Android高级UI之京东淘宝首页二级联动怎么实现
本文的主题是了解View的事件分发整体流程，然后重点实现京东淘宝首页二级联动，解决滑动冲突。
1、事件的种类和手势
1.1 单点触摸
根据面向对象思想，事件被封装成 MotionEvent 对象
1.2 多点触摸
多点触控 ( Multitouch，也称 Multi-touch )，即同时接受屏幕上多个点的人机交互操作，多点触 控是从 Android 2.0 开始引入的功能
1.3 手势
1.4 多点手势手指操作流程
2、View的体系结构和事件分发的框架
2.1 View和ViewGroup的关系
2.2 Android页面View的体系结构
2.3 事件的处理函数
2.4 事件的处理函数的关系
2.5.1 事件分发大流程
上图的注意点：
1.事件返回时 dispatchTouchEvent 直接指向了父View的 onTouchEvent 这一部分是不合理的，实际上它仅仅是给了父View的 dispatchTouchEvent 一个 false 返回值， 父View根据返回值来调用自身的 onTouchEvent。
2.ViewGroup的dispatchTouchEvent是根据 onInterceptTouchEvent 的返回值来确定是调用子View的 dispatchTouchEvent 还是调用自身的 onTouchEvent， 并没有将调用交给onInterceptTouchEvent
ViewGroup的dispatchTouchEvent事件分发简化流程：
public boolean dispatchTouchEvent(MotionEvent ev) {
boolean result = false; // 默认状态为没有消费过
if (!onInterceptTouchEvent(ev)) {
// 如果没有拦截，则交给子View
result = child.dispatchTouchEvent(ev);
}
if (!result) {
// 如果事件没有被消费，则询问自身onTouchEvent
result = onTouchEvent(ev);
}
return result;
}
2.5.2 事件分发大流程(View消费了事件）
2.5.3 事件分发大流程(ViewGroup消费了事件）
3、View和ViewGroup的分发流程
3.1 事件分发相关概念
3.2.1 事件分发极简流程
public boolean dispatchTouchEvent(MotionEvent ev) {
boolean result = false; // 默认状态为没有消费过
if (!onInterceptTouchEvent(ev)) {
// 如果没有拦截，则交给子View
result = child.dispatchTouchEvent(ev);
}
if (!result) {
// 如果事件没有被消费，则询问自身onTouchEvent
result = onTouchEvent(ev);
}
return result;
}
3.2.2 事件分发进阶流程
public boolean dispatchTouchEvent(MotionEvent ev) {
// 默认状态为没有消费过
boolean result = false;
//决定是否拦截
final boolean intercepted = false;
if (!requestDisallowInterceptTouchEvent()) {
intercepted = onInterceptTouchEvent(ev);
}
//找出最适合接收的孩子
if (!intercepted && (DOWN || POINTER_DOWN || HOVER_MOVE)) {
// 如果没有拦截交给子View
for (int i = childrenCount - 1; i >= 0; i--) {
mFirstTouchTarget = child.dispatchTouchEvent(ev);
}
}
//分发事件
if (mFirstTouchTarget == null) {
// 如果事件没有被消费,询问自身onTouchEvent
result = onTouchEvent(ev);
} else {
for(TouchTarget touchTarget : mFirstTouchTarget) {
result = touchTarget.child.dispatchTouchEvent(ev);
}
}
return result;
}
3.2.3 事件分发简单源码分析
/**
整体流程就是分3步：
步骤1：判断事件是否拦截
步骤2：遍历所有的子View，寻找targets
步骤3： 将事件分发给targets
*/
@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
...
boolean handled = false;
if (onFilterTouchEventForSecurity(ev)) {
final int action = ev.getAction();
final int actionMasked = action & MotionEvent.ACTION_MASK;
// ACTION_DOWN事件是一个手势的开始，所以这里会清空之前的手势的所有状态
// 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();
}
//步骤1：判断事件是否拦截
// 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;
}
...
//步骤2：遍历所有的子View，寻找targets
if (!canceled && !intercepted) {
...
//如果是一个手势的开始事件，MotionEvent.ACTION_DOWN、 MotionEvent.ACTION_POINTER_DOWN、MotionEvent.ACTION_HOVER_MOVE都是一个手势的开始事件
if (actionMasked == MotionEvent.ACTION_DOWN
|| (split && actionMasked == MotionEvent.ACTION_POINTER_DOWN)
|| actionMasked == MotionEvent.ACTION_HOVER_MOVE) {
...
for (int i = childrenCount - 1; i >= 0; i--) {
//遍历所有的子View，找到TouchTarget
if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {
...
//找到TouchTarget后，加入mFirstTouchTarget链表
newTouchTarget = addTouchTarget(child, idBitsToAssign);
...
}
...
}
...
}
// 步骤3： 将事件分发给targets
// 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.
...
}
}
}
所有的TouchTarget构成一个链表，mFirstTouchTarget指向的是链表的头节点：
// First touch target in the linked list of touch targets.
@UnsupportedAppUsage
private TouchTarget mFirstTouchTarget;
/* Describes a touched view and the ids of the pointers that it has captured.
*
* This code assumes that pointer ids are always in the range 0..31 such that
* it can use a bitfield to track which pointer ids are present.
* As it happens, the lower layers of the input dispatch pipeline also use the
* same trick so the assumption should be safe here...
*/
private static final class TouchTarget {
private static final int MAX_RECYCLED = 32;
private static final Object sRecycleLock = new Object[0];
private static TouchTarget sRecycleBin;
private static int sRecycledCount;
public static final int ALL_POINTER_IDS = -1; // all ones
// The touched child view.
@UnsupportedAppUsage
public View child;
// The combined bit mask of pointer ids for all pointers captured by the target.
public int pointerIdBits;
// The next target in the target list.
public TouchTarget next;
@UnsupportedAppUsage
private TouchTarget() {
}
public static TouchTarget obtain(@NonNull View child, int pointerIdBits) {
if (child == null) {
throw new IllegalArgumentException("child must be non-null");
}
final TouchTarget target;
synchronized (sRecycleLock) {
if (sRecycleBin == null) {
target = new TouchTarget();
} else {
target = sRecycleBin;
sRecycleBin = target.next;
sRecycledCount--;
target.next = null;
}
}
target.child = child;
target.pointerIdBits = pointerIdBits;
return target;
}
public void recycle() {
if (child == null) {
throw new IllegalStateException("already recycled once");
}
synchronized (sRecycleLock) {
if (sRecycledCount < MAX_RECYCLED) {
next = sRecycleBin;
sRecycleBin = this;
sRecycledCount += 1;
} else {
next = null;
}
child = null;
}
}
}
记录着每个手指触摸屏幕时的接收down事件的那些View，所以用链表。
如果多个手指跨越了多个View，则mFirstTouchTarget指向的链表就有多个节点；如果多个手指都在一个View上，则mFirstTouchTarget指向的链表只有一个结点。
3.3.1 View dispatchTouchEvent
1: 为什么 View 会有 dispatchTouchEvent ?
我们知道 View 可以注册很多事件监听器，例如：单击事件(onClick)、长按事件(onLongClick)、触摸事件(onTouch)，并且View自身也有 onTouchEvent() 方法，那么问题来了，这么多与事件相关的方法应该由谁管理？毋庸置疑就是 dispatchTouchEvent()，所以 View 也会有事件分发。
2: 与 View 事件相关的各个方法的调用顺序是怎样的？
•单击事件(onClickListener) 需要两个两个事件(ACTION_DOWN 和 ACTION_UP )才能触发，如果先分配给onClick判断，等它判断完，用户手指已经离开屏幕，黄花菜都凉了，定然造成 View 无法响应其他事件，应该最后调用。(最后)
•长按事件(onLongClickListener) 同理，也是需要长时间等待才能出结果，肯定不能排到前面，但因为不需要ACTION_UP，应该排在 onClick 前面。(onLongClickListener > onClickListener)
•触摸事件(onTouchListener) 如果用户注册了触摸事件，说明用户要自己处理触摸事件了，这个应该排在最前面。(最前)
•View自身处理(onTouchEvent()方法)
View提供了一种默认的处理方式，如果用户已经处理好了，也就不需要了，所以应该排在 onTouchListener 后面。(onTouchListener > onTouchEvent())
所以调用顺序是：onTouchListener > onTouchEvent() > onLongClickListener > onClickListener
3.3.2 View和ViewGroup的onTouchEvent
3.4.1 ViewGroup的onInterceptTouchEvent
/*
* ....
*
* @param ev The motion event being dispatched down the hierarchy.
* @return Return true to steal motion events from the children and have
* them dispatched to this ViewGroup through onTouchEvent().
* The current target will receive an ACTION_CANCEL event, and no further
* messages will be delivered here.
*/
public boolean onInterceptTouchEvent(MotionEvent ev) {
if (ev.isFromSource(InputDevice.SOURCE_MOUSE)
&& ev.getAction() == MotionEvent.ACTION_DOWN
&& ev.isButtonPressed(MotionEvent.BUTTON_PRIMARY)
&& isOnScrollbarThumb(ev.getX(), ev.getY())) {
return true;
}
return false;
}
3.5.1 ViewGroup的dispatchTouchEvent的成员变量
TouchTarget和mFirstTouchTarget, 这个变量是给手势设置的，跨越事件保留的。
/* Describes a touched view and the ids of the pointers that it has captured.
*
* This code assumes that pointer ids are always in the range 0..31 such that
* it can use a bitfield to track which pointer ids are present.
* As it happens, the lower layers of the input dispatch pipeline also use the
* same trick so the assumption should be safe here...
*/
private static final class TouchTarget {
private static final int MAX_RECYCLED = 32;
private static final Object sRecycleLock = new Object[0];
private static TouchTarget sRecycleBin;
private static int sRecycledCount;
public static final int ALL_POINTER_IDS = -1; // all ones
// The touched child view.
@UnsupportedAppUsage
public View child;
// The combined bit mask of pointer ids for all pointers captured by the target.
public int pointerIdBits;
// The next target in the target list.
public TouchTarget next;
@UnsupportedAppUsage
private TouchTarget() {
}
public static TouchTarget obtain(@NonNull View child, int pointerIdBits) {
if (child == null) {
throw new IllegalArgumentException("child must be non-null");
}
final TouchTarget target;
synchronized (sRecycleLock) {
if (sRecycleBin == null) {
target = new TouchTarget();
} else {
target = sRecycleBin;
sRecycleBin = target.next;
sRecycledCount--;
target.next = null;
}
}
target.child = child;
target.pointerIdBits = pointerIdBits;
return target;
}
public void recycle() {
if (child == null) {
throw new IllegalStateException("already recycled once");
}
synchronized (sRecycleLock) {
if (sRecycledCount < MAX_RECYCLED) {
next = sRecycleBin;
sRecycleBin = this;
sRecycledCount += 1;
} else {
next = null;
}
child = null;
}
}
}
3.5.2局部变量: newTouchTarget & alreadyDispatchedToNewTouchTarget
3.6.1 辅助功能view的分发逻辑
1、如果现在分发的这个事件就是分发给我这个辅助功能获焦的view的，那么我们立即进行正常的分发，同时清除这个MotionEvent事件FLAG_TARGET_ACCESSIBILITY_FOCUS标志， 请注意这个标志是事件的，而不是view的
2、如果这个事件被该ViewGroup拦截，或者已经有子view正在处理这个手势，我们清除这个事件的FLAG_TARGET_ACCESSIBILITY_FOCUS标志，进行正常的分发
4、滑动冲突解决方案
4.1 滑动冲突解决方案
4.2 外部拦截
•当ViewPager接收到DOWN事件，ViewPager默认不拦截DOWN事件，DOWN事件交由ListView处理，由于ListView可以滚动，即可以消费事件，则ViewPager的 mFirstTouchTarget会被赋值，即找到处理事件的子View。然后ViewPager接收到MOVE事件，
•若此事件是ViewPager不需要，则同样会将事件交由ListView去处理，然后ListView处理事件； •若此事件ViewGroup需要，因为DOWN事件被ListView处理，mFirstTouchEventTarget会被赋值，也就会调用onInterceptedTouchEvent,此时由于ViewPager对此事件感兴趣 ，则onInterceptedTouchEvent方法会返回true，表示ViewPager会拦截事件，此时当前的MOVE事件会消失，变为CANCEL事件，往下传递或者自己处理，同时 mFirstTouchTarget被重置为null。
•当MOVE事件再次来到时，由于mFristTouchTarget为null，所以接下来的事件都交给了ViewPager。
4.3 内部拦截
4.4 嵌套滑动
页面布局：
嵌套滑动有两个角色，一个是父亲，一个是孩子，这里的父亲是ScrollView，孩子是RecyclerView。
在滑动ViewPager里面的RecyclerView时，需要先判断ScrollView需不需要滑动。
Android事件分发之ACTION_CANCEL机制及作用
深入理解事件分发 ViewGroup.mFirstTouchTarget的设计
Android事件分发mFirstTouchTarget的思考