一、ViewGroup的onInterceptTouchEvent原始碼分析

onInterceptTouchEvent比較簡單先看他的原始碼

    public boolean onInterceptTouchEvent(MotionEvent ev) {
    //1、判斷是否是滑鼠裝置操作
    //2、ACTION_DOWN事件
    //3、是否是首要按鈕按下，如滑鼠左鍵
    //4、是否是滑動條     
    //看意思以上是相容安卓非手機裝置用的判斷，滿足的話，就返回true，攔截事件分發
        if (ev.isFromSource(InputDevice.SOURCE_MOUSE)
                && ev.getAction() == MotionEvent.ACTION_DOWN
                && ev.isButtonPressed(MotionEvent.BUTTON_PRIMARY)
                && isOnScrollbarThumb(ev.getX(), ev.getY())) {
            return
 
 true;
        }
        //預設返回false不攔截事件分發
        return false;
    }

onInterceptTouchEvent 作用就是要不要攔截向子View分發事件，如果想攔截的話，需要重寫onInterceptTouchEvent。

二、ViewGroup的dispatchTouchEvent原始碼分析

    @Override
    public boolean dispatchTouchEvent(MotionEvent ev) {
    //如果輸入一致性檢查事件不為null，先執行輸入前後一致性檢查
    //原始碼英文註釋說是為了debug的目的，估計是設定斷點後，再次執行的時候，為了不丟失事件，檢查是否是同一個輸入，可忽略
 

        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.
        //如果事件物件是容易獲取焦點的view，開始普通事件分發，可能是某個後代會處理點選。
 

        //貌似是用來判斷 motion event是否設定了容易獲取焦點Flags
        //這個flags是FLAG是_WINDOW_IS_OBSCURED，原始碼意思大約是設定事件物件，是否部分或者全部被遮擋到了
        if (ev.isTargetAccessibilityFocus() && isAccessibilityFocusedViewOrHost()) {
            ev.setTargetAccessibilityFocus(false);
        }

        //handled 是最終要返回的結果值
        boolean handled = false;
        //onFilterTouchEventForSecurity 對事件進行安全檢查，如果true可執行，false就放棄
        //onFilterTouchEventForSecurity 裡面進行了是否要遮擋進行過濾，以及event的物件是否被遮擋的兩個判斷
        if (onFilterTouchEventForSecurity(ev)) {
        //未被遮擋進入了此if內部
            final int action = ev.getAction();
            final int actionMasked = action & MotionEvent.ACTION_MASK;

            // Handle an initial down.
            //處理最初的ACTION_DOWN事件
            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.
                //當開始了一個新的手勢，拋棄了之前所有的狀態
                //由於app切換，ANR或者其他狀態改變，框架可能已經放棄了對前一個手勢的Up 和cancel的處理
                //cancelAndClearTouchTargets 清空了之前所有的找到的觸控物件，裡面呼叫了dispatchTransformedTouchEvent(event, true, target.child, target.pointerIdBits);對子View的事件進行了取消，傳遞的第二個引數true 是是否取消的標誌
                //裡面呼叫了clearTouchTargets 對比較重要的mFirstTouchTarget物件賦值null
               //mFirstTouchTarget的英文註釋 First touch target in the linked list of touch targets.是第一個找到的觸控物件
                cancelAndClearTouchTargets(ev);
                resetTouchState();
            }
            //檢查是否攔截
            // Check for interception.
            final boolean intercepted;
            //第一種情況ACTION_DOWN的時候判斷攔截，
            //第二種情況，已經找到了要處理事件的字View或者子ViewGroup
            if (actionMasked == MotionEvent.ACTION_DOWN
                    || mFirstTouchTarget != null) {
                    //之後又對disallowIntercept進行了判斷，disallowIntercept的意思是，是否禁用攔截，disallowIntercept可以使用requestDisallowInterceptTouchEvent方法自己進行設定，後面會具體分析
                final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
                if (!disallowIntercept) {
                //沒有禁用攔截，就會根據攔截函式onInterceptTouchEvent的返回值，進行攔截判斷，是否攔截
                    intercepted = onInterceptTouchEvent(ev);
                    ev.setAction(action); // restore action in case it was changed恢復action，防止action被改變了
                } else {//禁止攔截，所以設定攔截標識為不攔截
                    intercepted = false;
                }
            } else {
                // There are no touch targets and this action is not an initial down
                // so this view group continues to intercept touches.
                //沒有找到處理MotionEvent的目標物件，而且也不是最初的ACTION_DOWN，也就是現在是up 或者move事件了
                //mFirstTouchTarget 為null說明自己沒有需要處理事件的物件，也就是自己這個ViewGroup下沒有View要處理事件，所以攔截
                //
                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.
            //檢查事件是否被取消
            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;
            //定義了一個新的觸控物件，下面會用到
            TouchTarget newTouchTarget = null;
            boolean alreadyDispatchedToNewTouchTarget = false;
            if (!canceled && !intercepted) {//沒被取消或攔截

                // 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;

//ACTION_POINTER_DOWN 是說在有一個觸控點的情況下，又有手指觸控式螢幕幕。多點觸控的判斷
//主要關心ACTION_DOWN事件就可以了，
                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);
//有效的子View的數量
                    final int childrenCount = mChildrenCount;
                    //newTouchTarget 為null且子view數量不為0
                    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.
                        //找到一個能接收事件的子View，檢視層次上從上層到下層的順序查詢
                        final ArrayList<View> preorderedList = buildTouchDispatchChildList();
                        final boolean customOrder = preorderedList == null
                                && isChildrenDrawingOrderEnabled();
                        final View[] children = mChildren;
                        //是子View倒序遍歷子View，因為在新增子view到group的過程中，先新增的顯示在底層，後新增的顯示在上層，所以查詢的時候，先從可見的上層查詢起
                        for (int i = childrenCount - 1; i >= 0; i--) {
                            final int childIndex = getAndVerifyPreorderedIndex(
                                    childrenCount, i, customOrder);
                            final View child = getAndVerifyPreorderedView(
                                    preorderedList, children, 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;
                            }
//判斷子View能不能收到觸控事件
//判斷自VIew是不是在觸控範圍內  如果不滿足，就不檢查了
                            if (!canViewReceivePointerEvents(child)
                                    || !isTransformedTouchPointInView(x, y, child, null)) {
                                ev.setTargetAccessibilityFocus(false);
                                continue;
                            }
//子View滿足可觸控和在觸控範圍內
//找到某一指定的子view的TouchTarget物件，傳遞引數有子View，返回子VIew的TouchTarget物件
                            newTouchTarget = getTouchTarget(child);
                            //沒有找到子View的TouchTarget物件的處理
                            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);
                            //把經過一定改變的MotionEvent傳遞給子View的相對應的區域，是在這個方法裡，進行的子View的分發事件的呼叫。稍後單獨分析。
                            if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {
                                // Child wants to receive touch within its bounds.
                                //返回true表示子View想處理消費事件，
                                mLastTouchDownTime = ev.getDownTime();
                                //找到消費事件的view的index
                                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();
                                //把要消費事件的子View的TouchTarget物件放到list開頭
                                //addTouchTarget 裡面對mFirstTouchTarget進行了賦值，並返回，mFirstTouchTarget和newTouchTarget 此時指向相同
                                newTouchTarget = addTouchTarget(child, idBitsToAssign);
         //已經分發標識設定為true                       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();
                    }
//沒有找到消費事件的子View
                    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;
                    }
                }
            }
//以上是對 不攔截，未取消事件的處理和分發  ，如果找到了要消費事件的view， 那麼mFirstTouchTarget 就不應該為空了

//mFirstTouchTarget 為null 一是可能事件都被攔截了，二是沒有找到要消費事件的子View
            // Dispatch to touch targets.
            if (mFirstTouchTarget == null) {
                // No touch targets so treat this as an ordinary view.
                //沒有觸控物件，把它當成原始的view來處理
                //第三個引數傳遞是null，在裡面判斷了null會呼叫view的dispatchTouchEvent方法，也就是把ViewGroup當成view處理
                handled = dispatchTransformedTouchEvent(ev, canceled, null,
                        TouchTarget.ALL_POINTER_IDS);
            } else {
            //有子View消費事件
            //
                // 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;
                    //alreadyDispatchedToNewTouchTarget為true是在找到newTouchTarget的時候設定的
                    //且alreadyDispatchedToNewTouchTarget設定為true是在ACTION_DOWN的if判斷裡面的，它消費了ACTION_DOWN事件
                    //所以返回handled = true表示已經有子view消費事件了
                    if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) {
                        handled = true;
                    } else {
                    // 對於非ACTION_DOWN事件，則繼續傳遞給目標子元件進行處理(注意這裡的非ACTION_DOWN事件已經不需要再判斷是否攔截)
                        final boolean cancelChild = resetCancelNextUpFlag(target.child)
                                || intercepted;
                        if (dispatchTransformedTouchEvent(ev, cancelChild,
                                target.child, target.pointerIdBits)) {
                                // 如果target子元件進行處理，符合某些條件的話，會傳遞ACTION_CANCEL給target子元件
                    // 條件是：如果ACTION_DOWN時沒有被攔截，而後面的touch事件被攔截，則需要傳送ACTION_CANCEL給target子元件
                            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.
            //ACTION_CANCEL或者ACTION_UP，重置touch 的狀態
            //mFirstTouchTarget重置為null
            if (canceled
                    || actionMasked == MotionEvent.ACTION_UP
                    || actionMasked == MotionEvent.ACTION_HOVER_MOVE) {
                resetTouchState();
            } else if (split && actionMasked == MotionEvent.ACTION_POINTER_UP) {
            //多點觸控的取消處理
                final int actionIndex = ev.getActionIndex();
                final int idBitsToRemove = 1 << ev.getPointerId(actionIndex);
                removePointersFromTouchTargets(idBitsToRemove);
            }
        }

        if (!handled && mInputEventConsistencyVerifier != null) {
            mInputEventConsistencyVerifier.onUnhandledEvent(ev, 1);
        }
        return handled;
    }