Passing an array of closures to a swift method form objectiv-c file

Question

guys,

I created a swift class like this:

class SwiftClass
{

    override init()
    {
        super.init()
    }

    static func Test1(closure:()->Void){
        closure()
    }

    static func Test2(array:[()->Void]){
        for closure in array {
            closure()
        }
    }
}

Both 2 methods wok fine from swift file.

SwiftClass.Test1(closure:{print("Test1")})
SwiftClass.Test2(array:[{print("Test2_closure1")},{print("Test2_closure2")}])

The function Test1 work fine from objective-c file too.

[SwiftClass Test1WithClosure:^{
        NSLog(@"SwiftClass Test1") ;
    }] ;

but Test2 can't be used from objective-c file,the automatically generated -Swift.h file didn't declare it at all.

SWIFT_CLASS("_TtC17ArrayClosuresTest10SwiftClass")
@interface SwiftClass : NSObject
- (nonnull instancetype)init OBJC_DESIGNATED_INITIALIZER;
+ (void)Test1WithClosure:(void (^ _Nonnull)(void))closure;
@end

Is it possible to call Test2 from objective-c file?

Swift

5.3k

Posted by

Dogstarsuper

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Answer 1

If you explicitly declare the array-based method as "@objc", you'll get the following compilation error message: "method cannot be marked @objc because the type of the parameter cannot be represented in Objective-C".

Since you don't get this error with an array of (say) Int, and with a non-array closure parameter, this suggests the compiler could make an @objc-compatible method, but chooses not to try. Or, there may be a technical reason why this array/closure combination can't be represented in Obj-C.

As a workaround, you could change the Test2 parameter type to [AnyObject], or specify the parameter type as NSArray, and cast individual elements as the closure type.

Either way, it's probably worth a bug report, in case this is something that is really supposed to work.

Posted by

QuinceyMorris

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Answer 2

THX,

I tried the [AnyObject] and NSArray workaround ,

static func Test3(array:[AnyObject]){
        for closure in array {
            (closure as! ()->Void)()
        }
    }

It build successlly but I got a runtime error

"Could not cast value of type '__NSGlobalBlock__' (0x1188e9040) to '(()) -> ()' (0x1188ef030)."

Is there any mistake in the casting?

Posted by

Dogstarsuper

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Answer 3

The reason why your first attempt didn’t work is that Swift closures and Objective-C blocks are not identical (although they are closely related). You can get your

test2(array:)

method exposed to Objective-C by adopting

@convention(block)

, as shown here:

typealias Callback = @convention(block) () -> Void

class Test : NSObject {
    class func test2(array: [Callback]) {
        …
    }
}

Or, as you’ve already discovered, using NSArray. However, the cast required to call the callbacks still fails. If you expand the above to this:

@objc class func test2(array: [Callback]) {
    for item in array {
        item()
    }
}

you get:

fatal error: NSArray element failed to match the Swift Array Element type

And, as you’ve noted, in the NSArray case, where you have to do an explicit cast, you get a more focused error (even if you use

@convention(block)

).

Unfortunately I was unable to find a way around this, which brings me to the same conclusion as QuinceyMorris: this is definitely worth a bug.

Please post your bug number, just for the record.

Share and Enjoy
—
Quinn “The Eskimo!”
Apple Developer Relations, Developer Technical Support, Core OS/Hardware

let myEmail = "eskimo" + "1" + "@apple.com"

Posted by

eskimo

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Answer 4

I posted the bug and got a bug number 28995623. Can other peaple browse it via the number?

btw,I found a not so good workaround :

var closureArray:[()->Void]=[]

public func add(closure:@escaping ()->Void)
{
   closureArray.append(closure)
}

😕

Posted by

Dogstarsuper

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Answer 5

I posted the bug …

Thanks.

Can other [people] browse it via the number?

Not directly. You can publish its status via Open Radar or, alternatively, file a bug in Swift’s bug system, which is world readable (I shoulda sent you that link the first time around but wasn’t thinking clearly).

Share and Enjoy
—
Quinn “The Eskimo!”
Apple Developer Relations, Developer Technical Support, Core OS/Hardware

let myEmail = "eskimo" + "1" + "@apple.com"

Posted by

eskimo

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Answer 6

any news on the fix for this bug?

Posted by

tsanjoto

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Answer 7

any news on the fix for this bug?

No. If you’re feeling enthusiastic you could try fixing it yourself (-:

Share and Enjoy
—
Quinn “The Eskimo!”
Apple Developer Relations, Developer Technical Support, Core OS/Hardware

let myEmail = "eskimo" + "1" + "@apple.com"

Posted by

eskimo

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Passing an array of closures to a swift method form objectiv-c file

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