Override delegate in supplied Obj-C library in C#

pauldpauld USMember ✭✭
edited January 2013 in Xamarin.iOS


I'm working on integrating with a third-party hardware accessory. The iOS library that I'm binding to (successfully) also has some UI elements (UIAlertViews) that it displays, and I have requested that I be able to override these messages. The developers altered the library so it calls a number of class methods on a Obj-C class called AlertMessages, which is a UIAlertViewDelegate. This class appears to be linked into the supplied hardware library I'm binding to, but the example Xcode project also has an AlertMessages.m class that re-implements the class. The Objective-C runtime seems to know that it should use this external AlertMessages.m for the class, rather than the one embedded in the library - if I remove the .m file, the messages still work as they come from the library, and if I change the message text in AlertMessages.m then I see the changed text. All fine.

So, I'm now trying to implement my own AlertMessages class in C#, but I can't get the library or runtime to see my class, it just defaults back to the one in the .a I've bound to - note that I've bound to other classes in the library, but not specifically the AlertMessage class.

Here's what I have so far:

using MonoTouch.ObjCRuntime;
using MonoTouch.Foundation;

namespace Cosmos.iOS
    public class AlertMessages : UIAlertViewDelegate
        static Selector selInit       = new Selector("init");

        public AlertMessages ()
            : base(NSObjectFlag.Empty)
            Handle = Messaging.IntPtr_objc_msgSend(this.Handle,

        public AlertMessages (IntPtr handle)
            : base(handle)

        public static void alertMessagesplayAlarm()
            Console.WriteLine("alertmessage 1");

                /// ... other class methods

At the moment, none of the methods in my class are getting called, but I'm not sure why. I can't see anything in their sample code that specifies which delegate to use, and I don't think that would explain the ability to override it from the .m file.




  • ChrisHardyChrisHardy GBForum Administrator, Xamarin Team Xamurai

    How come you are writing the bindings manually yourself? Why do you use the binding library tools available to do the hard work for you: http://docs.xamarin.com/ios/Guides/Advanced_Topics/Binding_Objective-C_Libraries ?

  • pauldpauld USMember ✭✭

    Yes, I did use the binding tools to bind to the Obj-C library, I followed that guide you linked to. So I can call into the Obj-C library from C#.

    Where I'm stuck is creating a C# class that Obj-C can call into - the opposite direction. The information on doing that isn't as clear (to me). I ended up basing my AlertMessages (UIAlertViewDelegate) on the last section on this page: http://docs.xamarin.com/ios/Guides/Advanced_Topics/Binding_Objective-C_Libraries/binding_details

    I started off just using the RegisterAttribute and ExportAttribute but in desperation added the selector stuff, but I think that's not helping.

    Thinking about it some more, I think the code in the library I'm binding to must be working out if an AlertMessages class exists and uses it if present - I need to work out how it's doing that and get my class to appear in the same way.

    This is the class I need to implement in C#:

    @interface AlertMessages : NSObject <UIAlertViewDelegate>{
    // ...more class methods
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