VelocityTracker
public
final
class
VelocityTracker
extends Object
java.lang.Object | |
↳ | 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 | |
---|---|
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 |
void
|
computeCurrentVelocity(int units, float maxVelocity)
Compute the current velocity based on the points that have been collected. |
float
|
getAxisVelocity(int axis, int id)
Retrieve the last computed velocity for a given motion axis. |
float
|
getAxisVelocity(int axis)
Equivalent to calling |
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. |
boolean
|
isAxisSupported(int axis)
Checks whether a given velocity-trackable |
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 | |
---|---|
Public methods
addMovement
public void addMovement (MotionEvent event)
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
public void clear ()
Reset the velocity tracker back to its initial state.
computeCurrentVelocity
public void computeCurrentVelocity (int units)
Equivalent to invoking computeCurrentVelocity(int, float)
with a maximum
velocity of Float.MAX_VALUE.
Parameters | |
---|---|
units |
int |
See also:
computeCurrentVelocity
public 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 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
public float getAxisVelocity (int axis, int id)
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 startingBuild.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 MotionEvent.AXIS_X , MotionEvent.AXIS_Y , or MotionEvent.AXIS_SCROLL |
id |
int : Which pointer's velocity to return. |
Returns | |
---|---|
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:
getAxisVelocity
public float getAxisVelocity (int axis)
Equivalent to calling getAxisVelocity(int, int)
for axis
and the active
pointer.
Parameters | |
---|---|
axis |
int : Which axis' velocity to return.
Value is MotionEvent.AXIS_X , MotionEvent.AXIS_Y , or MotionEvent.AXIS_SCROLL |
Returns | |
---|---|
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. |
getXVelocity
public 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
public 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
public 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
public 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. |
isAxisSupported
public boolean isAxisSupported (int axis)
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 MotionEvent.AXIS_X , MotionEvent.AXIS_Y , or MotionEvent.AXIS_SCROLL |
Returns | |
---|---|
boolean |
true if axis is supported for velocity tracking, or false
otherwise. |
obtain
public static 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
public 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
protected 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 Java 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 | |
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Throwable |