Gesture Detector

[toiglak]

Frui needs an easy way to handle pointer events.
In this issue I wanted to document the overall API I am planning.

✅ Pointer Listener

Widget that responds to low-level pointer events.

PointerListener::builder()
    .on_pointer_down(|e| { ... })
    .on_pointer_up(|e| { ... })
    .on_pointer_scroll(|e| { ... })

✅ Pointer Region

Widget that responds to pointer entering child's widget region.
It also allows changing cursor icon (e.g. to a pointer icon when hovering over a button).

PointerRegion::builder()
    .cursor(Cursor::Pointer)
    .on_enter(|e| { ... })
    .on_move(|e| { ... })
    .on_exit(|e| { ... })

Gesture Detector

Widget that distinguishes between different high-level gestures.

GestureDetector::builder()
    .on_tap(|e| { ... })
    .on_double_tap(|e| { ... })
    .on_vertical_drag(|e| { ... })
    // ...

I was thinking about adding an ability to directly extend that widget with custom gestures:

fn add_gesture<G: Gesture>(self, gesture: G, callback: impl Fn(G::Event)) -> Self {
    // ...
}

Above builder method could then be abstracted using extension trait and used like:

GestureDetector::builder()
    .on_tap(|_| { ... })
    .on_my_custom_gesture(|_| { ... })

---

As of now, I am working on implementing PointerListener and KeyboardListener (other issue).

GestureDetector is a little bit tricky to implement and not that useful on PC, so I plan to implement it later when we will focus on supporting mobile devices. For now, PointerListener and PointerRegion should be enough for handling input on PC.

[toiglak]

PointerListener and PointerRegion were implemented in #10.