Forward declarations in Objective-C headers and Xamarin.iOS

Question

Forward declarations in Objective-C headers and Xamarin.iOS

Luc MOUSSIEGT 1

Hi everyone,

I've been working recently on integrating an Objective-C framework in a Xamarin.iOS application. The framework was not delivered through CocoaPads but directly as a zip archive, containing the .framework folder. The framework itself looks very similar to this one.

The main issue I have is that one header from this framework contains forward declarations ( @class statements) of classes whose definitions are missing from the public headers. I don't know whether this is expected from Objective Sharpie or not, but the ApiDefinitions.cs files contains no interface corresponding to those forward declared classes and as a result, other classes that have properties of those type can't be built. Example:

Objective-C header

@class SecretViewController;

@protocol MainViewControllerDelegate <NSObject>
@property (nonatomic, weak) SecretViewController* secretController;
@end

Resulting ApiDefinitions.cs generated by Objective Sharpie

[Protocol, Model (AutoGeneratedName = true)]
[BaseType (typeof(NSObject))]
interface MainViewControllerDelegate
{
    // @required @property (nonatomic, weak) SecretViewController * secretController
    [Abstract]
    [Export ("secretController", ArgumentSemantic.Weak)]
    SecretViewController secretController { get; set; }
}

But of course, this can't be built successfully. I get a CS0246 error: The type or namespace 'SecretViewController' could not be found (are you missing a using directive or an assembly reference?)

How am I supposed to adapt these kind of declarations in the binding project? I have no access to the internal code of the framework.

Wenyan Zhang (Shanghai Wicresoft Co,.Ltd.) 36,436 Reputation points Microsoft External Staff

2022-01-13T10:16:27.393+00:00

Would you mind sharing the whole ApiDefinitions.cs file to me? Usually, the Namespace at ApiDefinition.cs and Structs.cs need be same as the name of iOSBindingLibrary, you could check it. You also could check if your .framework (OC) references other third-party libraries( .a or .framework).
Matt Johnson-Pint 1 Reputation point

2023-03-01T19:15:52.5666667+00:00
I'm a bit late here, but FWIW I just ran into this recently as well. The way I solved it was to define the private type-forwarded types manually as empty stubs. So in your example, it would be like this:

[DisableDefaultCtor] [BaseType (typeof(NSObject))] interface SecretViewController { }

Put that in your API definitions (or in any file referenced with <ObjcBindingApiDefinition> in your csproj, and it should compile.

Now, you won't have much actual use for the instance of the object returned, but it should have the correct pointers to the real ObjC object - so if you pass it back in to another API that requires one, it will work.

Your answer

Wenyan Zhang (Shanghai Wicresoft Co,.Ltd.) 36,436 Reputation points Microsoft External Staff

2022-01-13T10:16:27.393+00:00

Would you mind sharing the whole ApiDefinitions.cs file to me? Usually, the Namespace at ApiDefinition.cs and Structs.cs need be same as the name of iOSBindingLibrary, you could check it. You also could check if your .framework (OC) references other third-party libraries( .a or .framework).
Matt Johnson-Pint 1 Reputation point

2023-03-01T19:15:52.5666667+00:00

I'm a bit late here, but FWIW I just ran into this recently as well. The way I solved it was to define the private type-forwarded types manually as empty stubs. So in your example, it would be like this:

[DisableDefaultCtor] [BaseType (typeof(NSObject))] interface SecretViewController { }

Put that in your API definitions (or in any file referenced with <ObjcBindingApiDefinition> in your csproj, and it should compile.

Now, you won't have much actual use for the instance of the object returned, but it should have the correct pointers to the real ObjC object - so if you pass it back in to another API that requires one, it will work.

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Forward declarations in Objective-C headers and Xamarin.iOS

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