This lesson teaches you to
You should also read
- Input Events API Guide
- Sensors Overview
- Making the View Interactive
- Design Guide for Gestures
- Design Guide for Touch Feedback
Try it out
InteractiveChart.zip
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 Detecting 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.
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 and the
VelocityTrackerCompat
class in the
Support Library.
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:
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: " + VelocityTrackerCompat.getXVelocity(mVelocityTracker, pointerId)); Log.d("", "Y velocity: " + VelocityTrackerCompat.getYVelocity(mVelocityTracker, 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; } }
Note: Note that you should calculate velocity after an
ACTION_MOVE
event,
not after ACTION_UP
. After an ACTION_UP
,
the X and Y velocities will be 0.