Tap detection

Normally, a Canvas-based MIDlet starts returning drag and touch release events immediately after a touch down event has taken place. This functionality can cause problems in situations where tapping a UI element requires a high degree of accuracy from the user. Since both the touch down event and release event must be received from the same UI element for a tap to be registered, a slight movement of the finger or stylus can result in the release event being received from outside the intended UI element, causing the intended tap action to fail.

This problem is compounded when a finger is used to navigate the MIDlet instead of a stylus. Because the finger covers a greater area than the stylus, a finger press can generate a lot of unnecessary drag events in addition to inaccurate touch down and release events.

The Series 40 and Symbian platforms provide different solutions for ensuring that taps are detected correctly in these situations.

Tap detection on Series 40 devices

To ensure that taps are detected correctly on Series 40 devices, use the Gesture API to define sufficient-sized interactive zones for touchable UI elements. Make the interactive zone slightly larger than the corresponding element, so that any slight moves the user accidentally makes between touch down and touch release are nevertheless registered for the element.

Tap detection on Symbian devices

On Symbian devices, from Java Runtime 1.4 for S60 onwards, Canvas elements generate a tap detection area around the touch event coordinates from which drag events are suppressed for a set amount of time. By default, the tap detection area is a square with sides that are 400 twips (roughly 7 mm) in length, and the drag events are suppressed for 500 milliseconds (JRT 1.4 for S60 and early JRT 2.1 for Symbian devices) or 350 milliseconds (newer JRT 2.1 for Symbian devices and from JRT 2.2 for Symbian onwards).

Figure: Default tap detection area

You can modify the tap detection area size and timeout value with the Nokia-MIDlet-Tap-Detection-Options JAD attribute. The size (side length) is specified in twips and the timeout in milliseconds. If you set both the size and timeout to 0, tap detection is disabled and all drag events are received normally. The settings apply to all Canvas elements in the MIDlet.

Note: The side length is given as the distance between the center point of the square and the side of the square. The actual width and height of the square are therefore twice the specified length.

The tap suppression process works as follows:

pointerPressed coordinates are saved whenever a touch down event occurs.
pointerDragged events are suppressed if they occur within the tap detection area around the pointerPressed coordinates and the timeout has not expired.
If the finger or stylus is dragged outside the tap detection area, drag events are passed normally to the MIDlet.
If the timeout expires, drag events are passed normally to the MIDlet.
If a pointerReleased event occurs inside the tap detection area within the timeout limit, the returned coordinates are identical to the original pointerPressed coordinates.