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# Objective #14098 added the `FunctionRegistry` for registering functions such that they can be retrieved by name and used dynamically. One thing we chose to leave out in that initial PR is support for closures. Why support closures? Mainly, we don't want to prohibit users from injecting environmental data into their registered functions. This allows these functions to not leak their internals to the public API. For example, let's say we're writing a library crate that allows users to register callbacks for certain actions. We want to perform some actions before invoking the user's callback so we can't just call it directly. We need a closure for this: ```rust registry.register("my_lib::onclick", move |event: ClickEvent| { // ...other work... user_onclick.call(event); // <-- Captured variable }); ``` We could have made our callback take a reference to the user's callback. This would remove the need for the closure, but it would change our desired API to place the burden of fetching the correct callback on the caller. ## Solution Modify the `FunctionRegistry` to store registered functions as `DynamicClosure<'static>` instead of `DynamicFunction` (now using `IntoClosure` instead of `IntoFunction`). Due to limitations in Rust and how function reflection works, `DynamicClosure<'static>` is functionally equivalent to `DynamicFunction`. And a normal function is considered a subset of closures (it's a closure that doesn't capture anything), so there shouldn't be any difference in usage: all functions that satisfy `IntoFunction` should satisfy `IntoClosure`. This means that the registration API introduced in #14098 should require little-to-no changes on anyone following `main`. ### Closures vs Functions One consideration here is whether we should keep closures and functions separate. This PR unifies them into `DynamicClosure<'static>`, but we can consider splitting them up. The reasons we might want to do so are: - Simplifies mental model and terminology (users don't have to understand that functions turn into closures) - If Rust ever improves its function model, we may be able to add additional guarantees to `DynamicFunction` that make it useful to separate the two - Adding support for generic functions may be less confusing for users since closures in Rust technically can't be generic The reasons behind this PR's unification approach are: - Reduces the number of methods needed on `FunctionRegistry` - Reduces the number of lookups a user may have to perform (i.e. "`get_function` or else `get_closure`") - Establishes `DynamicClosure<'static>` as the de facto dynamic callable (similar to how most APIs in Rust code tend to prefer `impl Fn() -> String` over `fn() -> String`) I'd love to hear feedback on this matter, and whether we should continue with this PR's approach or switch to a split model. ## Testing You can test locally by running: ``` cargo test --package bevy_reflect ``` --- ## Showcase Closures can now be registered into the `FunctionRegistry`: ```rust let punct = String::from("!!!"); registry.register_with_name("my_crate::punctuate", move |text: String| { format!("{}{}", text, punct) }); ``` |
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