編輯:關於Android編程
一直想寫篇關於Android事件派發機制的文章,卻一直沒寫,這兩天剛好是周末,有時間了,想想寫一篇吧,不然總是只停留在會用的層次上但是無法了解其內部機制。我用的是4.4源碼,打開看看,挺復雜的,尤其是事件是怎麼從Activity派發出來的,太費解了。了解Windows消息機制的人會發現,覺得Android的事件派發機制和Windows的消息派發機制挺像的,其實這是一種典型的消息“冒泡”機制,很多平台采用這個機制,消息最先到達最底層View,然後它先進行判斷是不是它所需要的,否則就將消息傳遞給它的子View,這樣一來,消息就從水底的氣泡一樣向上浮了一點距離,以此類推,氣泡達到頂部和空氣接觸,破了(消息被處理了),當然也有氣泡浮出到頂層了,還沒破(消息無人處理),這個消息將由系統來處理,對於Android來說,會由Activity來處理。
點擊事件用MotionEvent來表示,當一個點擊操作發生時,事件最先傳遞給當前Activity,由Activity的dispatchTouchEvent來進行事件派發,具體的工作是由Activity內部的Window來完成的,Window會將事件傳遞給decor view,decor view一般就是當前界面的底層容器(即setContentView所設置的View),通過Activity.getWindow.getDecorView()可以獲得。另外,看下面代碼的的時候,主要看我注釋的地方,代碼很多很復雜,我無法一一說明,但是我注釋的地方都是關鍵點,是博主仔細讀代碼總結出來的。
源碼解讀:
事件是由哪裡傳遞給Activity的,這個我還不清楚,但是不要緊,我們從activity開始分析,已經足夠我們了解它的內部實現了。
Code:Activity#dispatchTouchEvent
/** * 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) { //這個函數其實是個空函數,啥也沒干,如果你沒重寫的話,不用關心 onUserInteraction(); } //這裡事件開始交給Activity所附屬的Window進行派發,如果返回true,整個事件循環就結束了 //返回false意味著事件沒人處理,所有人的onTouchEvent都返回了false,那麼Activity就要來做最後的收場。 if (getWindow().superDispatchTouchEvent(ev)) { return true; } //這裡,Activity來收場了,Activity的onTouchEvent被調用 return onTouchEvent(ev); }
Window是如何將事件傳遞給ViewGroup的
Code:Window#superDispatchTouchEvent
/** * Used by custom windows, such as Dialog, to pass the touch screen event * further down the view hierarchy. Application developers should * not need to implement or call this. * */ public abstract boolean superDispatchTouchEvent(MotionEvent event);這竟然是一個抽象函數,還注明了應用開發者不要實現它或者調用它,這是什麼情況?再看看如下類的說明,大意是說:這個類可以控制頂級View的外觀和行為策略,而且還說這個類的唯一一個實現位於android.policy.PhoneWindow,當你要實例化這個Window類的時候,你並不知道它的細節,因為這個類會被重構,只有一個工廠方法可以使用。好吧,還是很模糊啊,不太懂,不過我們可以看一下android.policy.PhoneWindow這個類,盡管實例化的時候此類會被重構,但是重構而已,功能是類似的。
Abstract base class for a top-level window look and behavior policy. An instance of this class should be used as the top-level view added to the window manager. It provides standard UI policies such as a background, title area, default key processing, etc.
The only existing implementation of this abstract class is android.policy.PhoneWindow, which you should instantiate when needing a Window. Eventually that class will be refactored and a factory method added for creating Window instances without knowing about a particular implementation.
Code:PhoneWindow#superDispatchTouchEvent@Override public boolean superDispatchTouchEvent(MotionEvent event) { return mDecor.superDispatchTouchEvent(event); }這個邏輯很清晰了,PhoneWindow將事件傳遞給DecorView了,這個DecorView是啥呢,請看下面
private final class DecorView extends FrameLayout implements RootViewSurfaceTaker // This is the top-level view of the window, containing the window decor. private DecorView mDecor; @Override public final View getDecorView() { if (mDecor == null) { installDecor(); } return mDecor; }
順便說一下,平時Window用的最多的就是((ViewGroup)getWindow().getDecorView().findViewById(android.R.id.content)).getChildAt(0)即通過Activity來得到內部的View。這個mDecor顯然就是getWindow().getDecorView()返回的View,而我們通過setContentView設置的View是它的一個子View。目前事件傳遞到了DecorView 這裡,由於DecorView 繼承自FrameLayout且是我們的父View,所以最終事件會傳遞給我們的View,原因先不管了,換句話來說,事件肯定會傳遞到我們的View,不然我們的應用如何響應點擊事件呢。不過這不是我們的重點,重點是事件到了我們的View以後應該如何傳遞,這是對我們更有用的。從這裡開始,事件已經傳遞到我們的頂級View了,注意:頂級View實際上是最底層View,也叫根View。
點擊事件到底層View(一般是一個ViewGroup)以後,會調用ViewGroup的dispatchTouchEvent方法,然後的邏輯是這樣的:如果底層ViewGroup攔截事件即onInterceptTouchEvent返回true,則事件由ViewGroup處理,這個時候,如果ViewGroup的mOnTouchListener被設置,則會onTouch會被調用,否則,onTouchEvent會被調用,也就是說,如果都提供的話,onTouch會屏蔽掉onTouchEvent。在onTouchEvent中,如果設置了mOnClickListener,則onClick會被調用。如果頂層ViewGroup不攔截事件,則事件會傳遞給它的在點擊事件鏈上的子View,這個時候,子View的dispatchTouchEvent會被調用,到此為止,事件已經從最底層View傳遞給了上一層View,接下來的行為和其底層View一致,如此循環,完成整個事件派發。另外要說明的是,ViewGroup默認是不攔截點擊事件的,其onInterceptTouchEvent返回false。
源碼解讀:
Code:ViewGroup#dispatchTouchEvent
@Override 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; // 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) { //這裡判斷是否攔截點擊事件,如果攔截,則intercepted=true 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; } // 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; //這裡面一大堆是派發事件到子View,如果intercepted是true,則直接跳過 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); // Find a child that can receive the event. // Scan children from front to back. final View[] children = mChildren; final boolean customOrder = isChildrenDrawingOrderEnabled(); 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) { // 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(); mLastTouchDownIndex = childIndex; mLastTouchDownX = ev.getX(); mLastTouchDownY = ev.getY(); //注意下面兩句,如果有子View處理了點擊事件,則newTouchTarget會被賦值, //同時alreadyDispatchedToNewTouchTarget也會為true,這兩個變量是直接影響下面的代碼邏輯的。 newTouchTarget = addTouchTarget(child, idBitsToAssign); alreadyDispatchedToNewTouchTarget = true; break; } } } 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. //這裡如果當前ViewGroup攔截了事件,或者其子View的onTouchEvent都返回了false,則事件會由ViewGroup處理 if (mFirstTouchTarget == null) { // No touch targets so treat this as an ordinary view. //這裡就是ViewGroup對點擊事件的處理 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; }
Code:ViewGroup#dispatchTransformedTouchEvent
/** * 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. final int oldAction = event.getAction(); if (cancel || oldAction == MotionEvent.ACTION_CANCEL) { event.setAction(MotionEvent.ACTION_CANCEL); if (child == null) { //這裡就是ViewGroup對點擊事件的處理,其調用了View的dispatchTouchEvent方法 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; }再看
Code:View#dispatchTouchEvent
/** * 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; if (li != null && li.mOnTouchListener != null && (mViewFlags & ENABLED_MASK) == ENABLED && li.mOnTouchListener.onTouch(this, event)) { return true; } if (onTouchEvent(event)) { return true; } } if (mInputEventConsistencyVerifier != null) { mInputEventConsistencyVerifier.onUnhandledEvent(event, 0); } return false; }這段代碼比較簡單,View對事件的處理是這樣的:如果設置了OnTouchListener就調用onTouch,否則就直接調用onTouchEvent,而onClick是在onTouchEvent內部通過performClick觸發的。簡單來說,事件如果被ViewGroup攔截或者子View的onTouchEvent都返回了false,則事件最終由ViewGroup處理。
如果一個點擊事件,子View的onTouchEvent返回了false,則父View的onTouchEvent會被直接調用,以此類推。如果所有的View都不處理,則最終會由Activity來處理,這個時候,Activity的onTouchEvent會被調用。這個問題已經在1和2中做了說明。
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