Android Tutorial Android multitouch tutorial

Discussion in 'Tutorials & Examples' started by Erel, Jul 25, 2011.

  1. Erel

    Erel Administrator Staff Member Licensed User

    Using Agraham's Gestures library we can handle multitouch events.

    This is an example of a simple drawing application that supports multitouch:


    As you can see in the above screenshot, each finger draws with a different color and it also displays the current id and position of each touch.

    In order to support multitouch a Gestures object is used to add a listener to a specific view.
    The listener is responsible for raising an event with every movement or change in the multitouch state.

    Inside this event we are handling the gesture. Each touch gets a pointer id during a gesture. The pointer id is assigned when a finger touches the screen and is released when the same finger leaves the screen.

    I find it useful to maintain a Map with the pointer ids as the keys and the the extra data that you want to assign to each touch as values.
    In our example we store the color and the previous position as the value.

    Maintaining the map is done with the following code:
    Sub GesturesTouch(View As Object, PointerID As Int, Action As Int, X As Float, Y As Float) As Boolean
    Dim p As Point
    Select Action
    'New Point is assigned to the new touch
                p.Id = PointerID
                p.Color = 
    Colors.RGB(Rnd(0255), Rnd(0255), Rnd(0255))
                TouchMap.Put(PointerID, p)
    Case g.ACTION_UP
    End Select
    The Point type is declared in Sub Globals:
    Type Point(Id As Int, prevX As Int, prevY As Int, Color As Int)
    The result is that TouchMap holds information about the active touches.
    Now all that is left to do is to iterate over the TouchMap values and connect the previous coordinates with the new ones and then update the previous coordinates.
    The new coordinates are retrieved by calling g.GetX/Y with the pointer id.
    Dim px, py As Int
    For i = 0 To TouchMap.Size - 1
            p = TouchMap.GetValueAt(i)
            px = g.GetX( 
    'Get the current coordinates of this touch
            py = g.GetY(
    Dim s As String
            s = p.Id & 
    ": " & px & " x " & py
    Canvas.DrawText(s, 10dip20dip + i * RowHeight, Typeface.DEFAULT, TextSize, p.Color, "LEFT")
    If p.prevX > 0 AND p.prevY > 0 Then
    Canvas.DrawLine(p.prevX, p.prevY, px, py, p.Color, 5dip)
    End If
            p.prevX = px
            p.prevY = py
    p.prevX and p.prevY will equal 0 when the touch starts. In this case we do not draw anything.

    The gesture sub should return true or otherwise no more events will be raised for this gesture.

    Attached Files:

    Last edited: Aug 29, 2017
    Jones Hone, ilan, hibrid0 and 2 others like this.
  2. hackhack

    hackhack Active Member Licensed User

    So, were you going to do this anyway or is this your way of trying to get me to use the gesture library? ;)

    Nice that its a small working demo (more comments), works on my phone and tablet - wee.
  3. Erel

    Erel Administrator Staff Member Licensed User

    Actually it was a coincidence. I believe that the tutorial was posted before your question...
  4. Erel

    Erel Administrator Staff Member Licensed User

    What do you mean? Multitouch only works if you specifically handle it in your application (except of WebView).
  5. Erel

    Erel Administrator Staff Member Licensed User

    You should not use this library if you don't want to handle multitouch.

    If it is a different issue then please start a new thread for this question.
  6. Nachtfalke75

    Nachtfalke75 Member Licensed User

    When i start the MultitouchExample on my emulator or on my lg p920 device i get the message "The application xxx has stopped unexpectedly" :(
  7. Erel

    Erel Administrator Staff Member Licensed User

    Can you check the logs? There should be an error message with a stack trace.
  8. Nachtfalke75

    Nachtfalke75 Member Licensed User

    Thanks for your fast answer. I used an old lib and installed the 1.2 one. now i get no error message, but the screen get black and nothing happens. at the emulator and the real device. When i compile it in debug modus, it works well. i dont understand that :(
    Release works too
    Release (obfuscated) don't work!
    Last edited: Apr 14, 2012
  9. Erel

    Erel Administrator Staff Member Licensed User

    The problem is that GesturesTouch gets obfuscated and then the event cannot be raised.

    Rename the event (and the SetListener line) to Gestures_Touch and it will work. The obfuscator doesn't obfuscate identifiers with underscore.
  10. Nachtfalke75

    Nachtfalke75 Member Licensed User

    Ok, thank you :)
  11. KiloOne

    KiloOne Member Licensed User

    After analyzing the multipoint data in vb6 I have noticed that when two fingers get close together, two distinct contact points merge into one.

    The two fingers still have about 6 mm of 'non-contact on the touchscreen' between them.

    This makes it very difficult to write a program that tracks 5 fingers.

    This behavior can be observed with this tutorial code if you look closely.

    Is there a way to prevent this 'finger merging' issue?

  12. Erel

    Erel Administrator Staff Member Licensed User

    I see it too. I think that this is how the touches are recognized by the OS. Probably to prevent false multitouches from a single finger.
  13. KiloOne

    KiloOne Member Licensed User

    Is there something like an operating system api that could be used to adjust the parameters involved in the blob merging issue?
  14. Erel

    Erel Administrator Staff Member Licensed User

    I'm afraid not.
  15. Troberg

    Troberg Well-Known Member Licensed User

    I keep getting events on a gesture even though I've returned False. In my case, I want to detect a two finger click as well as an ordinary one finger click.
    If TouchMap.Size = 2 Then
    Return False
    Else If TouchMap.Size > 2 Then 'Not interested, end gesture
            Return False
    Else If TouchMap.Size = 0 AND Action = MultiTouch.ACTION_UP Then 'Size=0 because it has already been removed earlier in the code
    Return False
    Else 'Wait for more
            Return True
    End If
    What I would expect on a two finger click was this seqence:

    However, what I get is:

    Not that hard to get around with a state variable, but still nice if it was fixed.
  16. Erel

    Erel Administrator Staff Member Licensed User

    Returning false from this event doesn't end the gesture. It tells the system that the event was not consumed and it may be forward to a different view.
  17. Informatix

    Informatix Expert Licensed User

    You should look at the Gesture Detector lib.
  18. Troberg

    Troberg Well-Known Member Licensed User

    Ah, now I understand. The wording was a bit ambiguous. No problem, I just added a state variable, problem solved.
  19. Troberg

    Troberg Well-Known Member Licensed User

    I've missed that, but it wouldn't really help me in this instance. I cut down my example a bit, what I really do is to use two finger click with fingers side by side as toggle play/pause, and with fingers "over & under" to toggle mute.

    I will probably use it in the future, though, for other uses.
  20. Informatix

    Informatix Expert Licensed User

    Yes, it would help because it ensures when you tap that the up event is in the same area than the down event (so it's really a click). That avoids issues when you click with two fingers on areas close to each other. But if that works fine with your current solution, so you don't really need it, of course.
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