Android的Touch系统简介（一）

一、Android touch事件的相关概念

用户的Touch事件被包装成MotionEvent

用户当前的touch事件主要类型有：

ACTION_DOWN: 表示用户开始触摸.

ACTION_MOVE: 表示用户在移动(手指或者其他)

ACTION_UP:表示用户抬起了手指

ACTION_CANCEL:表示手势被取消了,一些关于这个事件类型的讨论见:http://stackoverflow.com/questions/11960861/what-causes-a-motionevent-action-cancel-in-android

ACTION_OUTSIDE: 表示用户触碰超出了正常的UI边界.

ACTION_POINTER_DOWN:有一个非主要的手指按下了.

ACTION_POINTER_UP:一个非主要的手指抬起来了

touch事件的元数据包括：

touch的位置

手指的个数

touch事件的时间

一个touch手势被定义为以ACTION_DOWN开始和以 ACTION_UP结束。

二、Touch事件的处理流程

当用户触摸屏幕时，触发Activity调用dispatchTouchEvent

事件对象会按自顶向下的顺序在View Tree中传递

父View（ViewGroups）会调用dispatchTouchEvent将Event传递给子View

Event在任何时候都可能被拦截

事件流会顺着View链递归向下传递直到被消耗

若某个View想处理touch事件，必须先消耗ACTION_DOWN。考虑到效率，后续的事件将不会向下传递。

若某个事件未被消耗，最后会被Activity的onTouchEvent()消耗

若任何View或ViewGroup设置了OnTouchListener，touch事件将被拦截。

Activity.dispathcTouchEvent()的源码分析：

 /**     * 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) {            onUserInteraction();        }        if (getWindow().superDispatchTouchEvent(ev)) {            return true;        }        return onTouchEvent(ev);    }

由代码可以看出，对于应用层，该函数在touch事件发生后首先被调用。onUserInteraction()是一个空函数，可被用户重载以进行相关处理。Event随后将被传递到关联到root view的window。若子view消耗了该Event，则返回true，否则Event最后被Activity的onTouchEvent()消耗。

ViewGroup.dispatchTouchEvent()的源码分析如下：

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;            // 处理初始的down事件            if (actionMasked == MotionEvent.ACTION_DOWN) {                //当新开始一个touch事件时，抛弃先前的touch状态                //当app切换，发生ANR或一些其他的touch状态发生时，framework会丢弃或取消先前的touch状态                cancelAndClearTouchTargets(ev);                resetTouchState();            }            // 检查是否进行事件拦截            final boolean intercepted;            if (actionMasked == MotionEvent.ACTION_DOWN                    || mFirstTouchTarget != null) {                final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;                if (!disallowIntercept) {                    //回调onInterceptTouchEvent()，返回false表示不拦截touch，否则拦截touch事件。                    intercepted = onInterceptTouchEvent(ev);                    ev.setAction(action); // restore action in case it was changed                } else {                    intercepted = false;                }            } else {                //没有touch事件的传递对象，同时touch动作不是初始动作down，所以ViewGroup继续拦截事件                intercepted = true;            }            // 检查cancel事件            final boolean canceled = resetCancelNextUpFlag(this)                    || actionMasked == MotionEvent.ACTION_CANCEL;            // 如果有第二个手指touch，更新touch目标列表。touch目标列表是一个View数组            final boolean split = (mGroupFlags & FLAG_SPLIT_MOTION_EVENTS) != 0;            TouchTarget newTouchTarget = null;            boolean alreadyDispatchedToNewTouchTarget = false;            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);                        // 找到一个能接受Event的子View，再对子View的View树进行遍历                        final View[] children = mChildren;                        final boolean customOrder = isChildrenDrawingOrderEnabled();                        //判断每个子View是否是TouchTarget，若是则添加到TouchTarget链表中                        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) {                                // 若子View处于touch目标中，同时已经接收了touch事件，则为器增加新的touch点                                newTouchTarget.pointerIdBits |= idBitsToAssign;                                break;                            }                            resetCancelNextUpFlag(child);                            //把MotionEvent的点坐标转换到子View的坐标系中，为ViewGroup创建一个新TouchTarget，TouchTarget包含了子View                            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();                                newTouchTarget = addTouchTarget(child, idBitsToAssign);                                alreadyDispatchedToNewTouchTarget = true;                                break;                            }                        }                    }                    if (newTouchTarget == null && mFirstTouchTarget != null) {                        // 没有发现接收event的子View，把Touch点赋给最早添加到TouchTarget链中的对象                        newTouchTarget = mFirstTouchTarget;                        while (newTouchTarget.next != null) {                            newTouchTarget = newTouchTarget.next;                        }                        newTouchTarget.pointerIdBits |= idBitsToAssign;                    }                }            }            // 传递给touch目标            if (mFirstTouchTarget == null) {                // 若没有Touch目标，则把自己当成一个View，调用                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;                    //若已被处理，则忽略。                    if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) {                        handled = true;                    } else {                        final boolean cancelChild = resetCancelNextUpFlag(target.child)                                || intercepted;                        //传递给子View处理                        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;                }            }            // 若在触摸点发生了up或cancel，则更新TouchTarget链表            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;    }

ViewGroup中将TouchEvent传递给子View的函数为dispatchTransformedTouchEvent()，源代码如下：

 /**     * 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.        // cancel动作是个特列，无需坐标转换或过滤。        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;        }        // 计算将被传递的点的数量。        final int oldPointerIdBits = event.getPointerIdBits();        final int newPointerIdBits = oldPointerIdBits & desiredPointerIdBits;        // Motion事件没有对应点，则丢弃这个Motion        if (newPointerIdBits == 0) {            return false;        }        /*若点的数量一致则无需进行不相关的点坐标转换，调用子View的dispatchTouchEvent*/        // 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.        /*该变量用于保存坐标转换后的MoetionEvent*/        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;                        /*直接对MotionEvent进行坐标变换，将MotionEvent传递下去*/                    event.offsetLocation(offsetX, offsetY);                    handled = child.dispatchTouchEvent(event);                    /*回复MotionEvent*/                    event.offsetLocation(-offsetX, -offsetY);                }                return handled;            }            transformedEvent = MotionEvent.obtain(event);        } else {            transformedEvent = event.split(newPointerIdBits);        }        // Perform any necessary transformations and dispatch.        if (child == null) {                /*调用父类即View的dispatchTouchEvent方法，该方法会调用onTouchEvent*/            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());            }            /*传递给子View处理*/            handled = child.dispatchTouchEvent(transformedEvent);        }        // Done.        transformedEvent.recycle();        return handled;    }

View对象的dispatchTouchEvent代码如下：

  /**     * 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;            /*先调用listener接口*/            if (li != null && li.mOnTouchListener != null && (mViewFlags & ENABLED_MASK) == ENABLED                    && li.mOnTouchListener.onTouch(this, event)) {                return true;            }           /*若MotionEvent未被消耗，则调用View的onTouchEvent *            * ViewGroup中没有定义onTouchEvent，故做后调用View中的onTouchEvent*/            if (onTouchEvent(event)) {                return true;            }        }        if (mInputEventConsistencyVerifier != null) {            mInputEventConsistencyVerifier.onUnhandledEvent(event, 0);        }        return false;    }

小结：

onInterceptTouchEvent:

onInterceptTouchEvent是在ViewGroup里面定义的，被ViewGroup.dispatchTouchEvent()调用，用于拦截所有的touch事件。默认返回false，表示不拦截touch事件，ViewGroup.dispatchTouchEvent()会调用子View的dispatchTouchEvent,将touch事件传递到子View中。若子View的dispatchTouchEvent 返回false，则ViewGroup的onTouchEvent会被调用；若子View的dispatchTouchEvent 返回true，表示消耗了手势事件，ViewGroup的onTouchEvent则不会被调用。若ViewGroup.onInterceptTouchEvent()返回true，表示Touch事件被拦截，ViewGroup. dispatchTransformedTouchEvent()函数将被调用，该函数会调用super.dispatchTouchEvent(event)，即View的dispatchEvent()，该函数首先会调用View.OnTouchListener.onTouch().若listener未消耗Touch事件，则会调用View.onTouchEvent().

onTouchEvent:

view中定义的方法onTouchEvent默认返回true,表示消耗了一个touch事件，ViewGroup中定义的onTouchEvent默认返回false，表示不处理Touch手势事件。手势事件类型包括ACTION_DOWN,ACTION_MOVE,ACTION_UP,ACTION_CANCEL等事件。

本节及后续都是参考了一篇国外讲义，下载地址：http://download.csdn.net/detail/bigconvience/7376431

更多相关文章

随机推荐