This lesson describes how to track movement in touch events.
A new onTouchEvent()
is triggered with
an ACTION_MOVE
event whenever the current touch
contact position, pressure, or size changes. As described in Detect common
gestures, all of these events are recorded in
the MotionEvent
parameter of onTouchEvent()
.
Because finger-based touch isn't always the most precise form of interaction, detecting touch events is often based more on movement than on simple contact. To help apps distinguish between movement-based gestures (such as a swipe) and non-movement gestures (such as a single tap), Android includes the notion of "touch slop". Touch slop refers to the distance in pixels a user's touch can wander before the gesture is interpreted as a movement-based gesture. For more discussion of this topic, see Managing Touch Events in a ViewGroup.
There are several different ways to track movement in a gesture, depending on the needs of your application. For example:
- The starting and ending position of a pointer (for example, move an on-screen object from point A to point B).
- The direction the pointer is traveling in, as determined by the x and y coordinates.
- History. You can find the size of a gesture's history by calling the
MotionEvent
methodgetHistorySize()
. You can then obtain the positions, sizes, time, and pressures of each of the historical events by using the motion event'sgetHistorical<Value>
methods. History is useful when rendering a trail of the user's finger, such as for touch drawing. See theMotionEvent
reference for details. - The velocity of the pointer as it moves across the touch screen.
Refer to the following related resources:
Track velocity
You could have a movement-based gesture that is simply based on the distance
and/or direction the pointer traveled. But velocity often is a determining
factor in tracking a gesture's characteristics or even deciding whether the
gesture occurred. To make velocity calculation easier, Android provides the
VelocityTracker
class. VelocityTracker
helps you track the velocity of touch events. This
is useful for gestures in which velocity is part of the criteria for the
gesture, such as a fling.
Here is a simple example that illustrates the purpose of the methods in the
VelocityTracker
API:
Kotlin
private const val DEBUG_TAG = "Velocity" class MainActivity : Activity() { private var mVelocityTracker: VelocityTracker? = null override fun onTouchEvent(event: MotionEvent): Boolean { when (event.actionMasked) { MotionEvent.ACTION_DOWN -> { // Reset the velocity tracker back to its initial state. mVelocityTracker?.clear() // If necessary retrieve a new VelocityTracker object to watch the // velocity of a motion. mVelocityTracker = mVelocityTracker ?: VelocityTracker.obtain() // Add a user's movement to the tracker. mVelocityTracker?.addMovement(event) } MotionEvent.ACTION_MOVE -> { mVelocityTracker?.apply { val pointerId: Int = event.getPointerId(event.actionIndex) addMovement(event) // When you want to determine the velocity, call // computeCurrentVelocity(). Then call getXVelocity() // and getYVelocity() to retrieve the velocity for each pointer ID. computeCurrentVelocity(1000) // Log velocity of pixels per second // Best practice to use VelocityTrackerCompat where possible. Log.d("", "X velocity: ${getXVelocity(pointerId)}") Log.d("", "Y velocity: ${getYVelocity(pointerId)}") } } MotionEvent.ACTION_UP, MotionEvent.ACTION_CANCEL -> { // Return a VelocityTracker object back to be re-used by others. mVelocityTracker?.recycle() mVelocityTracker = null } } return true } }
Java
public class MainActivity extends Activity { private static final String DEBUG_TAG = "Velocity"; ... private VelocityTracker mVelocityTracker = null; @Override public boolean onTouchEvent(MotionEvent event) { int index = event.getActionIndex(); int action = event.getActionMasked(); int pointerId = event.getPointerId(index); switch(action) { case MotionEvent.ACTION_DOWN: if(mVelocityTracker == null) { // Retrieve a new VelocityTracker object to watch the // velocity of a motion. mVelocityTracker = VelocityTracker.obtain(); } else { // Reset the velocity tracker back to its initial state. mVelocityTracker.clear(); } // Add a user's movement to the tracker. mVelocityTracker.addMovement(event); break; case MotionEvent.ACTION_MOVE: mVelocityTracker.addMovement(event); // When you want to determine the velocity, call // computeCurrentVelocity(). Then call getXVelocity() // and getYVelocity() to retrieve the velocity for each pointer ID. mVelocityTracker.computeCurrentVelocity(1000); // Log velocity of pixels per second // Best practice to use VelocityTrackerCompat where possible. Log.d("", "X velocity: " + mVelocityTracker.getXVelocity(pointerId)); Log.d("", "Y velocity: " + mVelocityTracker.getYVelocity(pointerId)); break; case MotionEvent.ACTION_UP: case MotionEvent.ACTION_CANCEL: // Return a VelocityTracker object back to be re-used by others. mVelocityTracker.recycle(); break; } return true; } }
Use pointer capture
Some apps, such as games, remote desktop, and virtualization clients, greatly benefit from getting control over the mouse pointer. Pointer capture is a feature available in Android 8.0 (API level 26) and later that provides such control by delivering all mouse events to a focused view in your app.
Request pointer capture
A view in your app can request pointer capture only when the view hierarchy that
contains it has focus. For this reason, you should request pointer capture when
there's a specific user action on the view, such as during an onClick()
event, or in the onWindowFocusChanged()
event handler
of your activity.
To request pointer capture, call the requestPointerCapture()
method on the view. The following code
example shows how to request pointer capture when the user clicks a view:
Kotlin
fun onClick(view: View) { view.requestPointerCapture() }
Java
@Override public void onClick(View view) { view.requestPointerCapture(); }
Once the request to capture the pointer is successful, Android calls onPointerCaptureChange(true)
.
The system delivers the mouse events to the focused view in your app as long as
it's in the same view hierarchy as the view that requested the capture. Other
apps stop receiving mouse events until the capture is released, including ACTION_OUTSIDE
events. Android delivers pointer events
from sources other than the mouse normally, but the mouse pointer is not visible
anymore.
Handle captured pointer events
Once a view has successfully acquired the pointer capture, Android starts delivering the mouse events. Your focused view can handle the events by performing one of the following tasks:
- If you're using a custom view, override
onCapturedPointerEvent(MotionEvent)
. - Otherwise, register an
OnCapturedPointerListener
.
The following code example shows how to implement onCapturedPointerEvent(MotionEvent)
:
Kotlin
override fun onCapturedPointerEvent(motionEvent: MotionEvent): Boolean { // Get the coordinates required by your app val verticalOffset: Float = motionEvent.y // Use the coordinates to update your view and return true if the event was // successfully processed return true }
Java
@Override public boolean onCapturedPointerEvent(MotionEvent motionEvent) { // Get the coordinates required by your app float verticalOffset = motionEvent.getY(); // Use the coordinates to update your view and return true if the event was // successfully processed return true; }
The following code example shows how to register an OnCapturedPointerListener
:
Kotlin
myView.setOnCapturedPointerListener { view, motionEvent -> // Get the coordinates required by your app val horizontalOffset: Float = motionEvent.x // Use the coordinates to update your view and return true if the event was // successfully processed true }
Java
myView.setOnCapturedPointerListener(new View.OnCapturedPointerListener() { @Override public boolean onCapturedPointer (View view, MotionEvent motionEvent) { // Get the coordinates required by your app float horizontalOffset = motionEvent.getX(); // Use the coordinates to update your view and return true if the event was // successfully processed return true; } });
Whether you use a custom view or register a listener, your view receives a
MotionEvent
with pointer coordinates that specify relative
movements such as X/Y deltas, similar to the coordinates delivered by a
trackball device. You can retrieve the coordinates by using getX()
and getY()
.
Release pointer capture
The view in your app can release the pointer capture by calling releasePointerCapture()
, as shown in the following code
example:
Kotlin
override fun onClick(view: View) { view.releasePointerCapture() }
Java
@Override public void onClick(View view) { view.releasePointerCapture(); }
The system can take the capture away from the view without you explicitly
calling releasePointerCapture()
, most commonly because
the view hierarchy that contains the view that requested capture has lost focus.