* Android provides a class to help handle touch screen sequences,and that class is VelocityTracker.
* When a finger is in motion on a touch screen,it might be nice to know how fast it is moving
across the surfaces.
* For Example, if the user is dragging an object across the screen and lets go,your application
probably wants to show that object flying across the screen accordingly.
* Android provides VelocityTracker to help with the math involved.
* To use VelocityTracker, you first get an instance of a VelocityTracker by calling the static method
* You can then add MotionEvent objects to it with the addMovement(MotionEvent ev) method.
* You would call this method in your handler that receives MotionEvent objects,from a handler
method such as onTouch(),or from a view's onTouchEvent().
* The VelocityTracker uses the MotionEvent objects to figure out what is going on with the user's
* Once VelocityTracker has at least two MotionEvent objects in it.
* The two VelocityTracker methods- getXVelocity() and getYVelocity() - return the corresponding velocity of the finger in the X and Y directions.