Resource Not Available in Later System Call

[asb9189]

Hi All,

I have a setup system that adds a resource containing the primary window's width and height so that I don't need to query for it every time. In other words, I want my future systems to have Res<WinSize> rather than Query<&Window, With<PrimaryWindow>> in their function definition.

However, my spawn_player system runs after setup and panics because the Res<WinSize has not yet been added as an available resource. The ordering of system execution is correct, however, the commands have not yet been executed by the time spawn_player executes.

I found this useful discussion (#3174) regarding the same issue, however it's not for Bevy 0.10. I noticed that Bevy 0.10's migration notes says the following:

Stages have been removed. Replace these with system sets, and then add command flushes using the apply_system_buffers exclusive system where needed.

I'm new to the Bevy ecosystem so I've been having a hard time finding examples of system sets and using the apply_system_buffers function. How could I easily ensure a resource that is injected by one system is available by a later system?

Thanks!

[nicopap]

How are you adding Res<WinSize> to your app? If you add setup to your app using .add_system(Startup, setup) and spawn_player in Update afaik, it should work, there is a synchronization step between the two. So it's something else.

Let me suggest two alternatives

1. Instead of adding the resource in setup, implement FromWorld for it.

impl FromWorld for WinSize {
   fn from_world(world: &mut World) -> Self {
      // impl
   }
}

Then you need to add the resource to the app as follow: app.init_resource::<WinSize>().

2. Use Query<&Window, With<PrimaryWindow>>. After all, your resource would lie to you if the window size changes from external factors. WinSize doesn't follow the single-source-of-truth principle, and will likely lead to bugs.

To simplify the task, you can define a SystemParam as follow:

#[derive(SystemParam)]
struct WinSize<'w, 's> {
    windows: Query<'w, 's, &Window, With<PrimaryWindow>>,
}
impl<'w, 's> WinSize<'w, 's> {
    fn size(&self) -> (u32, u32) {
        let resolution = self.windows.get_single().unwrap().resolution;
        (resolution.width, resolution.height)
    }
}

Now if you want to query for window size in a system you do

fn win_size_system(win_size: WinSize) {
  let size = win_size.size();
  println!("{size:?}");
}