Horizontal vs. Vertical Fling (Swipe)

GestureDetector

The TextView has an OnTouchListener that detects a touch. But we want to detect more than a touch. We want to detect a fling, which consists of a series of touches in different places. (“Fling” is the Android word for “swipe”.)

Each time the onTouch method of the OnTouchListener receives a MotionEvent, it passes the MotionEvent to a GestureDetector. After the GestureDetector receives enough of these MotionEvents, the GestureDetector can conclude that the MotionEvents constitute a swipe. The GestureDetector then calls the onFling method of the SimpleOnGestureListener that was plugged into the GestureDetector.

The object plugged into the GestureDetector must be an OnGestureListener. But OnGestureListener is an interface, and to create an object that implements this interface we would have to override all six of the interface’s abstract methods. If we are interested in overriding only one of them (in our case, onFling), it’s easier to subclass SimpleOnGestureListener, which provides six overriding methods that do nothing. We only have to override the methods that interest us.

Source code in Fling.zip

1. MainActivity.java
2. activity_main.xml
3. strings.xml
4. AndroidManifest.xml
5. build.gradle (Module: app).

The angle

Math.atan2 gets the angle in which the fling is pointing (with the y value negated because the Android Y axis points downwards), and Math.toDegrees converts it from radians to degrees.

Pressure and size

Only the TextView is touch-sensitive, so we expand it to fill the entire screen by giving it a layout_width and layout_height of fill_parent in activity_main.xml. To center the text in the TextView, the TextView has android:gravity="center". When a fling is detected, we change the gravity to TOP.

getPressure and getSize return meaningful, or at least serious looking, values on my Motorola MB612 phone. On my Azpen A272 tablet, the pressure is always 1 and the size is always .0392157 (close to the reciprocal of 25.5). On my Amazon Fire HD 6 tablet, the pressure might be meaningful and the size is always zero. On the Nexus 5X emulator, the size is microscopic and the pressure is always .503906.

Unicode

For portability, I wrote the special characters as Unicode code numbers (\u followed by four hexadecimal digits) rather than as the characters themselves. Here are links to the code charts: