bevy/crates/bevy_app/src/plugin.rs

use downcast_rs::{impl_downcast, Downcast};

use crate::App;
use std::any::Any;

/// A collection of Bevy app logic and configuration.
///
/// Plugins configure an [`App`]. When an [`App`] registers a plugin,
/// the plugin's [`Plugin::build`] function is run. By default, a plugin
/// can only be added once to an [`App`].
///
/// If the plugin may need to be added twice or more, the function [`is_unique()`](Self::is_unique)
/// should be overridden to return `false`. Plugins are considered duplicate if they have the same
/// [`name()`](Self::name). The default `name()` implementation returns the type name, which means
/// generic plugins with different type parameters will not be considered duplicates.
pub trait Plugin: Downcast + Any + Send + Sync {
    /// Configures the [`App`] to which this plugin is added.
    fn build(&self, app: &mut App);
    /// Configures a name for the [`Plugin`] which is primarily used for debugging.
    fn name(&self) -> &str {
        std::any::type_name::<Self>()
    }
    /// If the plugin can be meaningfully instantiated several times in an [`App`](crate::App),
    /// override this method to return `false`.
    fn is_unique(&self) -> bool {
        true
    }
}

impl_downcast!(Plugin);

/// A type representing an unsafe function that returns a mutable pointer to a [`Plugin`].
/// It is used for dynamically loading plugins.
///
/// See `bevy_dynamic_plugin/src/loader.rs#dynamically_load_plugin`.
pub type CreatePlugin = unsafe fn() -> *mut dyn Plugin;