VelocityTracker简介

淩亂°似流年 2022-04-18 05:14 161阅读 0赞

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翻译自： [http://developer.android.com/reference/android/view/VelocityTracker.html][http_developer.android.com_reference_android_view_VelocityTracker.html]

参照自：  [http://blog.jrj.com.cn/4586793646,5298605a.html][http_blog.jrj.com.cn_4586793646_5298605a.html]

android.view.VelocityTracker 主要用跟踪触摸屏事件（flinging事件和其他gestures手势事件）的速率。用 addMovement(MotionEvent) 函数将Motion event加入到VelocityTracker类实例中.你可以使用 getXVelocity()  或 getXVelocity() 获得横向和竖向的速率到速率时，但是使用它们之前请先调用 computeCurrentVelocity (int) 来初始化速率的单位 。

主要函数

<table> 
 <tbody> 
  <tr> 
   <th colspan="12">Public Methods</th> 
  </tr> 
  <tr> 
   <td>void</td> 
   <td><a href="http://developer.android.com/reference/android/view/VelocityTracker.html#addMovement%28android.view.MotionEvent%29" rel="nofollow">addMovement</a>(<a href="http://developer.android.com/reference/android/view/MotionEvent.html" rel="nofollow">MotionEvent</a>&nbsp;event) 
    <div>
      Add a user's movement to the tracker. 
    </div></td> 
  </tr> 
  <tr> 
   <td>void</td> 
   <td><a href="http://developer.android.com/reference/android/view/VelocityTracker.html#clear%28%29" rel="nofollow">clear</a>() 
    <div>
      Reset the velocity tracker back to its initial state. 
    </div></td> 
  </tr> 
  <tr> 
   <td>void</td> 
   <td><a href="http://developer.android.com/reference/android/view/VelocityTracker.html#computeCurrentVelocity%28int,%20float%29" rel="nofollow">computeCurrentVelocity</a>(int&nbsp;units, float maxVelocity) 
    <div>
      Compute the current velocity based on the points that have been collected. 
    </div> 
    <div>
      int&nbsp; unitis表示速率的基本时间单位。unitis值为 1的表示是，一毫秒时间单位内运动了多少个像素， unitis值为 1000表示一秒（1000毫秒）时间单位内运动了多少个像素 
    </div> 
    <div>
      float Velocity表示速率的最大值 
    </div></td> 
  </tr> 
  <tr> 
   <td>void</td> 
   <td><a href="http://developer.android.com/reference/android/view/VelocityTracker.html#computeCurrentVelocity%28int%29" rel="nofollow">computeCurrentVelocity</a>(int units) 
    <div>
      Equivalent to invoking&nbsp; 
     <code><a href="http://developer.android.com/reference/android/view/VelocityTracker.html#computeCurrentVelocity%28int,%20float%29" rel="nofollow">computeCurrentVelocity(int,&nbsp;float)</a></code>&nbsp;with a maximum velocity of Float.MAX_VALUE. 
    </div></td> 
  </tr> 
  <tr> 
   <td>abstract T</td> 
   <td><a href="http://developer.android.com/reference/android/view/VelocityTracker.html#getNextPoolable%28%29" rel="nofollow">getNextPoolable</a>()</td> 
  </tr> 
  <tr> 
   <td>float</td> 
   <td><a href="http://developer.android.com/reference/android/view/VelocityTracker.html#getXVelocity%28%29" rel="nofollow">getXVelocity</a>() 
    <div>
      Retrieve the last computed X velocity. 
    </div></td> 
  </tr> 
  <tr> 
   <td>float</td> 
   <td><a href="http://developer.android.com/reference/android/view/VelocityTracker.html#getXVelocity%28int%29" rel="nofollow">getXVelocity</a>(int id) 
    <div>
      Retrieve the last computed X velocity. 
    </div></td> 
  </tr> 
  <tr> 
   <td>float</td> 
   <td><a href="http://developer.android.com/reference/android/view/VelocityTracker.html#getYVelocity%28int%29" rel="nofollow">getYVelocity</a>(int id) 
    <div>
      Retrieve the last computed Y velocity. 
    </div></td> 
  </tr> 
  <tr> 
   <td>float</td> 
   <td><a href="http://developer.android.com/reference/android/view/VelocityTracker.html#getYVelocity%28%29" rel="nofollow">getYVelocity</a>() 
    <div>
      Retrieve the last computed Y velocity. 
    </div></td> 
  </tr> 
  <tr> 
   <td>abstract boolean</td> 
   <td><a href="http://developer.android.com/reference/android/view/VelocityTracker.html#isPooled%28%29" rel="nofollow">isPooled</a>()</td> 
  </tr> 
  <tr> 
   <td>static&nbsp;<a href="http://developer.android.com/reference/android/view/VelocityTracker.html" rel="nofollow">VelocityTracker</a></td> 
   <td><a href="http://developer.android.com/reference/android/view/VelocityTracker.html#obtain%28%29" rel="nofollow">obtain</a>() 
    <div>
      Retrieve a new VelocityTracker object to watch the velocity of a motion. 
    </div></td> 
  </tr> 
  <tr> 
   <td>void</td> 
   <td><a href="http://developer.android.com/reference/android/view/VelocityTracker.html#recycle%28%29" rel="nofollow">recycle</a>() 
    <div>
      Return a VelocityTracker object back to be re-used by others. 
    </div></td> 
  </tr> 
  <tr> 
   <td>abstract void</td> 
   <td><a href="http://developer.android.com/reference/android/view/VelocityTracker.html#setNextPoolable%28T%29" rel="nofollow">setNextPoolable</a>(T element)</td> 
  </tr> 
  <tr> 
   <td>abstract void</td> 
   <td><a href="http://developer.android.com/reference/android/view/VelocityTracker.html#setPooled%28boolean%29" rel="nofollow">setPooled</a>(boolean isPooled)</td> 
  </tr> 
 </tbody> 
</table>

示例：

private VelocityTracker mVelocityTracker;//生命变量

//在onTouchEvent(MotionEvent ev)中

if (mVelocityTracker == null) \{

mVelocityTracker = VelocityTracker. obtain () ;//获得VelocityTracker类实例

mVelocityTracker. addMovement (ev); //将事件加入到VelocityTracker类实例中

//判断当ev事件是MotionEvent.ACTION\_UP时：计算速率

final VelocityTracker velocityTracker = mVelocityTracker;

// 1000 provides pixels per second

velocityTracker.computeCurrentVelocity(1, (float)0.01) ;  //设置maxVelocity值为0.1时，速率大于0.01时，显示的速率都是0.01,速率小于0.01时，显示正常

Log.i("test","velocityTraker"+velocityTracker.getXVelocity());

velocityTracker.computeCurrentVelocity(1000);  //设置units的值为1000，意思为一秒时间内运动了多少个像素

Log.i("test","velocityTraker"+velocityTracker.getXVelocity());

大体的使用是这样的：

当你需要跟踪触摸屏事件的速度的时候,使用 obtain() 方法来获得 VelocityTracker 类的一个实例对象

在 onTouchEvent 回调函数中，使用 addMovement(MotionEvent) 函数将当前的移动事件传递给 VelocityTracker 对象

使用 computeCurrentVelocity  (int units) 函数来计算当前的速度，使用  getXVelocity  () 、  getYVelocity  () 函数来获得当前的速度

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![这里写图片描述][SouthEast]

[http_blog.csdn.net_jiangjunshow]: https://blog.csdn.net/jiangjunshow/article/details/77338485
[http_developer.android.com_reference_android_view_VelocityTracker.html]: http://developer.android.com/reference/android/view/VelocityTracker.html
[http_blog.jrj.com.cn_4586793646_5298605a.html]: http://blog.jrj.com.cn/4586793646,5298605a.html
[SouthEast]: /images/20220418/70a8c3401a954c29a615da53b5805276.png