(jiān)聽器。也就是我們平常用的xxx.setOnClickListener(listener);
/**
* Register a callback to be invoked when this view is clicked. If this view is not
* clickable, it becomes clickable.
*
* @param l The callback that will run
*
* @see #setClickable(boolean)
*/
public void setOnClickListener(@Nullable OnClickListener l) {
if (!isClickable()) {
setClickable(true);
}
getListenerInfo().mOnClickListener = l;
}
我們可以看到上面方法設(shè)置正是mListenerInfo的點擊監(jiān)聽器,驗證了上面的猜想。到了這里onTouch事件的傳遞機制基本已經(jīng)分析完成了,也算是告一段落了。
好了,這下我們可以解決開頭的問題了,順便我們再來小結(jié)一下:在dispatchTouchEvent中,如果設(shè)置了OnTouchListener并且View是enable的,那么首先被執(zhí)行的是OnTouchListener中的onTouch(View v, MotionEvent event) 。若onTouch返回true,則dispatchTouchEvent不再往下執(zhí)行并且返回true;不然會執(zhí)行onTouchEvent,在onTouchEvent中若View是可點擊的,則返回true,不然為false。還有在onTouchEvent中若View是可點擊以及當前觸摸事件為ACTION_UP,會執(zhí)行performClick() ,回調(diào)OnClickListener的onClick方法。
下面是我畫的一張草圖:

還有一點值得注意的地方是:假如當前事件是ACTION_DOWN,只有dispatchTouchEvent返回true了之后該View才會接收到接下來的ACTION_MOVE,ACTION_UP事件,也就是說只有事件被消費了才能接收接下來的事件。
總結(jié)
以上就是關(guān)于Android中onTouch事件傳遞機制的詳細分析,希望對各位Android開發(fā)者們的學(xué)習(xí)或者工作能有一定的幫助,如果有疑問大家可以留言交流。
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