Working around the reverse callback limitation on Xamarin.iOS

There's one annoying technical limitation of Xamarin.iOS if you have to pass a C# delegate instance to unmanaged code. It's not new, and it's well documented.

But still, having to flag the callback with an attribute and no instance method makes an API hard to use if you don't care that much about the internals of the library you're consuming.

I'm currently polishing the Chipmunk binding for Xamarin.iOS, and the cpSpace has some functions taking callbacks, like cpSpaceEachBody or cpSpaceAddPostStepCallback.



The C# API exposed looks like this (for the PostStep callback):

public class Space {    public void AddPostStepCallBack (Action action, Body body);}

If we could lift the restrictions, it could take lambdas or anonymous methods. This is how I did it behind the scenes:

public class Space {
    delegate void PostStepFunc (IntPtr space, IntPtr obj, IntPtr data); 

    [MonoTouch.MonoPInvokeCallback (typeof (PostStepFunc))]
    static void PostStepForBody (IntPtr space, IntPtr obj, IntPtr data)
    {
        var handle = GCHandle.FromIntPtr (data);
        var action = (Action)handle.Target;
        handle.Free ();
        action (obj == IntPtr.Zero ? null :  new Body (obj));
    } 

    [DllImport ("__Internal")]
    extern static void cpSpaceAddPostStepCallback (IntPtr space, PostStepFunc func, IntPtr key, IntPtr data);  

    public void AddPostStepCallback (Action action, Body obj)
    {
        var data = GCHandle.ToIntPtr(GCHandle.Alloc (action));
        cpSpaceAddPostStepCallback (Handle.Handle, PostStepForBody, obj.Handle.Handle, data);
    }
}

Static callback? : Check!
Flagged with attribute? : Check!

This is possible because of the free to use data pointer as last argument of the native function call. We don't expose it to the use, and hijack it to pass the GCHandle ptr of the callback provided by the user.

Hope it helps you.



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