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| Author | SHA1 | Message | Date | |
|---|---|---|---|---|
Gino Valente
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1042f09c2e
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bevy_reflect: Add DynamicClosure and DynamicClosureMut (#14141)
# Objective As mentioned in [this](https://github.com/bevyengine/bevy/pull/13152#issuecomment-2198387297) comment, creating a function registry (see #14098) is a bit difficult due to the requirements of `DynamicFunction`. Internally, a `DynamicFunction` contains a `Box<dyn FnMut>` (the function that reifies reflected arguments and calls the actual function), which requires `&mut self` in order to be called. This means that users would require a mutable reference to the function registry for it to be useful— which isn't great. And they can't clone the `DynamicFunction` either because cloning an `FnMut` isn't really feasible (wrapping it in an `Arc` would allow it to be cloned but we wouldn't be able to call the clone since we need a mutable reference to the `FnMut`, which we can't get with multiple `Arc`s still alive, requiring us to also slap in a `Mutex`, which adds additional overhead). And we don't want to just replace the `dyn FnMut` with `dyn Fn` as that would prevent reflecting closures that mutate their environment. Instead, we need to introduce a new type to split the requirements of `DynamicFunction`. ## Solution Introduce new types for representing closures. Specifically, this PR introduces `DynamicClosure` and `DynamicClosureMut`. Similar to how `IntoFunction` exists for `DynamicFunction`, two new traits were introduced: `IntoClosure` and `IntoClosureMut`. Now `DynamicFunction` stores a `dyn Fn` with a `'static` lifetime. `DynamicClosure` also uses a `dyn Fn` but has a lifetime, `'env`, tied to its environment. `DynamicClosureMut` is most like the old `DynamicFunction`, keeping the `dyn FnMut` and also typing its lifetime, `'env`, to the environment Here are some comparison tables: | | `DynamicFunction` | `DynamicClosure` | `DynamicClosureMut` | | - | ----------------- | ---------------- | ------------------- | | Callable with `&self` | ✅ | ✅ | ❌ | | Callable with `&mut self` | ✅ | ✅ | ✅ | | Allows for non-`'static` lifetimes | ❌ | ✅ | ✅ | | | `IntoFunction` | `IntoClosure` | `IntoClosureMut` | | - | -------------- | ------------- | ---------------- | | Convert `fn` functions | ✅ | ✅ | ✅ | | Convert `fn` methods | ✅ | ✅ | ✅ | | Convert anonymous functions | ✅ | ✅ | ✅ | | Convert closures that capture immutable references | ❌ | ✅ | ✅ | | Convert closures that capture mutable references | ❌ | ❌ | ✅ | | Convert closures that capture owned values | ❌[^1] | ✅ | ✅ | [^1]: Due to limitations in Rust, `IntoFunction` can't be implemented for just functions (unless we forced users to manually coerce them to function pointers first). So closures that meet the trait requirements _can technically_ be converted into a `DynamicFunction` as well. To both future-proof and reduce confusion, though, we'll just pretend like this isn't a thing. ```rust let mut list: Vec<i32> = vec![1, 2, 3]; // `replace` is a closure that captures a mutable reference to `list` let mut replace = |index: usize, value: i32| -> i32 { let old_value = list[index]; list[index] = value; old_value }; // Convert the closure into a dynamic closure using `IntoClosureMut::into_closure_mut` let mut func: DynamicClosureMut = replace.into_closure_mut(); // Dynamically call the closure: let args = ArgList::default().push_owned(1_usize).push_owned(-2_i32); let value = func.call_once(args).unwrap().unwrap_owned(); // Check the result: assert_eq!(value.take::<i32>().unwrap(), 2); assert_eq!(list, vec![1, -2, 3]); ``` ### `ReflectFn`/`ReflectFnMut` To make extending the function reflection system easier (the blanket impls for `IntoFunction`, `IntoClosure`, and `IntoClosureMut` are all incredibly short), this PR generalizes callables with two new traits: `ReflectFn` and `ReflectFnMut`. These traits mimic `Fn` and `FnMut` but allow for being called via reflection. In fact, their blanket implementations are identical save for `ReflectFn` being implemented over `Fn` types and `ReflectFnMut` being implemented over `FnMut` types. And just as `Fn` is a subtrait of `FnMut`, `ReflectFn` is a subtrait of `ReflectFnMut`. So anywhere that expects a `ReflectFnMut` can also be given a `ReflectFn`. To reiterate, these traits aren't 100% necessary. They were added in purely for extensibility. If we decide to split things up differently or add new traits/types in the future, then those changes should be much simpler to implement. ### `TypedFunction` Because of the split into `ReflectFn` and `ReflectFnMut`, we needed a new way to access the function type information. This PR moves that concept over into `TypedFunction`. Much like `Typed`, this provides a way to access a function's `FunctionInfo`. By splitting this trait out, it helps to ensure the other traits are focused on a single responsibility. ### Internal Macros The original function PR (#13152) implemented `IntoFunction` using a macro which was passed into an `all_tuples!` macro invocation. Because we needed the same functionality for these new traits, this PR has copy+pasted that code for `ReflectFn`, `ReflectFnMut`, and `TypedFunction`— albeit with some differences between them. Originally, I was going to try and macro-ify the impls and where clauses such that we wouldn't have to straight up duplicate a lot of this logic. However, aside from being more complex in general, autocomplete just does not play nice with such heavily nested macros (tried in both RustRover and VSCode). And both of those problems told me that it just wasn't worth it: we need to ensure the crate is easily maintainable, even at the cost of duplicating code. So instead, I made sure to simplify the macro code by removing all fully-qualified syntax and cutting the where clauses down to the bare essentials, which helps to clean up a lot of the visual noise. I also tried my best to document the macro logic in certain areas (I may even add a bit more) to help with maintainability for future devs. ### Documentation Documentation for this module was a bit difficult for me. So many of these traits and types are very interconnected. And each trait/type has subtle differences that make documenting it in a single place, like at the module level, difficult to do cleanly. Describing the valid signatures is also challenging to do well. Hopefully what I have here is okay. I think I did an okay job, but let me know if there any thoughts on ways to improve it. We can also move such a task to a followup PR for more focused discussion. ## Testing You can test locally by running: ``` cargo test --package bevy_reflect ``` --- ## Changelog - Added `DynamicClosure` struct - Added `DynamicClosureMut` struct - Added `IntoClosure` trait - Added `IntoClosureMut` trait - Added `ReflectFn` trait - Added `ReflectFnMut` trait - Added `TypedFunction` trait - `IntoFunction` now only works for standard Rust functions - `IntoFunction` no longer takes a lifetime parameter - `DynamicFunction::call` now only requires `&self` - Removed `DynamicFunction::call_once` - Changed the `IntoReturn::into_return` signature to include a where clause ## Internal Migration Guide > [!important] > Function reflection was introduced as part of the 0.15 dev cycle. This migration guide was written for developers relying on `main` during this cycle, and is not a breaking change coming from 0.14. ### `IntoClosure` `IntoFunction` now only works for standard Rust functions. Calling `IntoFunction::into_function` on a closure that captures references to its environment (either mutable or immutable), will no longer compile. Instead, you will need to use either `IntoClosure::into_closure` to create a `DynamicClosure` or `IntoClosureMut::into_closure_mut` to create a `DynamicClosureMut`, depending on your needs: ```rust let punct = String::from("!"); let print = |value: String| { println!("{value}{punct}"); }; // BEFORE let func: DynamicFunction = print.into_function(); // AFTER let func: DynamicClosure = print.into_closure(); ``` ### `IntoFunction` lifetime Additionally, `IntoFunction` no longer takes a lifetime parameter as it always expects a `'static` lifetime. Usages will need to remove any lifetime parameters: ```rust // BEFORE fn execute<'env, F: IntoFunction<'env, Marker>, Marker>(f: F) {/* ... */} // AFTER fn execute<F: IntoFunction<Marker>, Marker>(f: F) {/* ... */} ``` ### `IntoReturn` `IntoReturn::into_return` now has a where clause. Any manual implementors will need to add this where clause to their implementation. |
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|
Gino Valente
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276dd04001
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bevy_reflect: Function reflection (#13152)
# Objective
We're able to reflect types sooooooo... why not functions?
The goal of this PR is to make functions callable within a dynamic
context, where type information is not readily available at compile
time.
For example, if we have a function:
```rust
fn add(left: i32, right: i32) -> i32 {
left + right
}
```
And two `Reflect` values we've already validated are `i32` types:
```rust
let left: Box<dyn Reflect> = Box::new(2_i32);
let right: Box<dyn Reflect> = Box::new(2_i32);
```
We should be able to call `add` with these values:
```rust
// ?????
let result: Box<dyn Reflect> = add.call_dynamic(left, right);
```
And ideally this wouldn't just work for functions, but methods and
closures too!
Right now, users have two options:
1. Manually parse the reflected data and call the function themselves
2. Rely on registered type data to handle the conversions for them
For a small function like `add`, this isn't too bad. But what about for
more complex functions? What about for many functions?
At worst, this process is error-prone. At best, it's simply tedious.
And this is assuming we know the function at compile time. What if we
want to accept a function dynamically and call it with our own
arguments?
It would be much nicer if `bevy_reflect` could alleviate some of the
problems here.
## Solution
Added function reflection!
This adds a `DynamicFunction` type to wrap a function dynamically. This
can be called with an `ArgList`, which is a dynamic list of
`Reflect`-containing `Arg` arguments. It returns a `FunctionResult`
which indicates whether or not the function call succeeded, returning a
`Reflect`-containing `Return` type if it did succeed.
Many functions can be converted into this `DynamicFunction` type thanks
to the `IntoFunction` trait.
Taking our previous `add` example, this might look something like
(explicit types added for readability):
```rust
fn add(left: i32, right: i32) -> i32 {
left + right
}
let mut function: DynamicFunction = add.into_function();
let args: ArgList = ArgList::new().push_owned(2_i32).push_owned(2_i32);
let result: Return = function.call(args).unwrap();
let value: Box<dyn Reflect> = result.unwrap_owned();
assert_eq!(value.take::<i32>().unwrap(), 4);
```
And it also works on closures:
```rust
let add = |left: i32, right: i32| left + right;
let mut function: DynamicFunction = add.into_function();
let args: ArgList = ArgList::new().push_owned(2_i32).push_owned(2_i32);
let result: Return = function.call(args).unwrap();
let value: Box<dyn Reflect> = result.unwrap_owned();
assert_eq!(value.take::<i32>().unwrap(), 4);
```
As well as methods:
```rust
#[derive(Reflect)]
struct Foo(i32);
impl Foo {
fn add(&mut self, value: i32) {
self.0 += value;
}
}
let mut foo = Foo(2);
let mut function: DynamicFunction = Foo::add.into_function();
let args: ArgList = ArgList::new().push_mut(&mut foo).push_owned(2_i32);
function.call(args).unwrap();
assert_eq!(foo.0, 4);
```
### Limitations
While this does cover many functions, it is far from a perfect system
and has quite a few limitations. Here are a few of the limitations when
using `IntoFunction`:
1. The lifetime of the return value is only tied to the lifetime of the
first argument (useful for methods). This means you can't have a
function like `(a: i32, b: &i32) -> &i32` without creating the
`DynamicFunction` manually.
2. Only 15 arguments are currently supported. If the first argument is a
(mutable) reference, this number increases to 16.
3. Manual implementations of `Reflect` will need to implement the new
`FromArg`, `GetOwnership`, and `IntoReturn` traits in order to be used
as arguments/return types.
And some limitations of `DynamicFunction` itself:
1. All arguments share the same lifetime, or rather, they will shrink to
the shortest lifetime.
2. Closures that capture their environment may need to have their
`DynamicFunction` dropped before accessing those variables again (there
is a `DynamicFunction::call_once` to make this a bit easier)
3. All arguments and return types must implement `Reflect`. While not a
big surprise coming from `bevy_reflect`, this implementation could
actually still work by swapping `Reflect` out with `Any`. Of course,
that makes working with the arguments and return values a bit harder.
4. Generic functions are not supported (unless they have been manually
monomorphized)
And general, reflection gotchas:
1. `&str` does not implement `Reflect`. Rather, `&'static str`
implements `Reflect` (the same is true for `&Path` and similar types).
This means that `&'static str` is considered an "owned" value for the
sake of generating arguments. Additionally, arguments and return types
containing `&str` will assume it's `&'static str`, which is almost never
the desired behavior. In these cases, the only solution (I believe) is
to use `&String` instead.
### Followup Work
This PR is the first of two PRs I intend to work on. The second PR will
aim to integrate this new function reflection system into the existing
reflection traits and `TypeInfo`. The goal would be to register and call
a reflected type's methods dynamically.
I chose not to do that in this PR since the diff is already quite large.
I also want the discussion for both PRs to be focused on their own
implementation.
Another followup I'd like to do is investigate allowing common container
types as a return type, such as `Option<&[mut] T>` and `Result<&[mut] T,
E>`. This would allow even more functions to opt into this system. I
chose to not include it in this one, though, for the same reasoning as
previously mentioned.
### Alternatives
One alternative I had considered was adding a macro to convert any
function into a reflection-based counterpart. The idea would be that a
struct that wraps the function would be created and users could specify
which arguments and return values should be `Reflect`. It could then be
called via a new `Function` trait.
I think that could still work, but it will be a fair bit more involved,
requiring some slightly more complex parsing. And it of course is a bit
more work for the user, since they need to create the type via macro
invocation.
It also makes registering these functions onto a type a bit more
complicated (depending on how it's implemented).
For now, I think this is a fairly simple, yet powerful solution that
provides the least amount of friction for users.
---
## Showcase
Bevy now adds support for storing and calling functions dynamically
using reflection!
```rust
// 1. Take a standard Rust function
fn add(left: i32, right: i32) -> i32 {
left + right
}
// 2. Convert it into a type-erased `DynamicFunction` using the `IntoFunction` trait
let mut function: DynamicFunction = add.into_function();
// 3. Define your arguments from reflected values
let args: ArgList = ArgList::new().push_owned(2_i32).push_owned(2_i32);
// 4. Call the function with your arguments
let result: Return = function.call(args).unwrap();
// 5. Extract the return value
let value: Box<dyn Reflect> = result.unwrap_owned();
assert_eq!(value.take::<i32>().unwrap(), 4);
```
## Changelog
#### TL;DR
- Added support for function reflection
- Added a new `Function Reflection` example:
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