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Swift classes can inherit from traits #2363

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Dec 24, 2024
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Swift classes can inherit from traits #2363

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5 changes: 3 additions & 2 deletions CHANGELOG.md
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Expand Up @@ -33,8 +33,9 @@ We have [detailed upgrade notes](https://mozilla.github.io/uniffi-rs/next/Upgrad
[Detailed upgrade notes](https://mozilla.github.io/uniffi-rs/next/Upgrading.html)
### What's new?

- Kotlin: Proc-macros exporting an `impl Trait for Struct` block now has a class inheritance
hierarcy to reflect that. [#2297](https://github.com/mozilla/uniffi-rs/pull/2297)
- Kotlin and Swift: Proc-macros exporting an `impl Trait for Struct` block now has a class inheritance
hierarcy to reflect that.
[#2297](https://github.com/mozilla/uniffi-rs/pull/2297), [#2363](https://github.com/mozilla/uniffi-rs/pull/2363)

- Removed the `log` dependency and logging statements about FFI calls. These were not really useful
to consumers and could have high overhead when lots of FFI calls are made. Instead, the
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21 changes: 21 additions & 0 deletions fixtures/coverall/tests/bindings/test_coverall.swift
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Expand Up @@ -464,6 +464,27 @@ do {
traits[0].setParent(parent: nil)
}

// A struct which implements the node trait.
do {
let n = Node(name: "node")
assert(String(describing: n).starts(with: "Node { name: Some(\"node\"), parent: Mutex { "))
assert(n.getParent()?.name() == "via node")

n.setParent(parent: n.getParent())
// doubly-wrapped :(
// Get: "Some(UniFFICallbackHandlerNodeTrait { handle: 19 })"
// Want: Like the Rust node above.
// debugPrint("parent \(n.describeParent())")

let rustParent = Node(name: "parent")
n.setParent(parent: rustParent)
assert(n.getParent()?.name() == "parent")

let swiftParent = SwiftNode()
rustParent.setParent(parent: swiftParent)
assert(ancestorNames(node: n) == ["parent", "node-swift"])
}

// Test round tripping
do {
let rustGetters = makeRustGetters()
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21 changes: 21 additions & 0 deletions uniffi_bindgen/src/bindings/swift/gen_swift/mod.rs
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Expand Up @@ -272,6 +272,27 @@ impl Config {
}
}

// Given a trait, work out what the protocol name we generate for it.
// This differs based on whether the trait supports foreign impls (ie,
// whether is has a "callback interface".
fn trait_protocol_name(ci: &ComponentInterface, name: &str) -> Result<String> {
let (obj_name, has_callback_interface) = match ci.get_object_definition(name) {
Some(obj) => (obj.name(), obj.has_callback_interface()),
None => (
ci.get_callback_interface_definition(name)
.ok_or_else(|| anyhow::anyhow!("no interface {}", name))?
.name(),
true,
),
};
let class_name = SwiftCodeOracle.class_name(obj_name);
if has_callback_interface {
Ok(class_name)
} else {
Ok(format!("{class_name}Protocol"))
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Is there a good reason why this has "Protocol" appended? IIUC, this is a Rust trait without callback interface support, so Swift can't implement it. In that case, it seems like protocol should get {class_name} which feels like a nicer name, and the class that implements the protocol should get something like{class_name}Impl. I think that's how the other languages handle it.

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This is exactly the same as kotlin which has "Interface" appended

Note that this change doesn't change the hierarchy of any existing objects, it just controls which of the 2 generated classes is used in the inheritance chain.

I'm inclined to agree that the existing hierarchy is odd in these cases, I fear that trying to change that here will accidentally introduce new regressions.

Is this also your understanding - ie, that this patch doesn't change the class hierarchy for existing types, and that doing so is something we'd need to be very careful about?

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@bendk bendk Dec 23, 2024 •
edited
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Is this also your understanding - ie, that this patch doesn't change the class hierarchy for existing types

Yes agreed.

..and that doing so is something we'd need to be very careful about?

Maybe I have a slight difference of opinion here. I think it could be okay to make the change, since most end-users aren't going to see a difference. If they store a Foo and only use it to call methods from the protocol, it shouldn't matter if Foo is a concrete type that implements the protocol or the protocol itself.

I agree we should be careful, but maybe I'd remove the "very" part.

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Also, I think I got confused and believed that we had already made the change for other languages. If Kotlin works this way, then I think Swift should too. If we decide to change them, let's change both at the same time.

}
}

/// Generate UniFFI component bindings for Swift, as strings in memory.
pub fn generate_bindings(config: &Config, ci: &ComponentInterface) -> Result<Bindings> {
let header = BridgingHeader::new(config, ci)
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6 changes: 5 additions & 1 deletion uniffi_bindgen/src/bindings/swift/templates/ObjectTemplate.swift
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Expand Up @@ -25,7 +25,11 @@ open class {{ impl_class_name }}:
{%- if is_error %}
Swift.Error,
{% endif %}
{{ protocol_name }} {
{%- for t in obj.trait_impls() %}
{{ self::trait_protocol_name(ci, t.trait_name)? }},
{% endfor %}
{{ protocol_name }}
{
fileprivate let pointer: UnsafeMutableRawPointer!

/// Used to instantiate a [FFIObject] without an actual pointer, for fakes in tests, mostly.
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