Handle swift throw in closure in objective-c

Question

I've an objective-c function that looks like below

+ (void) invokeSortingWithClassName: (NSString*) className functionName: (NSString*) functionName closure: (id(^)(NSArray* arr))closure;

and I've to call this method from a swift class. The swift code that calls this method looks like below

SortHandler.invokeSorting(withClassName: className, functionName: functionName, closure:
{
    args in 
    let something = unwrap(args!)
    do
    {
        let returnValue = try closure(something as NSArray!) //this is another closure coming as a parameter in the function in which this code is written and this throws
        return returnValue;
    }
    catch
    {
        throw error

    }
    return "-1" as! T
})

At the start of closure definition, I get this attached error

Essentially, this closure throws an error and I am not sure, how to handle this error in objective-c definition of this function. Please help me fix this error

Your closure can't throw an error. You will need to resort to simply returning `nil` to indicate failure or even causing a runtime exception depending on how severe the problem is. Also a return after a throw makes no sense. — Paulw11, Mar 30 '17 at 06:29
Why cannot a closure throw an error? Is there any Apple document supporting it or link to any other blog/SO post? — prabodhprakash, Mar 30 '17 at 06:42
*A closure* can throw, but *this closure* cannot; it is not declared to throw (which is what the error message is telling you) and it can't be declared to throw since it is declared in an Objective-C class and Objective-C knows nothing of Swift exceptions. — Paulw11, Mar 30 '17 at 06:46
Exactly this is my question - how can I declare the same in objective-c? Is there a way to do it? — prabodhprakash, Mar 30 '17 at 06:47
There is a way to handle it: https://www.uraimo.com/2015/11/03/error-handling-from-objective-c-to-swift-2-and-back/ - but, I am not able to solve it for closures. — prabodhprakash, Mar 30 '17 at 06:48

score 3 · Answer 1 · answered Mar 30 '17 at 07:03

While a closure can be declared with throws, this closure has not been declared with throws, so your closure cannot throw - this is exactly what the error message is telling you.

Since the function is declared in Objective-C, the function signature can't be changed to include throws as Objective-C doesn't know anything about Swift exceptions.

The standard way in which error handling is translated between Swift and Objective-C is for the Objective-C block to receive an &NSError parameter.

If you have the ability to change the Objective-C method signature, it should be declared as

+(void) invokeSortingWithClassName: (NSString* _Nullable) className functionName: (NSString* _Nullable) functionName closure: (id _Nullable (^_Nullable)(NSArray* _Nullable arr, NSError* _Nullable * _Nullable error))closure;

This will allow you to throw from swift with the error being received via the error parameter in Objctive-C.

If you cannot change the method signature, then you will need to catch the exception in your closure and simply return an 'error' value, such as nil

Is there an importance in using nullable here? because, I tried same solution without nullable and it did not work. — prabodhprakash, Mar 30 '17 at 07:44
The nullable lets Swift know which parameters are optional. It is best practice to include it even with pure Objective-C — Paulw11, Mar 30 '17 at 08:24
I tried the same thing, but it is not working - I'm still getting the same compile error. — prabodhprakash, Mar 30 '17 at 08:32

Handle swift throw in closure in objective-c

1 Answers1