how to return array using closure

Question

I am new on Swift and I am trying to use closure for getting results back from viewcontroller

Here is the example

class MD5Calculator {

    static func imageChecksum(imageArray: [UIImage], onCalculated: @escaping () -> [String]){

        DispatchQueue.global(qos: .userInitiated).async {

            var array: [String] = []

            for chosenImage in imageArray {
                if let jpegData = UIImageJPEGRepresentation(chosenImage, 80) {
                    let checksum = jpegData.md5()
                    let chsum = checksum.toHexString()
                    array.append(chsum)
                }
            }

            DispatchQueue.main.async {
                //return array
            }
        }

    }
}

I want md5 calculation do in background and when its done pass it to viewController.

I created closure but I don't know how to return it. my question is:

How to pass string array with closure
How to call ImageChecksum in view controller

Matt · Accepted Answer · 2017-02-20 07:16:57Z

1

Your usage of closure is not effective. The array you are about to send back should be the parameter of the closure not the returning type.

This should work

class MD5Calculator {

    static func imageChecksum(imageArray: [UIImage], onCalculated: @escaping ([String]) -> ()){

        DispatchQueue.global(qos: .userInitiated).async {

            var array: [String] = []

            for chosenImage in imageArray {
                if let jpegData = UIImageJPEGRepresentation(chosenImage, 80) {
                    let checksum = jpegData.md5()
                    let chsum = checksum.toHexString()
                    array.append(chsum)
                }
            }

            DispatchQueue.main.async {
                //return array
                onCalculated(array)
            }
        }

    }
}

EDIT: Use it in your ViewController like this

MD5Calculator.imageChecksum(imageArray: [imageArray]) { array in
    // 'array' is your returned result
}

edited Feb 20, 2017 at 7:16

answered Feb 20, 2017 at 7:12

Matt

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1 Comment

pmb Over a year ago

how do I call on viewcontroller ?

Anil Varghese · Accepted Answer · 2017-02-20 07:17:22Z

You should send your result array as a parameter of closure, not as return value.

How to pass string array with closure?

class MD5Calculator {

static func imageChecksum(imageArray: [UIImage], onCalculated: @escaping (_ checksum: [String]) ->()){

    DispatchQueue.global(qos: .userInitiated).async {

        var array: [String] = []

        for chosenImage in imageArray {
            if let jpegData = UIImageJPEGRepresentation(chosenImage, 80) {
                let checksum = jpegData.md5()
                let chsum = checksum.toHexString()
                array.append(chsum)
            }
        }

        DispatchQueue.main.async {
            //return array
            onCalculated(array)
        }
    }

}
}

2.How to call ImageChecksum in view controller

MD5Calculator.imageChecksum(imageArray: []) { (checksum) in

}

Daumantas Versockas · Accepted Answer · 2017-02-20 07:24:52Z

0

The main problem is, that you have defined closure incorrectly. Basically, the idea is, that you should define a closure with a return value, when you want to get the value inside the function, not the outside. The usage (in this case) would be something like: let someConstant = onCalculated(). This is the way you get the value inside the function.

In your case (you want to return a value from function), the closure should be defined something like: onCalculated: @escaping (_ someArray: [String]) -> Void. in this case, you would just return the value from the function: onCalculated(someArray)

The usage:

MD5Calculator.imageChecksum(
    imageArray: inputImageArray, 
    onCalculated: { someArray in
        // enter code here
    }
)

P.S. Consider to use optional unwrap for closures!

edited Feb 20, 2017 at 7:24

answered Feb 20, 2017 at 7:10

Daumantas Versockas

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