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. Objects should always act in the same way that real objects do. For example, images should not immediately pop out of existence and reappear somewhere else, because objects in the real world don't do that. Instead, images should move from one place to another.
Users also sense 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, they should sense friction at the beginning that delays the motion, and then at the end sense momentum that carries the motion beyond the fling.
This lesson demonstrates how to use features of the Android framework to add these real-world behaviors to your custom view.
Like many other UI frameworks, Android supports an input event model. User actions are turned into events that trigger callbacks, and you can override the callbacks to customize how your application responds to the user. The most common input event in the Android system is touch, which triggers {@link android.view.View#onTouchEvent(android.view.MotionEvent)}. Override this method to handle the event:
@Override
public boolean onTouchEvent(MotionEvent event) {
return super.onTouchEvent(event);
}
Touch events by themselves are not 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 {@link android.view.GestureDetector}.
Construct a {@link android.view.GestureDetector} by passing in an instance of a class that implements {@link android.view.GestureDetector.OnGestureListener}. If you only want to process a few gestures, you can extend {@link android.view.GestureDetector.SimpleOnGestureListener} instead of implementing the {@link android.view.GestureDetector.OnGestureListener} interface. For instance, this code creates a class that extends {@link android.view.GestureDetector.SimpleOnGestureListener} and overrides {@link android.view.GestureDetector.SimpleOnGestureListener#onDown}.
class mListener extends GestureDetector.SimpleOnGestureListener {
@Override
public boolean onDown(MotionEvent e) {
return true;
}
}
mDetector = new GestureDetector(PieChart.this.getContext(), new mListener());
Whether or not you use {@link android.view.GestureDetector.SimpleOnGestureListener}, you must always implement an {@link android.view.GestureDetector.OnGestureListener#onDown onDown()} method that returns {@code true}. This step is necessary because all gestures begin with an {@link android.view.GestureDetector.OnGestureListener#onDown onDown()} message. If you return {@code false} from {@link android.view.GestureDetector.OnGestureListener#onDown onDown()}, as {@link android.view.GestureDetector.SimpleOnGestureListener} does, the system assumes that you want to ignore the rest of the gesture, and the other methods of {@link android.view.GestureDetector.OnGestureListener} never get called. The only time you should return {@code false} from {@link android.view.GestureDetector.OnGestureListener#onDown onDown()} is if you truly want to ignore an entire gesture. Once you've implemented {@link android.view.GestureDetector.OnGestureListener} and created an instance of {@link android.view.GestureDetector}, you can use your {@link android.view.GestureDetector} to interpret the touch events you receive in {@link android.view.GestureDetector#onTouchEvent onTouchEvent()}.
@Override
public boolean onTouchEvent(MotionEvent event) {
boolean result = mDetector.onTouchEvent(event);
if (!result) {
if (event.getAction() == MotionEvent.ACTION_UP) {
stopScrolling();
result = true;
}
}
return result;
}
When you pass {@link android.view.GestureDetector#onTouchEvent onTouchEvent()} a touch event that it doesn't recognize as part of a gesture, it returns {@code false}. You can then run your own custom gesture-detection code.
Gestures are a powerful way to control touchscreen devices, but they can be counterintuitive and difficult to remember unless they produce physically plausible results. A good example of this is the fling gesture, where the user quickly moves a finger across the screen and then lifts it. This gesture makes sense if the UI responds by moving quickly in the direction of the fling, then slowing down, as if the user had pushed on a flywheel and set it spinning.
However, simulating the feel of a flywheel isn't trivial. A lot of physics and math are required to get a flywheel model working correctly. Fortunately, Android provides helper classes to simulate this and other behaviors. The {@link android.widget.Scroller} class is the basis for handling flywheel-style fling gestures.
To start a fling, call {@link android.widget.Scroller#fling 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 for you by {@link android.view.GestureDetector}.
@Override
public boolean onFling(MotionEvent e1, MotionEvent e2, float velocityX, float velocityY) {
mScroller.fling(currentX, currentY, velocityX / SCALE, velocityY / SCALE, minX, minY, maxX, maxY);
postInvalidate();
}
Note: Although the velocity calculated by {@link android.view.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 4 to 8.
The call to {@link android.widget.Scroller#fling fling()} sets up the physics model for the fling gesture. Afterwards, you need to update the {@link android.widget.Scroller Scroller} by calling {@link android.widget.Scroller#computeScrollOffset Scroller.computeScrollOffset()} at regular intervals. {@link android.widget.Scroller#computeScrollOffset computeScrollOffset()} updates the {@link android.widget.Scroller 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 {@link android.widget.Scroller#getCurrX} and {@link android.widget.Scroller#getCurrY} to retrieve these values.
Most views pass the {@link android.widget.Scroller Scroller} object's x and y position directly to {@link android.view.View#scrollTo scrollTo()}. The PieChart example is a little different: it uses the current scroll y position to set the rotational angle of the chart.
if (!mScroller.isFinished()) {
mScroller.computeScrollOffset();
setPieRotation(mScroller.getCurrY());
}
The {@link android.widget.Scroller Scroller} class computes scroll positions for you, but it does not automatically apply those positions to your view. It's your responsibility to make sure you get and apply new coordinates often enough to make the scrolling animation look smooth. There are two ways to do this:
The PieChart example uses the second approach. This technique is slightly more complex to set up, but it works more closely with the animation system and doesn't require potentially unnecessary view invalidation. The drawback is that {@link android.animation.ValueAnimator} is not available prior to API level 11, so this technique cannot be used on devices running Android versions lower than 3.0.
Note: {@link android.animation.ValueAnimator} isn't available prior to API level 11, but you can still use it in applications that target lower API levels. You just need to make sure to check the current API level at runtime, and omit the calls to the view animation system if the current level is less than 11.
mScroller = new Scroller(getContext(), null, true);
mScrollAnimator = ValueAnimator.ofFloat(0,1);
mScrollAnimator.addUpdateListener(new ValueAnimator.AnimatorUpdateListener() {
@Override
public void onAnimationUpdate(ValueAnimator valueAnimator) {
if (!mScroller.isFinished()) {
mScroller.computeScrollOffset();
setPieRotation(mScroller.getCurrY());
} else {
mScrollAnimator.cancel();
onScrollFinished();
}
}
});
Users expect a modern UI to transition smoothly between states. UI elements fade in and out instead of appearing and disappearing. Motions begin and end smoothly instead of starting and stopping abruptly. The Android property animation framework, introduced in Android 3.0, makes smooth transitions easy.
To use the animation system, whenever a property changes that will affect your view's appearance, do not change the property directly. Instead, use {@link android.animation.ValueAnimator} to make the change. In the following example, modifying the currently selected pie slice in PieChart causes the entire chart to rotate so that the selection pointer is centered in the selected slice. {@link android.animation.ValueAnimator} changes the rotation over a period of several hundred milliseconds, rather than immediately setting the new rotation value.
mAutoCenterAnimator = ObjectAnimator.ofInt(PieChart.this, "PieRotation", 0); mAutoCenterAnimator.setIntValues(targetAngle); mAutoCenterAnimator.setDuration(AUTOCENTER_ANIM_DURATION); mAutoCenterAnimator.start();
If the value you want to change is one of the base {@link android.view.View} properties, doing the animation is even easier, because Views have a built-in {@link android.view.ViewPropertyAnimator} that is optimized for simultaneous animation of multiple properties. For example:
animate().rotation(targetAngle).setDuration(ANIM_DURATION).start();