Exposed a CursorPosition resource

[ManevilleF]

Objective

Addresses #5034

Solution

I added a CursorPosition resource, updated every frame if WindowPlugin::update_cursor_position is enabled.

Usage:

Instead of:

fn my_system(windows: Res<Windows>) {
     let position = windows.get_primary().unwrap().cursor_position;
     if let Some(pos) = position {
        // Do stuff
     }
}

We have:

fn my_system(cursor: Res<CursorPosition>) {
     if let Some(pos) = cursor.position() {
        // Do stuff
     }
}

Notes

I think the update system should probably be in a PreUpdate stage?

[DJMcNab]

Why not just have these be methods on Windows?

[ManevilleF]

I mean it could be but the related issue #5034 wants a separate resource

[tim-blackbird]

Only one window can have the cursor hover over it at a time, so window.cursor_position() only returns Some on one window each frame (I hope).
How about structuring the resource like this:

pub struct CursorPosition {
    pub(crate) position: Option<(Vec2, WindowId)>,
}

impl CursorPosition {
    // Most games only have one window. Make this case the simpler one.
    pub fn get(&self) -> Option<Vec2> {
        self.position.map(|(pos, _)| pos)
    }

    pub fn get_with_id(&self) -> Option<(Vec2, WindowId)> {
        self.position
    } 
}

Then it becomes:

fn my_system(cursor_position: Res<CursorPosition>) {
     if let Some(pos) = cursor_position.get() {
        // Do stuff
     }
}

[ManevilleF]

@devil-ira thanks, I didn't know that only one window could store the cursor position.

I updated the resource

[ManevilleF]

I'm not very happy with this API. I'm closing it

[tim-blackbird]

Whaaat. I like this though. It's certainly an improvement over the current state.

Can you say exactly what you're unhappy with?
Also, I don't mind adopting this if you don't want to work on it 😄

[ManevilleF]

Sure, you can adopt it !

I don't really like the extra system approach, I think Windows could just have a getter for this.

[Weibye]

The idea and API is good, I do think that this should be considered along with or after #4530
Or in other words, I believe #4530 is the "root cause" that this is trying to relieve a part of.