support multiple pointer devices

[karroffel]

The current Input<MouseButton> interface is very nice but implies the existence of a single mouse input device.

In future maybe multiple mice or touch inputs want to be supported (touch having a "backwards compatible" interface with MouseButton maybe?), for that a mechanism to select/retrieve the input device ID would be needed.

[inodentry]

Now that bevy has an API for touch input (and since mice and touch are different things anyway), can we adjust this issue (change the title, etc) to be about supporting multiple mice / pointer devices, specifically?

[bushrat011899]

This feels like a surprisingly straight-forward problem to be the oldest open issue on Bevy.

[Lege19]

I'm looking into working on this as my first contribution, and I'd like some affirmation that the way I have a sensible plan:

• MultiMouseButtonInput: event struct containing a MouseButtonInput and a winit::event::DeviceId
• MultiMouseMotion: event struct containing a MouseMotion and a winit::event::DeviceId
• MultiMouseWheel: event struct containing a MouseWheel and a winit::event::DeviceId
• MultiMouseButton: struct containing a MouseButton and a winit::event::DeviceId
• multi_mouse_button_input_system: the equivalent of mouse_button_input_system, but for ButtonInput<MultiMouseButton> (I'd separate these into different systems because they do not require access to any of the same resources or event readers and so they could run in parallel)
• Update <InputPlugin as Plugin>::build to include .init_event::<...> for the above events
• Update <InputPlugin as Plugin>::build to include .init_resource::<ButtonInput<MultiMouseButton>>
• Update <InputPlugin as Plugin>::build to include .add_systems(PreUpdate, multi_mouse_button_input_system.in_set(InputSystem))
• Update <InputPlugin as Plugin>::build to include .register_type::<MultiMouseButtonInput>
• Add variants to bevy_window::event::WindowEvent for the above events
• Add bevy_winit::converters::convert_multi_mouse_button(mouse_button: winit::event::MouseButton) -> MultiMouseButton
• Update bevy_winit::state::window_event to emit MultiMouseButtonInput and MultiMouseWheel in addition to the existing mouse events
• Update bevy_winit::state::device_event to emit MultiMouseMotion as well as MouseMotion

I think I'm not going to try and update bevy's pointer API's to support multiple pointers associated with different pointer devices, since (I think) those are supposed to imitate the underlying OS's pointer behavior, and most (all?) OS's don't this.

[NthTensor]

Hey, just so you're aware, winit (our windowing lib) is moving to a pointer abstraction in its next release. So I think we are probably just going to switch to a unified pointer api (and possibly scrap a big portion of the existing bevy_input) instead of making additions to what we already have.

[Lege19]

I was not aware, thanks for saving me the trouble, I'll look for another issue that seems doable.