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VelocityTracker

public final class VelocityTracker
extends Object

java.lang.Object
   ↳ android.view.VelocityTracker


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(int id)

Retrieve the last computed X velocity.

float getXVelocity()

Retrieve the last computed X velocity.

float getYVelocity()

Retrieve the last computed Y velocity.

float getYVelocity(int id)

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()

Called by the garbage collector on an object when garbage collection determines that there are no more references to the object.

Inherited methods

From class java.lang.Object

Public methods

addMovement

Added in API level 1
void addMovement (MotionEvent event)

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 MotionEvent: The MotionEvent you received and would like to track.

clear

Added in API level 1
void clear ()

Reset the velocity tracker back to its initial state.

computeCurrentVelocity

Added in API level 1
void computeCurrentVelocity (int units)

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

Parameters
units int

See also:

computeCurrentVelocity

Added in API level 4
void computeCurrentVelocity (int units, 
                float maxVelocity)

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

getXVelocity

Added in API level 8
float getXVelocity (int id)

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

Parameters
id int: Which pointer's velocity to return.
Returns
float The previously computed X velocity.

getXVelocity

Added in API level 1
float getXVelocity ()

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

Returns
float The previously computed X velocity.

getYVelocity

Added in API level 1
float getYVelocity ()

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

Returns
float The previously computed Y velocity.

getYVelocity

Added in API level 8
float getYVelocity (int id)

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

Parameters
id int: Which pointer's velocity to return.
Returns
float The previously computed Y velocity.

obtain

Added in API level 1
VelocityTracker obtain ()

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
VelocityTracker Returns a new VelocityTracker.

recycle

Added in API level 1
void recycle ()

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

Protected methods

finalize

Added in API level 1
void finalize ()

Called by the garbage collector on an object when garbage collection determines that there are no more references to the object. A subclass overrides the finalize method to dispose of system resources or to perform other cleanup.

The general contract of finalize is that it is invoked if and when the JavaTM virtual machine has determined that there is no longer any means by which this object can be accessed by any thread that has not yet died, except as a result of an action taken by the finalization of some other object or class which is ready to be finalized. The finalize method may take any action, including making this object available again to other threads; the usual purpose of finalize, however, is to perform cleanup actions before the object is irrevocably discarded. For example, the finalize method for an object that represents an input/output connection might perform explicit I/O transactions to break the connection before the object is permanently discarded.

The finalize method of class Object performs no special action; it simply returns normally. Subclasses of Object may override this definition.

The Java programming language does not guarantee which thread will invoke the finalize method for any given object. It is guaranteed, however, that the thread that invokes finalize will not be holding any user-visible synchronization locks when finalize is invoked. If an uncaught exception is thrown by the finalize method, the exception is ignored and finalization of that object terminates.

After the finalize method has been invoked for an object, no further action is taken until the Java virtual machine has again determined that there is no longer any means by which this object can be accessed by any thread that has not yet died, including possible actions by other objects or classes which are ready to be finalized, at which point the object may be discarded.

The finalize method is never invoked more than once by a Java virtual machine for any given object.

Any exception thrown by the finalize method causes the finalization of this object to be halted, but is otherwise ignored.

Throws
Throwable
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