Headless -> GUI Application based on condtions

[TheDarkThief]

I'm trying to conditionally create a window later in the application's lifecycle, but because there is no Window on startup WinitPlugin does not setup the schedule runner. It also interferes with ScheduleRunnerPlugin, and when it non-deterministically gives it control, it does not spawn the window. What would be the correct way to do what I'm trying to do?

fn main() {
    App::new()
        .insert_resource(WinitSettings::desktop_app())
        // .insert_resource(WinitSettings {
        //     focused_mode: bevy::winit::UpdateMode::Continuous,
        //     unfocused_mode: bevy::winit::UpdateMode::reactive_low_power(Duration::from_millis(10)),
        // })
        .add_plugins(TaskPoolPlugin::default())
        .add_plugins(ScheduleRunnerPlugin::run_loop(Duration::from_secs_f64(
            1.0 / 20.0,
        )))
        .add_plugins(PanicHandlerPlugin::default())
        .add_plugins(TimePlugin::default())
        .add_plugins(FrameCountPlugin::default())
        .add_plugins(InputPlugin::default())
        .add_plugins(AssetPlugin::default())
        .add_plugins(AccessibilityPlugin::default())
        .add_plugins(ImagePlugin::default())
        .add_plugins(WindowPlugin {
            primary_window: None,
            exit_condition: ExitCondition::DontExit,
            ..default()
        })
        .add_plugins(WinitPlugin::<WakeUp>::default())
        .add_plugins(RenderPlugin::default())
        .add_plugins(MeshPlugin::default())
        .add_plugins(CameraPlugin::default())
        .add_systems(Startup, setup)
        .add_systems(Update, my_update)
        .run();
}

const MAP_WIDTH: usize = 10;
const MAP_HEIGHT: usize = 10;
const MAP_SEED: u64 = 0;

fn my_update() {
    print!("Ran update ")
}

fn setup(mut commands: Commands) {
    commands.spawn(Camera2d);
    commands.spawn(Window {
        title: "Test".into(),
        ..default()
    });
    println!("I ran")
}

[DevOpsThinh]

Personally, I think you shouldn't "spawn" things like that within the lifecycle of an Activity/UIViewController. For example, on the Android operating system, the Window (Screen) is only created during the ONCREATE phase and rendered (visible) during the ONSTART phase. Each Activity can contain fragments, etc., and you can "spawn" those fragments, widgets, UI controls, etc. Furthermore, in a game application, what are the advantages and disadvantages of spawning multiple "windows"?

[TheDarkThief]

I don't care for Android, I'm using bevy to create a CLI tool, that on occasion needs to display graphics, think of something like R Studio. This isn't a game situation, I hope that explains why I only want to optionally create windows :)

[DevOpsThinh]

Your CLI applications don't need to be complicated by Bevy. As per your requirements, you should develop your CLI application using pure Rust or other multi-threaded programming languages, with one thread dedicated to calling Bevy for your simulation purposes!

[TheDarkThief]

I've come to the conclusion that Bevy's ECS and plugin system are best fit for my project. If you disagree that's fine, but regardless Bevy should be able to support you being able to open a window conditionally on supported platforms. Consider the simple situation where someone wants to host a headless server of your game. Without shipping two binaries, the individual could just pass a launch argument allowing that, but that by necessity requires you check the launch arguments, and then it open a window if that argument is not present.

I understand that Bevy is a popular project, resulting in less skilled individuals using it, but if you couldn't tell from me attempting to import the plugins manually, I've read the docs.

I don't claim to be a master at Bevy or Rust, in fact I'm using this project to improve my skills in Rust.
But you saying:

Your CLI applications don't need to be complicated by Bevy

is not helpful to anyone, like duh not every project requires X library or framework, and often times there are better tools for the project. In fact I considered using ggez, or winit directly instead of Bevy. But after looking into them I found it'd just be easier to use Bevy, as well as more helpful considering I have other projects planned where Bevy is the clear choice for them, as well as if I want to later expand on the project by adding some 3D functionality, Bevy's got me covered.
If you have nothing to add besides I think your wrong, and/or Don't use Bevy, then I hope you have a good rest of your day, your opinion has been heard.

[mockersf]

There shouldn't be a problem spawning a window conditionally, after your application started. As you're adding plugins manually instead of through DefaultPlugins, I'm guessing you're missing some.

If you try this it works for me:

use bevy::{diagnostic::FrameCount, prelude::*};

fn main() {
    App::new()
        .add_plugins((DefaultPlugins.set(WindowPlugin {
            primary_window: None,
            exit_condition: bevy::window::ExitCondition::DontExit,
            ..default()
        }),))
        .add_systems(Update, spawn_window)
        .run();
}

fn spawn_window(mut commands: Commands, frame_count: Res<FrameCount>) {
    if frame_count.0 == 100 {
        commands.spawn(Window {
            title: "Late window".to_owned(),
            ..default()
        });
    }
}

[it-me-joda]

It seems Bevy doesn't render anything if... there's nothing to render, and Wayland doesn't open a window if nothing is rendered.

The updated code displays something, and adds the PrimaryWindow component to the window so that it's used.

This is true and this PR just merged and fixed that for me for a different purpose. #22307

[it-me-joda]

I tried @mockersf snippet and it works for me on main but not on 0.17 and I believe it's because of the above PR I mentioned

[mockersf]

So that should work on the 0.18, starting with the 0.18.0-rc.2 (not yet released)!

[TheDarkThief]

I'll be patiently waiting :D

[it-me-joda]

I'll be patiently waiting :D

0.18.0-rc.2 is out now so you should be good if you get on that