use_callback should take a closure that accepts an argument

[jkelleyrtp]

Feature Request

Currently there's no way to pass in an argument to the closure that use_callback wraps. This should be easy enough to add. I think the only reason we didn't add it is to make the type signature of UseCallback a bit more compact, but that's not so important.

[spookyvision]

since I needed this I've hacked something together (mostly copy-paste of dioxus code, plus one argument) that appears to work for me. Let me know if this looks plausible and if so I'll create a PR!

pub fn use_callback1<T, O>(f: impl FnMut(T) -> O + 'static) -> UseCallback1<O, T> {
    // Create a copyvalue with no contents
    // This copyvalue is generic over F so that it can be sized properly
    let mut inner = use_hook(|| CopyValue::new(None));

    // Every time this hook is called replace the inner callback with the new callback
    inner.set(Some(f));

    // And then wrap that callback in a boxed callback so we're blind to the size of the actual callback
    use_hook(|| {
        let cur_scope = current_scope_id().unwrap();
        let rt = Runtime::current().unwrap();

        UseCallback1 {
            inner: CopyValue::new(Box::new(move |arg1| {
                // run this callback in the context of the scope it was created in.
                let run_callback = || inner.with_mut(|f: &mut Option<_>| f.as_mut().unwrap()(arg1));
                rt.on_scope(cur_scope, run_callback)
            })),
        }
    })
}

/// This callback is not generic over a return type so you can hold a bunch of callbacks at once
///
/// If you need a callback that returns a value, you can simply wrap the closure you pass in that sets a value in its scope
#[derive(PartialEq)]
pub struct UseCallback1<O: 'static + ?Sized, T: 'static> {
    inner: CopyValue<Box<dyn FnMut(T) -> O>>,
}

impl<O: 'static + ?Sized, T> Clone for UseCallback1<O, T> {
    fn clone(&self) -> Self {
        Self { inner: self.inner }
    }
}
impl<O: 'static, T: 'static> Copy for UseCallback1<O, T> {}

impl<O, T> UseCallback1<O, T> {
    /// Call the callback
    pub fn call(&self, arg1: T) -> O {
        (self.inner.write_unchecked())(arg1)
    }
}

// This makes UseCallback callable like a normal function
impl<O, T> std::ops::Deref for UseCallback1<O, T> {
    type Target = dyn Fn(T) -> O;

    fn deref(&self) -> &Self::Target {
        // https://github.com/dtolnay/case-studies/tree/master/callable-types

        // First we create a closure that captures something with the Same in memory layout as Self (MaybeUninit<Self>).
        let uninit_callable = MaybeUninit::<Self>::uninit();
        // Then move that value into the closure. We assume that the closure now has a in memory layout of Self.
        let uninit_closure = move |arg1| Self::call(unsafe { &*uninit_callable.as_ptr() }, arg1);

        // Check that the size of the closure is the same as the size of Self in case the compiler changed the layout of the closure.
        let size_of_closure = std::mem::size_of_val(&uninit_closure);
        assert_eq!(size_of_closure, std::mem::size_of::<Self>());

        // Then cast the lifetime of the closure to the lifetime of &self.
        fn cast_lifetime<'a, T>(_a: &T, b: &'a T) -> &'a T {
            b
        }
        let reference_to_closure = cast_lifetime(
            {
                // The real closure that we will never use.
                &uninit_closure
            },
            // We transmute self into a reference to the closure. This is safe because we know that the closure has the same memory layout as Self so &Closure == &Self.
            unsafe { std::mem::transmute(self) },
        );

        // Cast the closure to a trait object.
        reference_to_closure as &_
    }
}