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CHAPTER 9: User Interface
Multitouch
Multitouch events in the Android OS are surprisingly easy to handle. All you need is
to overwrite the
onTouchEvent()
method of a
View
or a
ViewGroup
element.
Inside the
onTouchEvent()
, you fetch the masked action and act upon it.
frog.setOnTouchListener { view, event ->
true
}
Note
In older versions of Android, you usually dispatch on the action as in
event.action
. With
multitouch
gestures, it is better to act on the
maskedAction
.
Inside the listener, you
get the masked action by
event.actionMasked
and pass it to a
when()
{ .. }
statement.
The magic now lies in this listener being invoked for all fingers (here called
pointers
)
consecutively. To find out how many fingers
currently are registered, you use
event.
pointerCount
, and if you want to know which finger the event belongs to you, use
val
index
= event.actionIndex
. A starting point is thus as follows:
theView.setOnTouchListener { view,event ->
fun actionToString(action:Int) : String = mapOf(
MotionEvent.ACTION_DOWN to "Down",
MotionEvent.ACTION_MOVE to "Move",
MotionEvent.ACTION_POINTER_DOWN to "Pointer Down",
MotionEvent.ACTION_UP to "Up",
MotionEvent.ACTION_POINTER_UP to "Pointer Up",
MotionEvent.ACTION_OUTSIDE to "Outside",
MotionEvent.ACTION_CANCEL to "Cancel").
getOrDefault(action,"")
val action = event.actionMasked
val index = event.actionIndex
var xPos = -1
var yPos = -1
Log.d("LOG", "The action is " +
actionToString(action))
if (event.pointerCount > 1) {
Log.d("LOG", "Multitouch event")
// The coordinates of the current screen contact,
// relative to the responding View or Activity.
xPos = event.getX(index).toInt()
yPos = event.getY(index).toInt()
} else {
// Single touch event
