Invoke Swift closure in Objective-C via `id` - objective-c

Question

This question has been inspired by this one: Swift closure in array becomes nil in Objective-c, which I answered with a workaround using a wrapper class, but I'm still curious as to why one can't call, in Objective-C, a Swift closure passed via id. A simple example follows.
Objective-C code:
// In a header
typedef void (^EmptyBlock)();
#interface MyClassOC : NSObject
-(void)invoke:(id)blk;
#end
// In an implementation (.m) file
#implementation MyClassOC
-(void)invoke:(id)blk {
EmptyBlock emptyBlock = blk;
emptyBlock();
}
#end
No problem providing an Objective-C block:
EmptyBlock block = ^{ puts("In OC empty block..."); };
MyClassOC * myClassOC = [[MyClassOC alloc] init];
[myClassOC invoke:block];
However, the Objective-C code in invoke... can't call a Swift closure passed in via id:
let myClassOC = MyClassOC()
let myBlock : EmptyBlock = {
print("In Swift EmptyBlock...")
}
myClassOC.invoke(myBlock)
I end up with EXC_BAD_ACCESS on this line:
EmptyBlock emptyBlock = blk;
Any idea what's going on here?

Answer 1

The reason is probably the support introduced in Swift 3 for any Swift type to be represented as an opaque object of id type in Objective-C. Read Swift Value Types in Objective-C in this Apple blog post for details. In that you will see that Swift types are passed as _SwiftValue * when the parameter type is id, this is an opaque type.
The argued benefit of this approach is that Swift value types can be stored in Objective-C collections; however the disadvantage is that you cannot convert to an Objective-C compatible value on the Objective-C side. Debug your code and you'll see the block is being passed as a _SwiftValue * and not an Objective-C block type.
Declare the bak parameter to have EmptyBlock type and your code works.
HTH

Answer 2

Because Swift closures and Objective-C blocks are not the same things. Swift automatically converts a closure to an Objective-C block if they see that you are calling a method with an Objective-C block type, but it doesn't do that otherwise. And as CRD's answer mentions, in Swift 3 any Swift value can be represented as an Objective-C object, so even though it sees that it expects an Objective-C object, it still doesn't know you want an Objective-C block because a Swift closure can still be bridged to an opaque object.
The only way I figured out to pass it from Swift and work is something like:
myClassOC.invoke(myBlock as #convention(block) () -> Void)