What's the best way to implement an Objective C protocol in D?

I see mention here https://dlang.org/changelog/2.085.0.html#4_deprecated_objc_interfaces but it's not clear where things are these days.

On Saturday, 16 May 2020 at 19:14:51 UTC, John Colvin wrote:
> What's the best way to implement an Objective C protocol in D?
>
> I see mention here https://dlang.org/changelog/2.085.0.html#4_deprecated_objc_interfaces but it's not clear where things are these days.

Based on some experimentation, I'm starting to wonder do protocols actually have any runtime component in Objective C? Because if I pass in an extern(Objective-C) class with the right interface to a function expecting a protocol everything just works.

On Saturday, 16 May 2020 at 19:14:51 UTC, John Colvin wrote:
> What's the best way to implement an Objective C protocol in D?
>
> I see mention here https://dlang.org/changelog/2.085.0.html#4_deprecated_objc_interfaces but it's not clear where things are these days.

I did it throught the Obj-C runtime a while ago:
https://github.com/AuburnSounds/Dplug/blob/dda1f80d69e8bfd4af0271721738ce827c2f0eae/au/dplug/au/cocoaviewfactory.d#L99

and the result is brittle, you need to replicate the protocol declaration, add all methods etc.

On 2020-05-17 11:32, John Colvin wrote:
> On Saturday, 16 May 2020 at 19:14:51 UTC, John Colvin wrote:
>> What's the best way to implement an Objective C protocol in D?
>>
>> I see mention here https://dlang.org/changelog/2.085.0.html#4_deprecated_objc_interfaces but it's not clear where things are these days.

It's the same these days. It's still not implemented.

> Based on some experimentation, I'm starting to wonder do protocols actually have any runtime component in Objective C?

No, not really.

> Because if I pass in  an extern(Objective-C) class with the right interface to a function expecting a protocol everything just works.

Yes, that works fine.

You can put the methods from the protocol directly in the class that implements them or in a base class. If you really want to have a specific type for the protocol you can use an abstract class to emulate an interface/protocol and cast your actual class to the abstract class:

extern (Objective-C)
abstract class Printer // the protocol
{
    void print(int value) @selector("print:");
}

extern (Objective-C)
class Foo : NSObject
{
    override static Foo alloc() @selector("alloc");
    override Foo init() @selector("init");

    void print(int value) @selector("print:")
    {
        writeln(value);
    }
}

extern (Objective-C) void print(Printer);

void main()
{
    auto foo = Foo.alloc.init;
    print(cast(Printer) cast(void*) foo); // need to cast through void*
}

-- 
/Jacob Carlborg