[swift-users] How to do Cocoa bindings in Swift? IB rejects optionals

[Jean-Denis Muys]

Hi,

I am exploring the use of Swift for a Mac Cocoa application - using Xcode 8.2.1, and it seems I hit a roadblock regarding Cocoa bindings.

In this toy Core Data document-based project, I added an NSArrayController in the StoryBoard. I need to bind it a NSManagedObjectContext so that my user interface works by itself. I followed the solution outlined in Technical Q&A QA1871 (https://developer.apple.com/library/content/qa/qa1871/_index.html).

So I want to add a property of type ManagedObjectContext to my ViewController class. 

I naturally declared it as an optional:

var moc: ManagedObjectContext?

But when I enter the property name in the bindings inspector of InterfaceBuilder, it complains: there is a red exclamation mark, and hovering over it pops up this error message:

“The Managed Object Context binding expects to be bound to an object of type NSObject, but mac is of type ManagedObjectContext?”

And it fails at run time too.

changing the type to ManagedObjectContext! doesn’t help: IB complains in exactly the same way.

changing the type to a non optional ManagedObjectContext silences the IB error, but now my ViewController class doesn’t compile anymore. The error I get is:

class ViewController has no initialiser 

I completely understand this error message. And I can add an initialiser. But I get this new error message:

property ‘self.moc’ not initialised at super.init call.

I understand that one too, but what can I do? At initialiser-time, the managedObjectContext is not yet known. In Objective-C I would set it to nil, which I cannot do since the property is not an optional any more.

Do I really need to allocate a dummy sentinel ManagedObjectContext, just to make the compiler happy?

This would be ugly as hell, far worse than the nil value we use in Objective-C. Swift in that case, would not be safer, but less safe than Obj-C.

I find this idea repulsive. Or did I miss something?

Or is Swift fundamentally incompatible with Cocoa bindings? That would be a pity too.

Thanks,

Jean-Denis