滑动速度跟踪类VelocityTracker介绍
VelocityTracker 是一个跟踪触摸事件滑动速度的帮助类,用于实现flinging以及其它类似的手势。它的原理是把触摸事件 MotionEvent 对象传递给VelocityTracker的addMovement(MotionEvent)方法,然后分析MotionEvent 对象在单位时间类发生的位移来计算速度。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
Public Methods |
---|
void |
Add a user's movement to the tracker. |
| | void | clear()
Reset the velocity tracker back to its initial state.
| | void | [computeCurrentVelocity](http://developer.android.com/reference/android/view/VelocityTracker.html#computeCurrentVelocity(int, float))(int units, float maxVelocity)
Compute the current velocity based on the points that have been collected.
intunitis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
floatVelocity表示速率的最大值
| | void | computeCurrentVelocity(int units)
Equivalent to invoking [computeCurrentVelocity(int, float)](http://developer.android.com/reference/android/view/VelocityTracker.html#computeCurrentVelocity(int, float))with a maximum velocity of Float.MAX_VALUE.
| | abstract T | getNextPoolable() | | float | getXVelocity()
Retrieve the last computed X velocity.
| | float | getXVelocity(int id)
Retrieve the last computed X velocity.
| | float | getYVelocity(int id)
Retrieve the last computed Y velocity.
| | float | getYVelocity()
Retrieve the last computed Y velocity.
| | abstract boolean | isPooled() | | static VelocityTracker | obtain()
Retrieve a new VelocityTracker object to watch the velocity of a motion.
| | void | recycle()
Return a VelocityTracker object back to be re-used by others.
| | abstract void | setNextPoolable(T element) | | abstract void | setPooled(boolean isPooled) |
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
下面是一个简单Demo:
package com.bxwu.demo.component.activity;
import android.app.Activity;
import android.graphics.Color;
import android.os.Bundle;
import android.view.MotionEvent;
import android.view.VelocityTracker;
import android.view.ViewConfiguration;
import android.view.ViewGroup.LayoutParams;
import android.widget.TextView;
public class VelocityTrackerTest extends Activity {
private TextView mInfo;
private VelocityTracker mVelocityTracker;
private int mMaxVelocity;
private int mPointerId;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
mVelocityTracker = VelocityTracker.obtain();
mMaxVelocity = ViewConfiguration.get(this).getMaximumFlingVelocity();
mInfo = new TextView(this);
mInfo.setLines(4);
mInfo.setLayoutParams(new LayoutParams(LayoutParams.FILL_PARENT, LayoutParams.FILL_PARENT));
mInfo.setTextColor(Color.WHITE);
setContentView(mInfo);
}
@Override
public boolean onTouchEvent(MotionEvent event) {
final int action = event.getAction();
mVelocityTracker.addMovement(event);
final VelocityTracker verTracker = mVelocityTracker;
switch (action) {
case MotionEvent.ACTION_DOWN:
//求第一个触点的id, 此时可能有多个触点,但至少一个
mPointerId = event.getPointerId(0);
break;
case MotionEvent.ACTION_MOVE:
//求伪瞬时速度
verTracker.computeCurrentVelocity(1000, mMaxVelocity);
final float velocityX = verTracker.getXVelocity(mPointerId);
final float velocityY = verTracker.getYVelocity(mPointerId);
recodeInfo(velocityX, velocityY);
break;
case MotionEvent.ACTION_UP:
releaseVelocityTracker();
break;
case MotionEvent.ACTION_CANCEL:
releaseVelocityTracker();
break;
default:
break;
}
return super.onTouchEvent(event);
}
//释放VelocityTracker
private void releaseVelocityTracker() {
if(null != mVelocityTracker) {
mVelocityTracker.clear();
mVelocityTracker.recycle();
mVelocityTracker = null;
}
}
private static final String sFormatStr = "velocityX=%f\\nvelocityY=%f";
/**
* 记录当前速度
*
* @param velocityX x轴速度
* @param velocityY y轴速度
*/
private void recodeInfo(final float velocityX, final float velocityY) {
final String info = String.format(sFormatStr, velocityX, velocityY);
mInfo.setText(info);
}
}
代码很简单,我们可以求出move过程中的伪瞬时速度, 这样在做很多控件的时候都是可以用到的,比如系统Launcher的分页,
ScrollView滑动等, 可根据此时的速度来计算ACTION_UP后的减速运动等。实现一些非常棒的效果。