編輯:關於Android編程
之前自認為對於Android的事件分發機制還算比較了解,直到前一陣偶然跟人探討該問題,才發現自己以前的理解有誤,慚愧之余遂決定研習源碼,徹底弄明白Android的事件分發機制,好了廢話少說,直接開干。
首先,我們對Android中的touch事件做一下總結,主要分為以下幾類:
1、Action_Down 用戶手指觸碰到屏幕的那一刻,會觸發該事件;
2、Action_Move 在觸碰到屏幕之後,手指開始在屏幕上滑動,會觸發Action_Move事件;
3、Action_Up 在用戶手指從屏幕上離開那一刻開始,會觸發Action_Up事件;
4、Action_Cancel Cancel事件一般跟Up事件的處理是一樣的,是由系統代碼自己去觸發,比如子view的事件被父view給攔截了,之前被分發的子view就會被發送cancel事件,或者用戶手指在滑動過程中移出了邊界。另外,在有多點觸控事件時,還會陸續觸發ACTION_POINTER_DOWN、ACTION_POINTER_UP等事件。
其次,我們知道Android中負責事件分發機制的方法主要有以下三個:
1、dispatchTouchEvent(MotionEvent event) --- 分發事件
我們知道當用戶觸摸到手機屏幕時,最先接收到事件並進行相應處理的應該是最外層的Activity,所以我們來看看Activity中是如何對事件進行分發的。
public boolean dispatchTouchEvent(MotionEvent ev) { if (ev.getAction() == MotionEvent.ACTION_DOWN) { onUserInteraction(); } if (getWindow().superDispatchTouchEvent(ev)) { return true; } return onTouchEvent(ev); }從以上代碼中我們可以看到調用getWindow().superDispatchTouchEvent(),而這裡的getWindow()返回的是Window抽象類,其實就是PhoneWindow類,繼承於Window抽象類,然後調用PhoneWindow的superDispatchTouchEvent(),
@Override public boolean superDispatchTouchEvent(MotionEvent event) { return mDecor.superDispatchTouchEvent(event); }
從superDispatchTouchEvent()方法中可以看到,它又調用了mDecor的superDispatchTouchEvent()方法,再看mDecor的superDispatchTouchEvent()方法,
public boolean superDispatchTouchEvent(MotionEvent event) { return super.dispatchTouchEvent(event); }
而mDecor是何許人也,其實就是PhoneWindow中的一個內部類DecorView的實例對象,是Activity的Window窗口中最根部的父容器,我們平時在Activity的onCreate()方法中,通過setContentView()給設置的布局容器,都屬於mDecor的子View mContentView對象的子view,而DecorView又繼承於FrameLayout,FrameLayout又繼承於ViewGroup,由此可知,Activity是如何將事件分發到相應的View當中去的:
Activity.dispatchTouchEvent(MotionEvent event) -> PhoneWindow.superDispatchTouchEvent(MotionEvent event) -> DecorView.superDispatchTouchEvent(MotionEvent event) -> FrameLayout.dispatchTouchEvent(MotionEvent event) -> ViewGroup.dispatchTouchEvent(MotionEvent event) -> 再逐級分發到各個ViewGroup/View當中去
另外從以上分析過程可以看出,有一點需注意,就是我們在繼承ViewGroup或其子類復寫dispatchTouchEvent時,在方法最後的返回值處,最好別直接寫成return true或者return false,而應寫成super.dispatchTouchEvent,否則無法對事件繼續進行逐級分發,因為在ViewGroup類的dispatchTouchEvent(MotionEvent event)方法中,會對該布局容器內的所有子View進行遍歷,然後再進行事件分發,詳細分發過程稍後會給出。
2、onInterceptTouchEvent(MotionEvent event) --- 攔截事件
onInterceptTouchEvent(MotionEvent event)方法只存在於ViewGroup當中,是用來對布局容器內子View的事件進行攔截的,如果父容器View對事件進行了攔截,即return true,則子View不會收到任何事件分發。
3、onTouchEvent(MotionEvent event) --- 處理消費事件
onTouchEvent(MotionEvent event)方法如果返回true,則表示該事件被當前View給消費掉了,它的父View的onTouchEvent()後續都不會得到調用,而是通過dispatchTouchEvent()逐級向上返回true到Activity;如果沒人消費該事件,都返回false,則最終會交給Activity去進行處理。
在大致了解了dispatchTouchEvent、onInterceptTouchEvent、onTouchEvent的作用之後,現在我們最需要理清的就是這三者之間的調用關系如何,為此我自己寫了一個測試Demo,界面如下:
屏幕中有ViewGroupA、ViewGroupB、ViewC,依次進行嵌套
測試代碼如下:
public class ViewGroupA extends LinearLayout { public ViewGroupA(Context context) { super(context); } public ViewGroupA(Context context, AttributeSet attrs) { super(context, attrs); } public ViewGroupA(Context context, AttributeSet attrs, int defStyleAttr) { super(context, attrs, defStyleAttr); } @Override public boolean onInterceptTouchEvent(MotionEvent ev) { Log.d("hanyee", this.getClass().getSimpleName() + " onInterceptTouchEvent -> " + ViewUtils.actionToString(ev.getAction())); boolean result = super.onInterceptTouchEvent(ev); Log.d("hanyee", this.getClass().getSimpleName() + " onInterceptTouchEvent return super.onInterceptTouchEvent(ev)=" + result); return result; } @Override public boolean dispatchTouchEvent(MotionEvent ev) { Log.d("hanyee", this.getClass().getSimpleName() + " dispatchTouchEvent -> " + ViewUtils.actionToString(ev.getAction())); boolean result = super.dispatchTouchEvent(ev); Log.d("hanyee", this.getClass().getSimpleName() + " dispatchTouchEvent return super.dispatchTouchEvent(ev)= " + result); return result; } @Override public boolean onTouchEvent(MotionEvent ev) { Log.d("hanyee", this.getClass().getSimpleName() + " onTouchEvent -> " + ViewUtils.actionToString(ev.getAction())); boolean result = super.onTouchEvent(ev); Log.d("hanyee", this.getClass().getSimpleName() + " onTouchEvent return super.onTouchEvent(ev)=" + result); return result; } } public class ViewGroupB extends LinearLayout { public ViewGroupB(Context context) { super(context); } public ViewGroupB(Context context, AttributeSet attrs) { super(context, attrs); } public ViewGroupB(Context context, AttributeSet attrs, int defStyleAttr) { super(context, attrs, defStyleAttr); } @Override public boolean onInterceptTouchEvent(MotionEvent ev) { Log.d("hanyee", this.getClass().getSimpleName() + " onInterceptTouchEvent -> " + ViewUtils.actionToString(ev.getAction())); boolean result = super.onInterceptTouchEvent(ev); Log.d("hanyee", this.getClass().getSimpleName() + " onInterceptTouchEvent return super.onInterceptTouchEvent(ev)=" + result); return result; } @Override public boolean dispatchTouchEvent(MotionEvent ev) { Log.d("hanyee", this.getClass().getSimpleName() + " dispatchTouchEvent -> " + ViewUtils.actionToString(ev.getAction())); boolean result = super.dispatchTouchEvent(ev); Log.d("hanyee", this.getClass().getSimpleName() + " dispatchTouchEvent return super.dispatchTouchEvent(ev)= " + result); return result; } @Override public boolean onTouchEvent(MotionEvent ev) { Log.d("hanyee", this.getClass().getSimpleName() + " onTouchEvent -> " + ViewUtils.actionToString(ev.getAction())); boolean result = super.onTouchEvent(ev); Log.d("hanyee", this.getClass().getSimpleName() + " onTouchEvent return super.onTouchEvent(ev)=" + result); return result; } } public class ViewC extends View { public ViewC(Context context) { super(context); } public ViewC(Context context, AttributeSet attrs) { super(context, attrs); } public ViewC(Context context, AttributeSet attrs, int defStyleAttr) { super(context, attrs, defStyleAttr); } @Override public boolean dispatchTouchEvent(MotionEvent ev) { Log.d("hanyee", this.getClass().getSimpleName() + " dispatchTouchEvent -> " + ViewUtils.actionToString(ev.getAction())); boolean result = super.dispatchTouchEvent(ev); Log.d("hanyee", this.getClass().getSimpleName() + " dispatchTouchEvent return super.dispatchTouchEvent(ev)= " + result); return result; } @Override public boolean onTouchEvent(MotionEvent ev) { Log.d("hanyee", this.getClass().getSimpleName() + " onTouchEvent -> " + ViewUtils.actionToString(ev.getAction())); boolean result = super.onTouchEvent(ev); Log.d("hanyee", this.getClass().getSimpleName() + " onTouchEvent return super.onTouchEvent(ev)=" + result); return result; } }
測試情景1:ViewGroupA、ViewGroupB、ViewC都沒有消費事件
測試結果如下圖:
由圖中log可以看出,如果沒有任何view消費事件的話,事件的傳遞順序如下:
ViewGroupA.dispatchTouchEvent ->ViewGroupA.onInterceptTouchEvent(return false, 沒有進行攔截) ->ViewGroupB.dispatchTouchEvent ->ViewGroupB.onInterceptTouchEvent(return false, 沒有進行攔截) ->ViewC.dispatchTouchEvent ->ViewC.onTouchEvent(return false, 沒有消費) -> ViewC.dispatchTouchEvent(return false, 將onTouchEvent的處理結果回傳給ViewGroupB) -> ViewGroupB.onTouchEvent(return false, 也沒有消費) -> ViewB.dispatchTouchEvent(return false, 將onTouchEvent的處理結果回傳給ViewGroupA) ->ViewGroupA.onTouchEvent(return false, 也沒有消費) -> ViewA.dispatchTouchEvent(return false, 最終將onTouchEvent的處理結果回傳給Activity) -> Activity對事件進行最終處理
看到這裡大伙可能會有些疑問,怎麼就只有Down事件,而沒有後續的Move、Up等事件,這是因為沒有任何子View消費Down事件,Down事件最終被最外層的Activity給處理掉了,所以後續的所有Move、Up等事件都不會再分發給子View了,這裡在後面的源碼分析時會提到。
測試情景2:ViewC消費了事件
測試結果如下圖:
由圖中的log可以看出,一旦ViewC消費了Down事件,它的父容器ViewGroupB,祖父容器ViewGroupA的onTouchEvent都不會被調用了,而是直接通過dispatchTouchEvent將Down以及後續的Move、Up事件的處理結果返回至Activity。
測試情景3:僅點擊ViewGroupB,讓ViewGroupB消費事件
測試結果如下圖:
從圖中log可以看出,如果點擊ViewGroupB,事件根本就不會傳遞到ViewC,ViewGroupB在消費了Down事件之後,再直接由父容器ViewGroupA的dispatchTouchEvent將ViewGroupB的onTouchEvent處理結果true回傳給Activity,接下來後續的Move、Up事件都只會傳遞至ViewGroupB,而不會分發給ViewC。
測試情景4:讓ViewGroupB對事件進行攔截
測試結果如圖:
從圖中log可以看出,如果ViewGroupB的onInterceptTouchEvent 返回true,對子view的事件進行攔截,則ViewC不會收到任何的點擊事件,事件流變成了ViewGroupA -->ViewGroupB --> ViewGroupA,而沒有經過ViewC
通過上述幾種情景,我們可以大致了解,ViewGroupA的dispatchTouchEvent最先被調用,主要負責事件分發,然後會調用其onInterceptTouchEvent,如果返回true,則後續的ViewGroupB、ViewC都不會收到任何的點擊事件,相反如果返回false,就放棄攔截事件,接著會遍歷調用子View的dispatchTouchEvent方法將事件分發給ViewGroupB,如果ViewGroupB也沒有攔截事件,則又會遍歷調用子View的dispatchTouchEvent方法將事件分發給ViewC,如果ViewC在onTouchEvent中消費了事件返回true,則會將true通過dispatchTouchEvent方法逐級返回給其父容器直至Activity中,而且不會調用各個父容器對應的onTouchEvent方法,如果子View在onTouchEvent中沒消費事件返回false,則通過dispatchTouchEvent方法將false返回給ViewGroupB,ViewGroupB就知道子View沒有消費事件,就會調用自己的onTouchEvent來處理該事件,然後同理遞歸著ViewC在onTouchEvent中對於事件的處理邏輯,直到ViewGroupA將事件處理完反饋給Activity。
前面列了這麼多現象,並歸納總結出以上dispatchTouchEvent、onInterceptTouchEvent、onTouchEvent之間的調用關系,相信大家對於Android事件的分發機制已經有了較為清晰的認識,但作為一名程序員,知其然,還得知其所以然,下面就帶領大家一起研讀下源碼,看看到底為啥是這樣的調用關系。從上面的情景log中大家應該可以看出,事件分發機制的最初始的入口就是ViewGroup的dispatchTouchEvent,下面就看看其代碼:
@Override public boolean dispatchTouchEvent(MotionEvent ev) { if (mInputEventConsistencyVerifier != null) { mInputEventConsistencyVerifier.onTouchEvent(ev, 1); } // If the event targets the accessibility focused view and this is it, start // normal event dispatch. Maybe a descendant is what will handle the click. if (ev.isTargetAccessibilityFocus() && isAccessibilityFocusedViewOrHost()) { ev.setTargetAccessibilityFocus(false); } boolean handled = false; if (onFilterTouchEventForSecurity(ev)) { final int action = ev.getAction(); final int actionMasked = action & MotionEvent.ACTION_MASK; // Handle an initial down. if (actionMasked == MotionEvent.ACTION_DOWN) { // Throw away all previous state when starting a new touch gesture. // The framework may have dropped the up or cancel event for the previous gesture // due to an app switch, ANR, or some other state change. cancelAndClearTouchTargets(ev); resetTouchState(); } // Check for interception. final boolean intercepted; if (actionMasked == MotionEvent.ACTION_DOWN || mFirstTouchTarget != null) { final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0; if (!disallowIntercept) { intercepted = onInterceptTouchEvent(ev); ev.setAction(action); // restore action in case it was changed } else { intercepted = false; } } else { // There are no touch targets and this action is not an initial down // so this view group continues to intercept touches. intercepted = true; } // If intercepted, start normal event dispatch. Also if there is already // a view that is handling the gesture, do normal event dispatch. if (intercepted || mFirstTouchTarget != null) { ev.setTargetAccessibilityFocus(false); } // Check for cancelation. final boolean canceled = resetCancelNextUpFlag(this) || actionMasked == MotionEvent.ACTION_CANCEL; // Update list of touch targets for pointer down, if needed. final boolean split = (mGroupFlags & FLAG_SPLIT_MOTION_EVENTS) != 0; TouchTarget newTouchTarget = null; boolean alreadyDispatchedToNewTouchTarget = false; if (!canceled && !intercepted) { // If the event is targeting accessiiblity focus we give it to the // view that has accessibility focus and if it does not handle it // we clear the flag and dispatch the event to all children as usual. // We are looking up the accessibility focused host to avoid keeping // state since these events are very rare. View childWithAccessibilityFocus = ev.isTargetAccessibilityFocus() ? findChildWithAccessibilityFocus() : null; 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); // Find a child that can receive the event. // Scan children from front to back. final ArrayListpreorderedList = buildOrderedChildList(); final boolean customOrder = preorderedList == null && isChildrenDrawingOrderEnabled(); final View[] children = mChildren; for (int i = childrenCount - 1; i >= 0; i--) { final int childIndex = customOrder ? getChildDrawingOrder(childrenCount, i) : i; final View child = (preorderedList == null) ? children[childIndex] : preorderedList.get(childIndex); // If there is a view that has accessibility focus we want it // to get the event first and if not handled we will perform a // normal dispatch. We may do a double iteration but this is // safer given the timeframe. if (childWithAccessibilityFocus != null) { if (childWithAccessibilityFocus != child) { continue; } childWithAccessibilityFocus = null; i = childrenCount - 1; } if (!canViewReceivePointerEvents(child) || !isTransformedTouchPointInView(x, y, child, null)) { ev.setTargetAccessibilityFocus(false); continue; } newTouchTarget = getTouchTarget(child); if (newTouchTarget != null) { // Child is already receiving touch within its bounds. // Give it the new pointer in addition to the ones it is handling. newTouchTarget.pointerIdBits |= idBitsToAssign; break; } resetCancelNextUpFlag(child); if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) { // Child wants to receive touch within its bounds. mLastTouchDownTime = ev.getDownTime(); if (preorderedList != null) { // childIndex points into presorted list, find original index for (int j = 0; j < childrenCount; j++) { if (children[childIndex] == mChildren[j]) { mLastTouchDownIndex = j; break; } } } else { mLastTouchDownIndex = childIndex; } mLastTouchDownX = ev.getX(); mLastTouchDownY = ev.getY(); newTouchTarget = addTouchTarget(child, idBitsToAssign); alreadyDispatchedToNewTouchTarget = true; break; } // The accessibility focus didn't handle the event, so clear // the flag and do a normal dispatch to all children. ev.setTargetAccessibilityFocus(false); } if (preorderedList != null) preorderedList.clear(); } if (newTouchTarget == null && mFirstTouchTarget != null) { // Did not find a child to receive the event. // Assign the pointer to the least recently added target. newTouchTarget = mFirstTouchTarget; while (newTouchTarget.next != null) { newTouchTarget = newTouchTarget.next; } newTouchTarget.pointerIdBits |= idBitsToAssign; } } } // Dispatch to touch targets. if (mFirstTouchTarget == null) { // No touch targets so treat this as an ordinary view. 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; if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) { handled = true; } else { final boolean cancelChild = resetCancelNextUpFlag(target.child) || intercepted; 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; } } // Update list of touch targets for pointer up or cancel, if needed. 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; }
這方法看似比較長,但我們只挑比較重要的點來看,在第32行會根據disallowIntercept來判斷是否對子view來進行事件攔截,子view可以通過調用requestDisallowInterceptTouchEvent()方法來改變其值,如果可以進行攔截,則會調用onInterceptTouchEvent()方法,根據其返回值來判斷需不需要對子View進行攔截,默認情況下onInterceptTouchEvent()方法返回的是false,所以如果我們在自定義View時如果想攔截的話,可以重寫這個方法返回true就行了。
然後在第58行的if條件中,會根據是否取消canceled以及之前的是否攔截的標志intercepted來判斷是否走進下面的邏輯代碼塊,這裡我們只看intercepted,如果沒有攔截,則會進入if後面的邏輯代碼塊,直到第89行的for循環,我們會看到ViewGroup在對所有子View進行遍歷,以方便接下來的事件分發,再看到107、108行的判斷,canViewReceivePointerEvents()用來判斷是否該View能夠接受處理事件,
private static boolean canViewReceivePointerEvents(View child) { return (child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null; }可以看到只有當view處於可見狀態且沒有做動畫時才能接收處理事件,再看isTransformedTouchPointInView()是用來判斷當前事件是否觸發在該view的范圍之內,這裡我們可以回想前面的測試情景3,當我們點擊ViewGroupB時,ViewC完全沒有收到任何事件,就是因為點擊事件不在ViewC的范圍之類,在isTransformedTouchPointInView()進行判斷時就給過濾掉了,所以ViewC不會收到任何分發的事件。再看看第122行,會調用dispatchTransformedTouchEvent()來將事件分發給對應的view進行處理,讓我們進入其方法體看看,
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. 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; } // Calculate the number of pointers to deliver. final int oldPointerIdBits = event.getPointerIdBits(); final int newPointerIdBits = oldPointerIdBits & desiredPointerIdBits; // If for some reason we ended up in an inconsistent state where it looks like we // might produce a motion event with no pointers in it, then drop the event. if (newPointerIdBits == 0) { return false; } // 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. 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; event.offsetLocation(offsetX, offsetY); handled = child.dispatchTouchEvent(event); event.offsetLocation(-offsetX, -offsetY); } return handled; } transformedEvent = MotionEvent.obtain(event); } else { transformedEvent = event.split(newPointerIdBits); } // Perform any necessary transformations and dispatch. if (child == null) { 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()); } handled = child.dispatchTouchEvent(transformedEvent); } // Done. transformedEvent.recycle(); return handled; }
我們看到在方法的末尾第55行,如果child為Null,則會調用ViewGroup的父類View的dispatchTouchEvent,否則就會調用child自身的dispatchTouchEvent方法進行事件分發處理。如果child是ViewGroup,則會又遞歸調用ViewGroup的dispatchTouchEvent方法邏輯進行事件分發,如果是View,則跟child為Null情況一樣,都是會調到View的dispatchTouchEvent方法,接下來我們看看View的dispatchTouchEvent方法,
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; }同樣我們撿重點的看,第23行用來做過濾,看是否有窗口覆蓋在上面,第27~29行三個判斷條件說明了,當View的touch事件監聽器不為空,View是enable狀態,且touch事件監聽回調方法onTouch方法返回true三個條件同時滿足時,則會最終返回true,而且第33行的onTouchEvent方法都不會得到執行,這說明View的OnTouchListener監聽回調的優先級要高於onTouchEvent,如果我們給View設置了OnTouchListener監聽,並且在回調方法onTouch()中返回true,View的onTouchEvent就得不到執行,其dispatchTouchEvent方法就會直接返回true給父容器,相反如果返回false,或者沒有設置OnTouchListener監聽,才會執行onTouchEvent()方法對分發來的事件進行處理。接著再去看看onTouchEvent()中如何對事件進行處理的,
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; }
從第7~16行可以看出,當View為disable狀態,而又clickable時,是會消費掉事件的,只不過在界面上沒有任何的響應。第18~22行,關於TouchDelegate,根據對官方文檔的理解就是說有兩個View, ViewB在ViewA中,ViewA比較大,如果我們想點擊ViewA的時候,讓ViewB去響應點擊事件,這時候就需要使用到TouchDelegate,簡單的理解就是如果該View有自己的事件委托處理人,就交給委托人處理。從第24~26行可以看出,只有當View是可點擊狀態時,才會進入對應各種事件的詳細處理邏輯,否則會直接返回false,表明該事件沒有被消費。在第59行,可以看到在Action_Up事件被觸發時,會執行performClick(),也就是View的點擊事件,由此可知,view的onClick()回調是在Action_Up事件中被觸發的。第134行直接返回了true,可以看出只要View處於可點擊狀態,並且進入了switch的判斷邏輯,就會被返回true,表明該事件被消費掉了,也就是說只要View是可點擊的,事件傳到了其OnTouchEvent,都會被消費掉。而平時我們在調用setOnClickListener方法給View設置點擊事件監聽時,都會將其點擊狀態修改為可點擊狀態。
public void setOnClickListener(@Nullable OnClickListener l) { if (!isClickable()) { setClickable(true); } getListenerInfo().mOnClickListener = l; }
追溯完View的事件分發流程,我們再返回到ViewGroup的dispatchTouchEvent方法的122行,如果對應得child消費了點擊事件,就會通過對應的dispatchTouchEvent方法返回true並最終在122行使得條件成立,然後會進入到138行,調用addTouchTarget對newTouchTarget進行賦值,並且mFirstTouchTarget跟newTouchTarget的值都一樣,然後將alreadyDispatchedToNewTouchTarget置為true
private TouchTarget addTouchTarget(View child, int pointerIdBits) { TouchTarget target = TouchTarget.obtain(child, pointerIdBits); target.next = mFirstTouchTarget; mFirstTouchTarget = target; return target; }
然後來到了163行,由於mFirstTouchTarget和newTouchTarget在addTouchTarget中都被賦值了,所以會直接進入172行的while循環,由於之前在138、139行對mFirstTouchTarget、newTouchTarget、alreadyDispatchedToNewTouchTarget都賦值了,使得174行條件成立,所以就直接返回true了,至此,ViewGroup就完成了對子View的遍歷及事件分發,由於事件被消費掉了,所以ViewGroup對應的所有外圍容器都會遞歸回調dispatchTouchEvent將true傳遞給Activity,到這也就解釋了測試情景2的產生原理。在Down相關事件被消費掉之後,後續的Move、Up事件在dispatchTouchEvent方法的68~70行不符合判斷條件,直接會來到179行的dispatchTransformedTouchEvent方法繼續進行分發,待子View進行消費。
如果在ViewGroup的dispatchTouchEvent方法第58行被攔截了(對應測試情景4),或者107~108行不成立(對應測試情景3),或者122行返回false(即子View沒有消費事件,對應測試情景1),則會直接進入到第163行,這時mFirstTouchTarget肯定為空,所以會又調用dispatchTransformedTouchEvent方法,而且傳進去的child為空,最終就會直接走到dispatchTransformedTouchEvent方法的55行,然後調用super.dispatchTouchEvent,之後的處理邏輯跟前面調View的dispatchTouchEvent邏輯一樣。
終上所述,整個Android的事件分發機制可以大致概括成如下的流程圖
PS:以上相關的系統代碼均為Android6.0的系統源碼,整個Android事件分發機制還算有點復雜,完全給整明白寫下這篇文章還費了些時間,中間查閱了一些資料,可能有些地方還存在些理解偏差,還請大家指出相互學習進步
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