Multi-touch API

[Kim Topley]

This is a great write-up! I have a couple of questions:

> ScrollEvent has types SCROLL_STARTED, SCROLL, SCROLL_FINISHED. The
started/finished notifications are generated only by touch gestures, mouse
wheel still generates only one-time SCROLL event.

Is there a reason for this difference? Isn't this going to make it hard to
write event handlers that are independent of the source of the scroll
events. In particular, how will the event handling code know that it should
not expect a SCROLL_FINISHED event?

> In addition to the formerly existing deltaX and deltaY fields it has
totalDeltaX and totalDeltaY (those contain zeros for mouse wheel
scrolling). There is also a new field touchCount that specifies how many
touch points are used for the gesture (a new gesture is started each time
the touch count changes).

Similar question about the reason for the difference between mouse wheel
scrolling and swipes.

Kim





On Thu, May 24, 2012 at 3:48 AM, Pavel Safrata <pavel.safrata at oracle.com>wrote:

> Hello,
> we are at the end of a long journey to introduce an initial multi-touch
> support. The API has been lying in 2.2 repository for some time already,
> but we haven't got much feedback, so we've decided to make a last call for
> your concerns that might result in API changes before it goes public. We
> are already past feature freeze though, so the window is going to be quite
> small. A summary of the API follows.
>
> The touch actions produce three types of events: MouseEvents,
> GestureEvents and TouchEvents. All the events are delivered simultaneously
> and independently.
>
>
> * Mouse events *
>
> Single touches are translated to normal MouseEvents, so the existing
> applications should work fine on touch screen. Sometimes you may need to
> identify and handle differently the mouse events synthesized from
> touch-screen, for that purpose they have a new isSynthesized flag.
>
>
> * Gesture events *
>
> Gesture events are: ScrollEvent, RotateEvent, ZoomEvent, SwipeEvent. They
> are generated by both touch screen and trackpad. Basic common
> characteristics are:
>  * each event has coordinates, for trackpad events mouse coordinates are
> used, four touch screen events the center point between all the touches is
> used
>  * each event has modifiers information
>  * each event has isDirect flag that distinguishes between direct events
> produced by touching on direct coordinates on touch screen and indirect
> events produced by trackpad or mouse.
>
> The ScrollEvent, RotateEvent and ZoomEvent are continuous. They have event
> types "started", "performed" and "finished". The "started" event comes when
> the gesture is detected, then the "performed" events are delivered,
> containing delta values (change since the previous event) and total delta
> values (change since the gesture start). The "finished" event is delivered
> when the gesture finishes (the touches are released). After that, another
> "performed" events may be delivered, with an isInertia flag set to true.
> Whole gesture is delivered to a single node that was picked on gesture
> coordinates in time of gesture start.
>
> The SwipeEvent is a one-time event. When all the touch points involved in
> a gesture are pressed, then moved in the same direction and released, we
> recognize the gesture and deliver it as a single SwipeEvent (containing the
> swipe direction). Note that the described gesture produces also
> ScrollEvents, they are not exlusive with swipe.
>
> The gestures specifically:
>
> ScrollEvent has types SCROLL_STARTED, SCROLL, SCROLL_FINISHED. The
> started/finished notifications are generated only by touch gestures, mouse
> wheel still generates only one-time SCROLL event. In addition to the
> formerly existing deltaX and deltaY fields it has totalDeltaX and
> totalDeltaY (those contain zeros for mouse wheel scrolling). There is also
> a new field touchCount that specifies how many touch points are used for
> the gesture (a new gesture is started each time the touch count changes).
>
> ZoomEvent has types ZOOM_STARTED, ZOOM, ZOOM_FINISHED. It contains
> zoomFactor and totalZoomFactor. The values are to be multiplied with node's
> scale - values greater than 1 mean zooming in, values between zero and one
> mean zooming out.
>
> RotateEvent has types ROTATION_STARTED, ROTATE, ROTATION_FINISHED. It
> contains angle and totalAngle. The angles are in degrees and are meant to
> be added to node's rotation - positive values mean clock-wise rotation.
>
> SwipeEvent has types SWIPE_LEFT, SWIPE_RIGHT, SWIPE_UP, SWIPE_DOWN and
> contains touchCount field. On touch screen it is delivered to the node
> picked on center point of the entire gesture.
>
>
> * Touch events *
>
> The TouchEvent can be used for tracking all the particular touch points.
> They are delivered for touch screen actions only, trackpad doesn't produce
> them.
>
> Each event carries a touch point (representing one pressed finger) and
> references to all other touch points. This design allows for handling and
> consuming each finger separately while making it possible to encapsulate
> handling of more complex multi-touch gestures in which not all touch points
> need to be over the handling node. In any moment of a multi-touch action,
> we have a set of touch points - for each of those touch points we create
> one touch event. This bunch of events is called "event set" and is marked
> by a common eventSetId number. All touch events from the set carry the same
> list of touch points, each of them carries different one as the "main"
> touch point.
>
> Each touch point has state (PRESSED, MOVED, STATIONARY, RELEASED),
> coordinates, id (unique in scope of a gesture) and target (the node to
> which this touch point's event is delivered). A method belongsTo(node)
> allows to test on the other touch points if they are delivered to the given
> target node (including bubbling).
>
> The event has types TOUCH_PRESSED, TOUCH_MOVED, TOUCH_STATIONARY,
> TOUCH_RELEASED, corresponding to the sate of its touch point. They also
> contain touchCount and modifiers.
>
> By default, each touch point is delivered to a single node during its
> whole trajectory, similarly to mouse dragging and gestures. This behavior
> is great for dragging nodes and is consistent with the rest of our events,
> but sometimes you want it different way - in this case it can be altered by
> using a grabbing API. The touch point provides methods ungrab() (since this
> call the touch point will be always delivered to a node picked under it),
> grab(node) (since this call the touch point will be always delivered to the
> given node) and grab() (since this call the touch point will be grabbed by
> the current source - the node whose handler calls it). In another words the
> grabbing/ungrabbing determines where to deliver each touch point and by
> default each newly pressed touch point is automatically grabbed by the node
> picked for it.
>
>
> * Notes *
>
> There is one thing that makes the existing apps behave wrong. When you
> drag one finger over touch screen, it generates both mouse dragging and
> scrolling. We have to deliver both of them for the majority of applications
> to work correctly. The few nodes that handle both of those events
> (ScrollBar is a typical example) usually don't work well and need to be
> updated (to ignore synthesized mouse events for instance).
>
> For a classic application it should be enough to use mouse events and
> gestures. For a touch-screen-only appliction that wants to do some complex
> multi-touch logic, TouchEvents provide all the power. Developing an
> application that uses all kinds of events requires considerable care to
> avoid conflicting handling of user actions.
>
> We tried to stick with the native behavior as much as possible for users
> to get what they are used to. Where the underlying platform knows a
> gesture, we use the native recognition (elsewhere we have our own). We
> produce inertia for gestures based on the native support on each platform
> (for instance if a platform doesn't support zooming inertia at all, we
> don't generate it there). The goal is to make the same application behave
> as much native-like as possible on all platforms while minimizing
> developer's need to consider the differences.
>
> The gestures can generally be used without considering differences between
> touch-screen and trackpad. The most significant exception is touchCount on
> SwipeEvent. On touch-screen you can generate swipe by any number of touch
> points. On trackpad one-finger siwpes make no sense (they are used just to
> move the mouse cursor). On Mac in particular we use the native swipe
> recognition that generates only three and more finger swipes. So an
> application that uses touchCount on SwipeEvent will probably need different
> handling for direct and indirect events (which such complex applications
> will likely do anyway).
>
>
> This is basically what we have in 2.2. Are there any objections?
> Thanks,
> Pavel
>