Make a custom view interactive

Drawing a UI is only one part of creating a custom view. You also need to make your view respond to user input in a way that closely resembles the real-world action you're mimicking.

Make the objects in your app act like real objects do. For example, don't let images in your app pop out of existence and reappear elsewhere, because objects in the real world don't do that. Instead, move your images from one place to another.

Users sense even subtle behavior or feel in an interface and react best to subtleties that mimic the real world. For example, when users fling a UI object, give them a sense of inertia at the beginning that delays the motion. At the end of the motion, give them a sense of momentum that carries the object beyond the fling.

This page demonstrates how to use features of the Android framework to add these real-world behaviors to your custom view.

You can find additional related information in Input events overview and Property animation overview.

Handle input gestures

Like many other UI frameworks, Android supports an input event model. User actions turn into events that trigger callbacks, and you can override the callbacks to customize how your app responds to the user. The most common input event in the Android system is touch, which triggers onTouchEvent(android.view.MotionEvent). Override this method to handle the event, as follows:

Kotlin

override fun onTouchEvent(event: MotionEvent): Boolean {
    return super.onTouchEvent(event)
}

Java

@Override
   public boolean onTouchEvent(MotionEvent event) {
    return super.onTouchEvent(event);
   }

Touch events by themselves aren't particularly useful. Modern touch UIs define interactions in terms of gestures such as tapping, pulling, pushing, flinging, and zooming. To convert raw touch events into gestures, Android provides GestureDetector.

Construct a GestureDetector by passing in an instance of a class that implements GestureDetector.OnGestureListener. If you only want to process a few gestures, you can extend GestureDetector.SimpleOnGestureListener instead of implementing the GestureDetector.OnGestureListener interface. For example, this code creates a class that extends GestureDetector.SimpleOnGestureListener and overrides onDown(MotionEvent).

Kotlin

private val myListener =  object : GestureDetector.SimpleOnGestureListener() {
    override fun onDown(e: MotionEvent): Boolean {
        return true
    }
}

private val detector: GestureDetector = GestureDetector(context, myListener)

Java

class MyListener extends GestureDetector.SimpleOnGestureListener {
   @Override
   public boolean onDown(MotionEvent e) {
       return true;
   }
}
detector = new GestureDetector(getContext(), new MyListener());

Whether or not you use GestureDetector.SimpleOnGestureListener, always implement an onDown() method that returns true. This is necessary because all gestures begin with an onDown() message. If you return false from onDown(), as GestureDetector.SimpleOnGestureListener does, the system assumes you want to ignore the rest of the gesture, and the other methods of GestureDetector.OnGestureListener aren't called. Only return false from onDown() if you want to ignore an entire gesture.

After you implement GestureDetector.OnGestureListener and create an instance of GestureDetector, you can use your GestureDetector to interpret the touch events you receive in onTouchEvent().

Kotlin

override fun onTouchEvent(event: MotionEvent): Boolean {
    return detector.onTouchEvent(event).let { result ->
        if (!result) {
            if (event.action == MotionEvent.ACTION_UP) {
                stopScrolling()
                true
            } else false
        } else true
    }
}

Java

@Override
public boolean onTouchEvent(MotionEvent event) {
   boolean result = detector.onTouchEvent(event);
   if (!result) {
       if (event.getAction() == MotionEvent.ACTION_UP) {
           stopScrolling();
           result = true;
       }
   }
   return result;
}

When you pass onTouchEvent() a touch event that it doesn't recognize as part of a gesture, it returns false. You can then run your own custom gesture-detection code.

Create physically plausible motion

Gestures are a powerful way to control touchscreen devices, but they can be counterintuitive and difficult to remember unless they produce physically plausible results.

For example, suppose you want to implement a horizontal fling gesture that sets the item drawn in the view spinning around its vertical axis. This gesture makes sense if the UI responds by moving quickly in the direction of the fling, then slowing down, as if the user pushes on a flywheel and makes it spin.

The documentation on how to animate a scroll gesture gives a detailed explanation about how to implement your own scoll behavior. But simulating the feel of a flywheel isn't trivial. A lot of physics and math is required to make a flywheel model work correctly. Fortunately, Android provides helper classes to simulate this and other behaviors. The Scroller class is the basis for handling flywheel-style fling gestures.

To start a fling, call fling() with the starting velocity and the minimum and maximum x and y values of the fling. For the velocity value, you can use the value computed by GestureDetector.

Kotlin

fun onFling(e1: MotionEvent, e2: MotionEvent, velocityX: Float, velocityY: Float): Boolean {
    scroller.fling(
            currentX,
            currentY,
            (velocityX / SCALE).toInt(),
            (velocityY / SCALE).toInt(),
            minX,
            minY,
            maxX,
            maxY
    )
    postInvalidate()
    return true
}

Java

@Override
public boolean onFling(MotionEvent e1, MotionEvent e2, float velocityX, float velocityY) {
   scroller.fling(currentX, currentY, velocityX / SCALE, velocityY / SCALE, minX, minY, maxX, maxY);
   postInvalidate();
    return true;
}

Note: Although the velocity calculated by GestureDetector is physically accurate, many developers feel that using this value makes the fling animation too fast. It's common to divide the x and y velocity by a factor of four to eight.

The call to fling() sets up the physics model for the fling gesture. Afterward, update the Scroller by calling Scroller.computeScrollOffset() at regular intervals. computeScrollOffset() updates the Scroller object's internal state by reading the current time and using the physics model to calculate the x and y position at that time. Call getCurrX() and getCurrY() to retrieve these values.

Most views pass the Scroller object's x and y positions directly to scrollTo(). This example is a little different: it uses the current scroll x position to set the rotational angle of the view.

Kotlin

scroller.apply {
    if (!isFinished) {
        computeScrollOffset()
        setItemRotation(currX)
    }
}

Java

if (!scroller.isFinished()) {
    scroller.computeScrollOffset();
    setItemRotation(scroller.getCurrX());
}

The Scroller class computes scroll positions for you, but it doesn't automatically apply those positions to your view. Apply new coordinates often enough to make the scrolling animation look smooth. There are two ways to do this:

Force a redraw by calling postInvalidate() after calling fling(). This technique requires that you compute scroll offsets in onDraw() and call postInvalidate() every time the scroll offset changes.
Set up a ValueAnimator to animate for the duration of the fling and add a listener to process animation updates by calling addUpdateListener(). This technique lets you animate properties of a View.

Make your transitions smooth

Users expect a modern UI to transition smoothly between states: UI elements fading in and out instead of appearing and disappearing, and motions beginning and ending smoothly instead of starting and stopping abruptly. The Android property animation framework makes smooth transitions easier.

To use the animation system, whenever a property changes what affects your view's appearance, don't change the property directly. Instead, use ValueAnimator to make the change. In the following example, modifying the selected child component in the view makes the entire rendered view rotate so that the selection pointer is centered. ValueAnimator changes the rotation over a period of several hundred milliseconds, rather than immediately setting the new rotation value.

Kotlin

autoCenterAnimator = ObjectAnimator.ofInt(this, "Rotation", 0).apply {
    setIntValues(targetAngle)
    duration = AUTOCENTER_ANIM_DURATION
    start()
}

Java

autoCenterAnimator = ObjectAnimator.ofInt(this, "Rotation", 0);
autoCenterAnimator.setIntValues(targetAngle);
autoCenterAnimator.setDuration(AUTOCENTER_ANIM_DURATION);
autoCenterAnimator.start();

If the value you want to change is one of the base View properties, doing the animation is even easier, because views have a built-in ViewPropertyAnimator that is optimized for simultaneous animation of multiple properties, as in the following example:

Kotlin

animate()
    .rotation(targetAngle)
    .duration = ANIM_DURATION
    .start()

Java

animate().rotation(targetAngle).setDuration(ANIM_DURATION).start();