Replace multiple calls to add_system with add_systems

[JoJoJet]

Objective

When multiple systems are added to an app or schedule, prefer using add_systems. This makes the code less repetitive, and usually results in nicer formatting.

Example

// Before:
schedule.add_system(ab).add_system(cd).add_system(ce);

// After:
schedule.add_systems((ab, cd, ce));

[hymm]

Generally in favor of these changes. Most things are easier to read and understand. Had some comments around a few places where chain could have been used.

When we start applying different restrictions to multiple systems like with

.add_system((
    system1.in_set(Prepare),
    system2.in_set(Prepare),
    system3.in_set(Queue),
));

I think there are stylistic choices where it isn't clear what the best way to write things is. The above could also be written as:

.add_systems((system1, system2).in_set(Prepare))
.add_system(system3.in_set(Queue))

Probably not worth debating at this point, since we have some open issues around limitations of add_systems that could make this point moot.

[hymm]

can this use chain?

[JoJoJet]

Not really. These sets aren't being ordered with respect to each other, they're ordered with respect to *Flush sets.

[hymm]

schedule.configure_sets((
    PreStartup,
    PreStartupFlush,
    Startup,
    StartupFlush,
    PostStartup,
    PostStartupFlush
).chain());

Isn't this the same?

[JoJoJet]

Ooh I like that

[hymm]

Suggested change

	.add_startup_systems((setup_camera_fog, setup_terrain_scene, setup_instructions))
	.add_systems((
	setup_camera_fog,
	setup_terrain_scene,
	setup_instructions
	).on_startup())

We've removed most of the other add_startup_system calls.

[JoJoJet]

My main issue with this syntax is that rustfmt makes it look horrible. It'll become something like

add_systems(
    (
        setup_camera_fog,
        setup_terrain_scene,
        setup_instructions,
    )
        .on_startup(),
)

It's not exactly a dealbreaker, but for the time being I prefer it the way it is now.

[JoJoJet]

Once we get recursive SystemConfigs working, we should be able to make this look really nice:

add_systems((
    (setup_camera_fog, setup_terrain_scene, setup_instructions).on_startup(),
    toggle_system,
))

[james7132]

This in theory requires a lot more codegen than the original add_system chain. Wonder if this has any measurable impact on cargo build --timings.