先看下图，平时简单的也就down，move和up拉。如果多点触控还point down，point up

image.png

如下代码，如果我们要禁止掉多点触控

    override fun dispatchTouchEvent(ev: MotionEvent): Boolean {
        println("dispatchTouchEvent==============${MotionEventCompat.getActionMasked(ev)}===${(ev.action )}==${ev.action and MotionEvent.ACTION_MASK}")
        if((ev.action and MotionEvent.ACTION_MASK)==MotionEvent.ACTION_POINTER_DOWN){
            return true
        }
        return super.dispatchTouchEvent(ev)
    }

一些常用的十六进制和二进制的关系以及作用

0x0100 对应的二进制1 0000 0000
0x0200 对应的二进制10 0000 0000
0x0300 对应的二进制11 0000 0000

0xff00对应的二进制 1111 1111 0000 0000 //这个与操作的话低八位就没了，只保留高八位，然后一般会进行位移操作
0x00ff对应的二进制1111 1111

public static final int ACTION_MASK = 0xff;
public static final int ACTION_POINTER_INDEX_MASK = 0xff00;

然后看这个与操作 ev.action and MotionEvent.ACTION_MASK
因为0xff 转换为二进制就是低八位全是1，所以和它进行操作的结果就是保留低八位，高八位就都成0了

然后看些过时的一些参数
ACTION_POINTER_DOWN =5
public static final int ACTION_POINTER_2_DOWN = ACTION_POINTER_DOWN | 0x0100;
十六进制的2个0就有八位了，也就是低八位是0，和down进行与操作的结果，就是高八位是1，低八位是5
public static final int ACTION_POINTER_3_DOWN = ACTION_POINTER_DOWN | 0x0200;
十六进制的2个0就有八位了，也就是低八位是0，和down进行与操作的结果，就是高八位是2，低八位是5
其实上边的1和2就是索引了，也就是第几根手指触摸屏幕的。
如果我们不关心这个，我们只关系它是不是多点触控，那么就可以与CTION_MASK = 0xff进行与操作，去掉高八位，那么就只保留低八位的，也就是ACTION_POINTER_DOWN 了。

如果你要获取索引是第几根手指，那么结果与ACTION_POINTER_INDEX_MASK = 0xff00进行与操作，就会去掉低八位，然后右移八位，把低八位都去掉，高八位就成低八位了
public static final int ACTION_POINTER_ID_SHIFT = 8;

也就是index=(mAction & ACTION_POINTER_INDEX_MASK) >> ACTION_POINTER_INDEX_SHIFT
index返回就是0,1,2的数字拉。

总结一下

获取普通 的action，比如down，move和up，point down，point up
用如下的方法，会去掉高八位的信息
(ev.action and MotionEvent.ACTION_MASK)
如果要获取高八位的索引，用如下的方法
var index=(ev.action and MotionEvent.ACTION_POINTER_INDEX_MASK) shr MotionEvent.ACTION_POINTER_INDEX_SHIFT//右移8位,获取index

另外进制转换这里有图，忘了可以看下
https://jingyan.baidu.com/article/597a0643614568312b5243c0.html

View的触摸事件

下边注释写了3种返回true的情况

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)) {
      //返回true情况1，鼠标在滚动条上拖动
            if ((mViewFlags & ENABLED_MASK) == ENABLED && handleScrollBarDragging(event)) {
                result = true;
            }
            //noinspection SimplifiableIfStatement
            ListenerInfo li = mListenerInfo;
//返回true情况2，view调用了setOnTouchListener，并且listener返回为true
            if (li != null && li.mOnTouchListener != null
                    && (mViewFlags & ENABLED_MASK) == ENABLED
                    && li.mOnTouchListener.onTouch(this, event)) {
                result = true;
            }
//返回true情况3，view自身的onTouchEvent方法返回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;
    }

ViewGroup的触摸事件

有如下三个方法

    override fun onInterceptTouchEvent(ev: MotionEvent): Boolean {
        val intercept=super.onInterceptTouchEvent(ev)
        println("onInterceptTouchEvent==============${ev.action}====$intercept")
        return true
    }

    override fun dispatchTouchEvent(ev: MotionEvent): Boolean {

        val result=super.dispatchTouchEvent(ev)

        return result
    }

    override fun onTouchEvent(event: MotionEvent): Boolean {
        println("onTouchEvent==============${event.action}")
        return super.onTouchEvent(event)
    }

一个ViewGroup能否处理触摸事件，或者如果想截断触摸事件，关键就是看dispatchTouchEvent在event等于ACTION_DOWN的时候是否返回true
而其他2个方法都是在super.dispatchTouchEvent(ev)里调用的，如果没有执行这个super方法，那么其他2个方法是不会执行的。

简单研究下dispatchTouchEvent源码

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) {//不允许中断为false的条件下
                    intercepted = onInterceptTouchEvent(ev);//看自己是否中断触摸事件
                    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) {//viewgroup自身没有中断触摸事件。

                // 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 ArrayList<View> preorderedList = buildTouchDispatchChildList();
                        final boolean customOrder = preorderedList == null
                                && isChildrenDrawingOrderEnabled();
                        final View[] children = mChildren;
                        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;
                            }

                            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);
//将event传递给child，看child是否处理，如果处理的话，最后一行alreadyDispatchedToNewTouchTarget =true
                            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) {//没有子child处理，那自己处理
                // 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;
    }

中间用到的方法

private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,
            View child, int desiredPointerIdBits)

if (child == null) {
//没有子类处理的时候child为null，这时候调用super方法，其实也就是view的默认dipatchTouchEvent方法拉，这个方法里会调用onTouch方法，上边View分析了
            handled = super.dispatchTouchEvent(transformedEvent);
        }
//如果child不为空，那么就child处理，然后就一层一层的往下边去拉。
handled = child.dispatchTouchEvent(transformedEvent);