Input devices as entities

[Brezak]

This proposal is heavily based on the ideas in #12770. I strongly suggest you read the PR description before continuing.

The current way of doing input doesn't differentiate between different sources of the same type very well. This isn't an issue for keyboards and mice where we expect there to be only one instance of each, but gamepads break this assumption. Our current solution is to store the gamepad associated with the button and to provide the user with a resource that keeps track of all gamepads we know about. As #12770 points out, this leads to a lot of unnecessary boilerplate around working with gamepad input. Another issue is that gamepads may not implement all possible features (e.g. rumble). Treating gamepads as entities provides a good solution for both problems. Components help us attach unique state to each gamepad entity and if a gamepad doesn't implement a feature we can just not give it a component for said feature.

The actual problem statement

The assumption that there will ever be only 1 keyboard or mouse is wrong. For example, macro pads exist. Also there's no good reason why a local co-op games shouldn't let you hand a second keyboard to your friend. Currently, we don't have the tools to deal with multiple input devices of the same type at the same time and we can't even let our users build these tools since we don't expose which device produced an input event.

My proposal

• ButtonInput, Touches and others get turned into components.
• Input devices get their own entities with input components added based on their capabilities.

For scenarios where we don't care about distinguishing between devices I propose we add a new system param called Input. This type would be generic over input component. We could then specify methods for each type of input we can read.

struct Input<'w, 's, D, F = ()>
where
    D: QueryData,
    F: QueryFilter,
{
    input_query: Query<'w, 's, D, F>
}

impl<'w, 's, F: QueryFilter> Input<'s, 'w, ButtonInput<KeyCode>, F> {
    pub fn just_pressed(&self, key_code: KeyCode) {
        self.input_query.iter().any(|input| input.just_pressed(key_code))
    }
    ...
}

Other solutions

• Store the DeviceId in KeyboardInput, MouseMotion, MouseWheel, MouseButtonInput, TouchInput, TouchpadMagnify and TouchepadRotate so our users can implement their own solutions.

or

• Do nothing.