Swift closure in array becomes nil in Objective-c

Question

I created an objective-c method which will invoke a method via NSInvocation:

typedef void (^ScriptingEmptyBlock)();
typedef void (^ScriptingErrorBlock)(NSErr         


        

Answer 1

I must admit that I don't fully understand why this is happening, but as far as I can tell this has nothing to do with using NSInvocation and would happen even if we just passed a Swift closure to an Objective-C function via a parameter of type id. Passing an Objective-C block via id works just fine, not sure why: Swift closures are supposed to be compatible with Objective-C blocks. As you know, elements of NSArray are of type id, so any Objective-C object can be an array element.

To work around this problem of accessing a Swift closure passed via id in Objective-C one can introduce a wrapper class.

// In a header:
@interface EmptyBlockWrapper : NSObject
@property EmptyBlock _blk;
@end

// In an implementation file (just an empty implementation):
@implementation EmptyBlockWrapper

@end

Then we can use a wrapper instance instead of a block as an array element in Swift:

let myBlock : EmptyBlock = {
    print("In Swift EmptyBlock...")
}

let myBlockWrapper = EmptyBlockWrapper()
myBlockWrapper._blk = myBlock

In an Objective-C method we can call it as follows, assuming args is NSArray *:

EmptyBlockWrapper * emptyBlockWrapper = args[1];
emptyBlockWrapper._blk();

Hopefully this is helpful. Of course, this is just a simplified example to give you an idea; this could be made much fancier.