Conversion of Block with callback in Objective C to closure in Swift

Question

I have following callback in block in Objective C

typedef void (^COMPLETION_BLOCK)(NSString *response, NSError *errorString);

+ (void)responseFromURL:(NSURL *)url completionBlock:(COMPLETION_BLOCK)completionBlock
{
   __block  NSError *error = nil;    
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
        NSString *response = [NSString stringWithContentsOfURL:url
                                                      encoding:NSUTF8StringEncoding
                                                         error:&error];

        dispatch_async(dispatch_get_main_queue(), ^{
                completionBlock(response, error);
        });
    });
}

Here is how i call it in Objective C

[Utilities responseFromURL:NSURL URLWithString:@""] completionBlock:^(NSString *response, NSError *errorString)
{

}]

I have tried converting this in Clousers in swift here is the code

class Utilities
{
    typealias COMPLETION_BLOCK = ( response : NSString, error : NSError) -> ()

    func responseFromURL(url: NSURL , completionBlock:COMPLETION_BLOCK)
    {
        let priority = DISPATCH_QUEUE_PRIORITY_DEFAULT
        var e : NSError?;

        dispatch_async(dispatch_get_global_queue(priority, 0))
        {
            var content:NSString? = NSString(contentsOfURL: url,
                encoding: NSUTF8StringEncoding,
                error: &e)

            dispatch_async(dispatch_get_main_queue())
            {
                completionBlock(response: content!,error:  e!);
            }
        }
    }
}

I am not sure if this is correct conversion. Secondly how am i suppose to call the block in swift?

var urlString = String(format: BASE_URL + "%d", categoryType);
        var url = NSURL(string :urlString);

SarahR · Accepted Answer · 2016-06-01 02:29:22Z

3

You are almost there. You don't need the typealias as you can specify the completionBlock in the function definition.

One thing that would cause you problems is the forced unwrapping of content & e one of which is going to be nil.

Here is my version of your function:

func responseFromURL(url : NSURL, completionBlock : (response : String?, error : NSError?) -> Void) {
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0)) {
        var error : NSError? = nil
        let response = String(contentsOfURL: url, encoding: NSUTF8StringEncoding, error: &error)

        dispatch_async(dispatch_get_main_queue()) {
            completionBlock(response: response, error: error)
        }
    }
}

And to call it:

responseFromURL(myURL) { (response, error) in
    if error != nil {
        // handle error
    }
    else if response != nil {
        // data came back
    }
}

A couple of points to notice:

in the completion block definition, I specified that both response and error parameters are optionals. You should get one or the other back, but you need to check which one.
unlike in Objective-C, you do not have to send in a pointer to the error. The completion handler will send you back an error if it needs to.
I used the shorter form of Swift closures but if you let auto-complete do its thing, you will get the longer form which may be more readable.

edited Jun 1, 2016 at 2:29

answered Feb 6, 2015 at 8:17

SarahR

9058 silver badges12 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Muhammad Umar Over a year ago

please check my edited code, this is the myUrl i am trying to pass in ur code however it is giving error, extra argument in call. Secondly there is orange line on entire code below it . I have tried to access it using Utilities.responseFromUrl See my code how i am defining function in utilities

Muhammad Umar Over a year ago

nvrmind needed to make it static, Fixed it, Thanks :)

Collectives™ on Stack Overflow

Conversion of Block with callback in Objective C to closure in Swift

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related