It would be great if egui could support pinch-to-zoom, multi-finger swipes and other touch gestures. For a start, the most important place to support it is the web (i.e. in egui_web).

(NOTE: full multi-touch support, e.g. dragging two sliders at once, is a separate issue).

I suggest we start by adding touch events to egui::Event. Let's do something similar to winit:

enum Event {
    …
    Touch(TouchEvent),
}

pub struct TouchId(pub u64);

struct TouchEvent {
    id: TouchId,
    pos: Pos2,
    phase: TouchPhase,
}

pub enum TouchPhase {
    Started,
    Moved,
    Ended,
    Cancelled,
}

egui::InputState can then keep track of the touches in a HashMap<TouchId, Pos2> and detect gestures. For instance, if there is exactly two touches then it could measure zoom, translation and rotation to support manipulating 2D surfaces. An egui application would then just do something like self.zoom *= ui.input().zoom(); to update its zoom-state each frame.