Android VelocityTracker使用总结
16lz
2021-01-23
VelocityTracker是一个什么东西呢,查看VelocityTracker源代码发现有如下注释:
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- /**
- * Helper for tracking the velocity of touch events, for implementing
- * flinging and other such gestures.
- *
- * Use {@link #obtain} to retrieve a new instance of the class when you are going
- * to begin tracking. Put the motion events you receive into it with
- * {@link #addMovement(android.view.MotionEvent)}. When you want to determine the velocity call
- * {@link #computeCurrentVelocity(int)} and then call {@link #getXVelocity(int)}
- * and {@link #getYVelocity(int)} to retrieve the velocity for each pointer id.
- *
- * 追踪触摸事件速率,实现flinging和其他手势的帮助类
- *
- * 1、当开始追踪的时候,使用obtain来获取VelocityTracker类的实例
- * 2、把接收到的MotionEvent放入到addMovement(android.view.MotionEvent)中
- * 3、当要确定速度时调用computeCurrentVelocity(int),
- * 使用getXVelocity(int)和getYVelocity(int)来检测每个触摸点id的速率
- */
VelocityTracker是一个帮助追踪触摸事件速率的追踪器,可以追踪fliinging和其他触摸手势。
如何使用VelocityTracker呢,注释中提到如下步骤:
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- 1、当开始追踪的时候,使用obtain来获取VelocityTracker类的实例
- 2、把接受到的MotionEvent放入到addMovement(android.view.MotionEvent)中
- 3、当要确定速度时调用computeCurrentVelocity(int),
- 使用getXVelocity(int)和getYVelocity(int)来检测每个触摸点id的速率
既然VelocityTracker是追踪触摸事件的速度追踪器,当然需要与触摸事件结合使用。
第一步:当你想要追踪触摸事件的速度时,使用private VelocityTracker mVelocityTracker = VelocityTracker.obtain();来获取一个实例,
obtain()方法的源代码:
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- /**
- * Retrieve a new VelocityTracker object to watch the velocity of a
- * motion. Be sure to call {@link #recycle} when done. You should
- * generally only maintain an active object while tracking a movement,
- * so that the VelocityTracker can be re-used elsewhere.
- *
- * @return Returns a new VelocityTracker.
- *
- * 获取一个新的VelocityTracker对象,用于检测一个动作的速率
- * 检测结束时确保调用了recycle()方法来回收VelocityTracker对象
- * 追踪一个移动事件时通常只需要维护一个活动对象,这样VelocityTracker可以被重用
- */
- static public VelocityTracker obtain() {
- VelocityTracker instance = sPool.acquire();
- return (instance != null) ? instance : new VelocityTracker(null);
- }
第二步:把接收到的MotionEvent放入到addMovement(MotionEvent event)方法中,
在初始化MotionEvent的ACTION_DOWN时调用addMovement(MotionEvent event)
然后在ACTION_MOVE和ACTION_UP动作中就可以检测到速度了。
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- /**
- * Add a user's movement to the tracker. You should call this for the
- * initial {@link MotionEvent#ACTION_DOWN}, the following
- * {@link MotionEvent#ACTION_MOVE} events that you receive, and the
- * final {@link MotionEvent#ACTION_UP}. You can, however, call this
- * for whichever events you desire.
- *
- * @param event The MotionEvent you received and would like to track.
- *
- * 添加一个移动事件到追踪器
- * 1.为初始化MotionEvent的ACTION_DOWN动作调用addMovement()
- * 2.接下来,在MotionEvent的ACTION_MOVE和ACTION_UP动作中接收。
- * 不管是哪一个events都可以调用本方法
- */
- public void addMovement(MotionEvent event) {
- if (event == null) {
- throw new IllegalArgumentException("event must not be null");
- }
- nativeAddMovement(mPtr, event);
- }
第三步:想要确定速度的时候调用computeCurrentVelocity(int units)方法。有两个同样的方法,一个带有最大值参数,默认为Float.MAX_VALUE
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- /**
- * Compute the current velocity based on the points that have been
- * collected. Only call this when you actually want to retrieve velocity
- * information, as it is relatively expensive. You can then retrieve
- * the velocity with {@link #getXVelocity()} and
- * {@link #getYVelocity()}.
- * 根据收集到的触摸点计算当前速率
- * 因为此方法相当消耗性能,所以只有当真的确实想要检测速度信息的时候才调用这个方法
- * 然后可以通过getXVelocity()和getYVelocity()方法获取到追踪的速度
- *
- * @param units The units you would like the velocity in. A value of 1
- * provides pixels per millisecond, 1000 provides pixels per second, etc.
- * units :代表速度的单位,
- * 值为1时:代表每毫秒运动一个像素,px/ms
- * 值为1000时:代表每秒运动1000个像素,1000px/s
- * @param maxVelocity The maximum velocity that can be computed by this method.
- * This value must be declared in the same unit as the units parameter. This value
- * must be positive.
- * maxVelocity :代表可以被本方法计算的最大速度,
- * 这个值必须用同一个作为速度参数的单位声明,而且值必须为正数
- */
- public void computeCurrentVelocity(int units, float maxVelocity) {
- nativeComputeCurrentVelocity(mPtr, units, maxVelocity);
- }
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- /**
- * Equivalent to invoking {@link #computeCurrentVelocity(int, float)} with a maximum
- * velocity of Float.MAX_VALUE.
- *
- * @see #computeCurrentVelocity(int, float)
- *
- * 等价调用带有最大值为Float.MAX_VALUE的computeCurrentVelocity(int, float)方法,
- */
- public void computeCurrentVelocity(int units) {
- nativeComputeCurrentVelocity(mPtr, units, Float.MAX_VALUE);
- }
第四步:使用getXVelocity()和getYVelocity()方法获取检测到的速度
获取X轴方向的速度:
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- /**
- * Retrieve the last computed X velocity. You must first call
- * {@link #computeCurrentVelocity(int)} before calling this function.
- *
- * 检测最后计算的X轴方向的速度,在调用getXVelocity()之前必须先调用computeCurrentVelocity(int)
- * @return The previously computed X velocity.
- */
- public float getXVelocity() {
- return nativeGetXVelocity(mPtr, ACTIVE_POINTER_ID);
- }
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- /**
- * Retrieve the last computed Y velocity. You must first call
- * {@link #computeCurrentVelocity(int)} before calling this function.
- *
- * 检测最后计算的X轴方向的速度,在调用getXVelocity()之前必须先调用computeCurrentVelocity(int)
- * @return The previously computed Y velocity.
- */
- public float getYVelocity() {
- return nativeGetYVelocity(mPtr, ACTIVE_POINTER_ID);
- }
第五步:回收VelocityTracker实例
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- /**
- * Return a VelocityTracker object back to be re-used by others. You must
- * not touch the object after calling this function.
- *
- * 回收一个VelocityTracker对象给其他动作事件使用
- * 在调用recycle()这个函数之后,你不能访问VelocityTracker对象
- */
- public void recycle() {
- if (mStrategy == null) {
- clear();
- sPool.release(this);
- }
- }
小实例:
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- package com.zwc.admin.scrollerdemo;
- import android.support.v7.app.ActionBarActivity;
- import android.os.Bundle;
- import android.util.Log;
- import android.view.MotionEvent;
- import android.view.VelocityTracker;
- public class VelocityTrackerTestActivity extends ActionBarActivity {
- private static final String TAG = "VelocityTrackerTestActivity";
- private VelocityTracker mVelocityTracker;
- private int mPointerId;
- @Override
- protected void onCreate(Bundle savedInstanceState) {
- super.onCreate(savedInstanceState);
- setContentView(R.layout.activity_velocity_tracker_test);
- }
- @Override
- public boolean onTouchEvent(MotionEvent event) {
- int action = event.getAction();
- if (null == mVelocityTracker) {
- mVelocityTracker = VelocityTracker.obtain();
- }
- mVelocityTracker.addMovement(event);
- switch (action) {
- case MotionEvent.ACTION_DOWN:
- //获取第一个触摸点的id
- mPointerId = event.getPointerId(0);
- break;
- case MotionEvent.ACTION_MOVE:
- mVelocityTracker.computeCurrentVelocity(1000);
- float xVelocity = mVelocityTracker.getXVelocity(event.getPointerId(mPointerId));
- float yVelocity = mVelocityTracker.getYVelocity(event.getPointerId(mPointerId));
- Log.e(TAG, "xVelocity = " + xVelocity + ",yVelocity = " + yVelocity);
- break;
- case MotionEvent.ACTION_UP:
- case MotionEvent.ACTION_CANCEL:
- if (null != mVelocityTracker) {
- mVelocityTracker.recycle();
- }
- break;
- }
- return super.onTouchEvent(event);
- }
- }
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