Duplicate Plugin Dependencies

[smessmer]

It seems app.add_plugin(MyPlugin) just directly calls MyPlugin::build(app). This means, if the same plugin gets called multiple times from different places, its build function would be called multiple times. What are best practices around this? I imagine it could break things since bevy relies so heavily on global state (in the form of resources).

One probably common example I can think of is if you build a game that uses multiple plugins, say PluginA and PluginB, and each of those plugins rely on a GUI plugin, say bevy-egui. Who should call app.add_plugin(bevy_egui::EguiPlugin)? Should PluginA and PluginB not call that and instead document that they rely on it and that the main app needs to call add_plugin(EguiPlugin)? Should PluginA and PluginB both call add_plugin(EguiPlugin) and should EguiPlugin be written in a way that it can handle being called multiple times? Should bevy itself somehow handle this and deduplicate those calls?

[harudagondi]

There is a PR (#6411) that solves this issue by disallowing addition of plugins multiple times unless explicitly allowed.

IMO I think the user should add EguiPlugin and not PluginA or PluginB to be more explicit.

[mockersf]

Also, https://dev-docs.bevyengine.org/bevy/app/struct.App.html#method.is_plugin_added let you check if a plugin has already been added. PluginA and PluginB could check that EguiPlugin has not been added before adding it

[smessmer]

Oh interesting, the plugin registry seems to have been added after the currently newest release 0.8.1, hadn't seen it.

[mockersf]

yup it has been added very recently after the move away from resources as settings

[smessmer]

Taking a look at common dependencies, they seem to follow a pattern of "if plugin not added yet, add it myself":

        if !app.world.contains_resource::<EguiSettings>() {
            app.add_plugin(EguiPlugin);
        }

Sources:
bevy-editor-pls
bevy-inspector-egui
bevy-console

Is this the pattern recommended for libraries offering plugins that depend on other third party plugins?