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Android APIs
public final class

VelocityTracker

extends Object
java.lang.Object
   ↳ android.view.VelocityTracker

Class Overview

Helper for tracking the velocity of touch events, for implementing flinging and other such gestures. Use 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 addMovement(MotionEvent) . When you want to determine the velocity call computeCurrentVelocity(int) and then call getXVelocity(int) and getYVelocity(int) to retrieve the velocity for each pointer id.

Summary

Public Methods
void addMovement ( MotionEvent event)
Add a user's movement to the tracker.
void clear ()
Reset the velocity tracker back to its initial state.
void computeCurrentVelocity (int units)
Equivalent to invoking computeCurrentVelocity(int, float) with a maximum velocity of Float.MAX_VALUE.
void computeCurrentVelocity (int units, float maxVelocity)
Compute the current velocity based on the points that have been collected.
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.
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.
Protected Methods
void finalize ()
Invoked when the garbage collector has detected that this instance is no longer reachable.
[Expand]
Inherited Methods
From class java.lang.Object

Public Methods

public void addMovement ( MotionEvent event)

Added in API level 1

Add a user's movement to the tracker. You should call this for the initial ACTION_DOWN , the following ACTION_MOVE events that you receive, and the final ACTION_UP . You can, however, call this for whichever events you desire.

Parameters
event The MotionEvent you received and would like to track.

public void clear ()

Added in API level 1

Reset the velocity tracker back to its initial state.

public void computeCurrentVelocity (int units)

Added in API level 1

Equivalent to invoking computeCurrentVelocity(int, float) with a maximum velocity of Float.MAX_VALUE.

See Also

public void computeCurrentVelocity (int units, float maxVelocity)

Added in API level 4

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 getXVelocity() and getYVelocity() .

Parameters
units The units you would like the velocity in. A value of 1 provides pixels per millisecond, 1000 provides pixels per second, etc.
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.

public float getXVelocity ()

Added in API level 1

Retrieve the last computed X velocity. You must first call computeCurrentVelocity(int) before calling this function.

Returns
  • The previously computed X velocity.

public float getXVelocity (int id)

Added in API level 8

Retrieve the last computed X velocity. You must first call computeCurrentVelocity(int) before calling this function.

Parameters
id Which pointer's velocity to return.
Returns
  • The previously computed X velocity.

public float getYVelocity (int id)

Added in API level 8

Retrieve the last computed Y velocity. You must first call computeCurrentVelocity(int) before calling this function.

Parameters
id Which pointer's velocity to return.
Returns
  • The previously computed Y velocity.

public float getYVelocity ()

Added in API level 1

Retrieve the last computed Y velocity. You must first call computeCurrentVelocity(int) before calling this function.

Returns
  • The previously computed Y velocity.

public static VelocityTracker obtain ()

Added in API level 1

Retrieve a new VelocityTracker object to watch the velocity of a motion. Be sure to call recycle() when done. You should generally only maintain an active object while tracking a movement, so that the VelocityTracker can be re-used elsewhere.

Returns
  • Returns a new VelocityTracker.

public void recycle ()

Added in API level 1

Return a VelocityTracker object back to be re-used by others. You must not touch the object after calling this function.

Protected Methods

protected void finalize ()

Added in API level 1

Invoked when the garbage collector has detected that this instance is no longer reachable. The default implementation does nothing, but this method can be overridden to free resources.

Note that objects that override finalize are significantly more expensive than objects that don't. Finalizers may be run a long time after the object is no longer reachable, depending on memory pressure, so it's a bad idea to rely on them for cleanup. Note also that finalizers are run on a single VM-wide finalizer thread, so doing blocking work in a finalizer is a bad idea. A finalizer is usually only necessary for a class that has a native peer and needs to call a native method to destroy that peer. Even then, it's better to provide an explicit close method (and implement Closeable ), and insist that callers manually dispose of instances. This works well for something like files, but less well for something like a BigInteger where typical calling code would have to deal with lots of temporaries. Unfortunately, code that creates lots of temporaries is the worst kind of code from the point of view of the single finalizer thread.

If you must use finalizers, consider at least providing your own ReferenceQueue and having your own thread process that queue.

Unlike constructors, finalizers are not automatically chained. You are responsible for calling super.finalize() yourself.

Uncaught exceptions thrown by finalizers are ignored and do not terminate the finalizer thread. See Effective Java Item 7, "Avoid finalizers" for more.

Throws
Throwable