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

[Jean-Denis Muys]

moc is declared inside the view controller class, which is a subclass of NSViewController which is an Objective-C class. So I believe the scope is @objc

In any case, I tried to prefix the declaration with “@objc”, but this is has no visible effect.

That being said, prefixing its declaration with “dynamic” sort of fixes the problem: Interface Builder still complains as described, but the program now works fine at runtime.

I have a hard time understanding clearly why: for bindings to work, the bound properties need to be KVO-compliant. But wouldn’t the @objc scope already do that?

What is the exact effect of the dynamic annotation?

JD

PS: I made a zip archive of the project available at http://dl.free.fr/pWkoKQLOc <http://dl.free.fr/pWkoKQLOc>. There is also an equivalent Objective-C project that works fine.


> On 17 Jan 2017, at 17:43, David Sweeris <davesweeris at mac.com <mailto:davesweeris at mac.com>> wrote:
> 
> Is `moc` declared in a non-"@objc" scope? I think there's a "@IBsomethingerather" that you have to put in front of those to let Interface Builder see them or something. Sorry I can't be more definitive, but I'm not in front of my computer and can't double-check.
> 
> HTH
> - Dave Sweeris 
> 
> Sent from my iPhone
> 
>> On Jan 16, 2017, at 10:11, Jean-Denis Muys via swift-users <swift-users at swift.org <mailto:swift-users at swift.org>> wrote:
>> 
>> 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 <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
>> 
>> 
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