VelocityTracker

Kotlin |Java

class VelocityTracker

kotlin.Any
↳	android.view.VelocityTracker

Helper for tracking the velocity of motion 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(android.view.MotionEvent). When you want to determine the velocity, call computeCurrentVelocity(int) and then call the velocity-getter methods like getXVelocity(int), getYVelocity(int), or getAxisVelocity(int,int) to retrieve velocity for different axes and/or pointer IDs.

Summary

Public methods
Unit	`addMovement(event: MotionEvent!)` Add a user's movement to the tracker.
Unit	`clear()` Reset the velocity tracker back to its initial state.
Unit	`computeCurrentVelocity(units: Int)` Equivalent to invoking `computeCurrentVelocity(int,float)` with a maximum velocity of Float.
Unit	`computeCurrentVelocity(units: Int, maxVelocity: Float)` Compute the current velocity based on the points that have been collected.
Float	`getAxisVelocity(axis: Int)` Equivalent to calling `getAxisVelocity(int,int)` for `axis` and the active pointer.
Float	`getAxisVelocity(axis: Int, id: Int)` Retrieve the last computed velocity for a given motion axis.
Float	`getXVelocity()` Retrieve the last computed X velocity.
Float	`getXVelocity(id: Int)` Retrieve the last computed X velocity.
Float	`getYVelocity()` Retrieve the last computed Y velocity.
Float	`getYVelocity(id: Int)` Retrieve the last computed Y velocity.
Boolean	`isAxisSupported(axis: Int)` Checks whether a given velocity-trackable `MotionEvent` axis is supported for velocity tracking by this `VelocityTracker` instance (refer to `getAxisVelocity(int,int)` for a list of potentially velocity-trackable axes).
static VelocityTracker!	`obtain()` Retrieve a new VelocityTracker object to watch the velocity of a motion.
Unit	`recycle()` Return a VelocityTracker object back to be re-used by others.

Protected methods
Unit	`finalize()`

Public methods

addMovement

Added in API level 1

fun addMovement(event: MotionEvent!): Unit

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

fun clear(): Unit

Reset the velocity tracker back to its initial state.

computeCurrentVelocity

Added in API level 1

fun computeCurrentVelocity(units: Int): Unit

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

See Also

#computeCurrentVelocity(int, float)

computeCurrentVelocity

Added in API level 4

fun computeCurrentVelocity(
    units: Int, 
    maxVelocity: Float
): Unit

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 units per millisecond, 1000 provides units per second, etc. Note that the units referred to here are the same units with which motion is reported. For axes X and Y, the units are pixels.
`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.

getAxisVelocity

Added in API level 34

fun getAxisVelocity(axis: Int): Float

Equivalent to calling getAxisVelocity(int,int) for axis and the active pointer.

Parameters
`axis`	Int: Which axis' velocity to return. Value is `android.view.MotionEvent#AXIS_X`, `android.view.MotionEvent#AXIS_Y`, or `android.view.MotionEvent#AXIS_SCROLL`

Return
`Float`	The previously computed velocity for `axis` for the active pointer if `axis` is supported for velocity tracking, or 0 if velocity tracking is not supported for the axis.

See Also

#isAxisSupported(int)
#getAxisVelocity(int, int)

getAxisVelocity

Added in API level 34

fun getAxisVelocity(
    axis: Int, 
    id: Int
): Float

Retrieve the last computed velocity for a given motion axis. You must first call computeCurrentVelocity(int) or computeCurrentVelocity(int,float) before calling this function.

In addition to MotionEvent.AXIS_X and MotionEvent.AXIS_Y which have been supported since the introduction of this class, the following axes can be candidates for this method:

MotionEvent.AXIS_SCROLL: supported starting android.os.Build.VERSION_CODES#UPSIDE_DOWN_CAKE

Before accessing velocities of an axis using this method, check that your VelocityTracker instance supports the axis by using isAxisSupported(int).

Parameters
`axis`	Int: Which axis' velocity to return. Value is `android.view.MotionEvent#AXIS_X`, `android.view.MotionEvent#AXIS_Y`, or `android.view.MotionEvent#AXIS_SCROLL`
`id`	Int: Which pointer's velocity to return.

Return
`Float`	The previously computed velocity for `axis` for pointer ID of `id` if `axis` is supported for velocity tracking, or 0 if velocity tracking is not supported for the axis.

See Also

#isAxisSupported(int)

getXVelocity

Added in API level 1

fun getXVelocity(): Float

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

Return
`Float`	The previously computed X velocity.

getXVelocity

Added in API level 8

fun getXVelocity(id: Int): Float

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.

Return
`Float`	The previously computed X velocity.

getYVelocity

Added in API level 1

fun getYVelocity(): Float

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

Return
`Float`	The previously computed Y velocity.

getYVelocity

Added in API level 8

fun getYVelocity(id: Int): Float

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.

Return
`Float`	The previously computed Y velocity.

isAxisSupported

Added in API level 34

fun isAxisSupported(axis: Int): Boolean

Checks whether a given velocity-trackable MotionEvent axis is supported for velocity tracking by this VelocityTracker instance (refer to getAxisVelocity(int,int) for a list of potentially velocity-trackable axes).

Note that the value returned from this method will stay the same for a given instance, so a single check for axis support is enough per a VelocityTracker instance.

Parameters
`axis`	Int: The axis to check for velocity support. Value is `android.view.MotionEvent#AXIS_X`, `android.view.MotionEvent#AXIS_Y`, or `android.view.MotionEvent#AXIS_SCROLL`

Return
`Boolean`	`true` if `axis` is supported for velocity tracking, or `false` otherwise.

See Also

#getAxisVelocity(int, int)
#getAxisVelocity(int)

obtain

Added in API level 1

static fun obtain(): VelocityTracker!

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.

Return
`VelocityTracker!`	Returns a new VelocityTracker.

recycle

Added in API level 1

fun recycle(): Unit

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

protected fun finalize(): Unit

Exceptions
`java.lang.Throwable`	the `Exception` raised by this method