HorizontalDragLayout-模仿QQ客户端的Item滑动删除

首先感谢http://blog.csdn.net/lmj623565791/article/details/46858663hongyang的文章，之前看过ViewDragHelper类也读过一些demo一直都是半知半解且之前一些自定义的ViewGroup大都不是按这种方式来写，这一次抓紧一次自己写一个demo熟悉ViewDragHelper。ViewDragHelper存在于v4包种，目的用于帮助我们自定义ViewGroup。

先上效果图： 

第一步,声明ViewDragHelper:

<code class="hljs cpp has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">ViewDragHelper.create(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1.0f</span>, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> ViewDragCallback());</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul>

其中1.0f代表最小滑动距离touchSlop是系统默认的多少倍数越大的话越灵敏。

第二步，继承ViewDragHelper.CallBack实现里面的方法来满足滑动需求。

<code class="hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> <span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">ViewDragCallback</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">extends</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">ViewDragHelper</span>.<span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">Callback</span> {</span>

        <span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">boolean</span> <span class="hljs-title" style="box-sizing: border-box;">tryCaptureView</span>(View view, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> i) {
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> view == mContentView || view == mDeleteView;
        }

        <span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> <span class="hljs-title" style="box-sizing: border-box;">clampViewPositionHorizontal</span>(View child, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> left, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> dx) {
            Log.w(TAG, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"clampViewPositionHorizontal = "</span> + left + <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"/"</span> + dx);
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> realLeft = left;
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (child == mContentView) {
                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (left > <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>) {<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// right scroll</span>
                    realLeft = left > mDeleteView.getWidth() ? mDeleteView.getWidth() : left;
                } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (left < <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>) {<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// left scroll</span>
                    realLeft = Math.abs(left) > mDeleteView.getWidth() ? -mDeleteView.getWidth() : left;
                }
            }
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> realLeft;
        }

        <span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">onViewReleased</span>(View releasedChild, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">float</span> xvel, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">float</span> yvel) {
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">super</span>.onViewReleased(releasedChild, xvel, yvel);
            Log.w(TAG, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"--- scroll finish when finger up ---"</span>);
            Log.w(TAG, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"onViewReleased = "</span> + xvel + <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"/"</span> + yvel);
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (releasedChild == mContentView) {
                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (xvel > <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>) {
                    mDragHlper.settleCapturedViewAt(point.x, point.y);
                } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> {
                    mDragHlper.settleCapturedViewAt(point.x - mDeleteView.getWidth(), point.y);
                }
                invalidate();
            }
        }

        <span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">onViewPositionChanged</span>(View changedView, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> left, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> top, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> dx, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> dy) {
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">super</span>.onViewPositionChanged(changedView, left, top, dx, dy);
            Log.w(TAG, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"--- when view position changed = "</span> + left + <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"/"</span> + dx);
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (changedView == mContentView) {
                mDeleteView.offsetLeftAndRight(dx);
            } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (changedView == mDeleteView) {
                mContentView.offsetLeftAndRight(dx);
            }
            invalidate();
        }
    }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li><li style="box-sizing: border-box; padding: 0px 5px;">37</li><li style="box-sizing: border-box; padding: 0px 5px;">38</li><li style="box-sizing: border-box; padding: 0px 5px;">39</li><li style="box-sizing: border-box; padding: 0px 5px;">40</li><li style="box-sizing: border-box; padding: 0px 5px;">41</li><li style="box-sizing: border-box; padding: 0px 5px;">42</li><li style="box-sizing: border-box; padding: 0px 5px;">43</li><li style="box-sizing: border-box; padding: 0px 5px;">44</li><li style="box-sizing: border-box; padding: 0px 5px;">45</li><li style="box-sizing: border-box; padding: 0px 5px;">46</li><li style="box-sizing: border-box; padding: 0px 5px;">47</li><li style="box-sizing: border-box; padding: 0px 5px;">48</li></ul>

tryCaptureView：这个方法只有返回true才能进行滑动，否则没有效果。 
onViewReleased：这个方法指当手放开后监听当速度满足需求的时候直接滑动到设定的地方。 
onViewPositionChanged：当被CaptureView滑动的时候设置DeleteView也移动到相对应的地方。 
clampViewPositionHorizontal：设置这个ViewGoup中的ContentView的滑动区域。

第三步，复写手指控制方法：

<code class="hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">boolean</span> <span class="hljs-title" style="box-sizing: border-box;">onInterceptTouchEvent</span>(MotionEvent ev) {
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> action = MotionEventCompat.getActionMasked(ev);
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (action == MotionEvent.ACTION_CANCEL || action == MotionEvent.ACTION_UP) {
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">false</span>;
        }
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> mDragHlper.shouldInterceptTouchEvent(ev);
    }

    <span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">boolean</span> <span class="hljs-title" style="box-sizing: border-box;">onTouchEvent</span>(MotionEvent event) {
        mDragHlper.processTouchEvent(event);
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">true</span>;
    }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li></ul>

关于ViewDragHelper有如下几点：

ViewDragHelper.Callback是连接ViewDragHelper与view之间的桥梁（这个view一般是指拥子view的容器即parentView）；

ViewDragHelper的实例是通过静态工厂方法创建的；

你能够指定拖动的方向；

ViewDragHelper可以检测到是否触及到边缘；

ViewDragHelper并不是直接作用于要被拖动的View，而是使其控制的视图容器中的子View可以被拖动，如果要指定某个子view的行为，需要在Callback中想办法；

ViewDragHelper的本质其实是分析onInterceptTouchEvent和onTouchEvent的MotionEvent参数，然后根据分析的结果去改变一个容器中被拖动子View的位置（ 通过offsetTopAndBottom(int offset)和offsetLeftAndRight(int offset)方法 ），他能在触摸的时候判断当前拖动的是哪个子View；

虽然ViewDragHelper的实例方法 ViewDragHelper create(ViewGroup forParent, Callback cb) 可以指定一个被ViewDragHelper处理拖动事件的对象 ，但ViewDragHelper类的设计决定了其适用于被包含在一个自定义ViewGroup之中，而不是对任意一个布局上的视图容器使用ViewDragHelper。