NSFastEnumeration in Swift

Question

I am trying to convert an Objective-C project to swift, but I am unable to find how to use NSFastEnumeration for an object of a class that conforms to NSFastEnumeration.

Here is the code in ObjC:

//  get the decode results
id<NSFastEnumeration> results = [info objectForKey: ZBarReaderControllerResults];

ZBarSymbol *symbol = nil;
for(symbol in results)
    // just grab the first barcode
    break;

so far I tried to find how to do this, but this doe not seems work, here is the swift code:

var results: ZBarSymbolSet = infoDictionary?.objectForKey(ZBarReaderControllerResults) as ZBarSymbolSet

    var symbol : ZBarSymbol? = nil;

    for symbol in results
    {    //just grab first barcode
        break;
    }

the error comes in for condition - "ZBarSymbolSet" does not have a member named "Generator"

What am I doing wrong?

Here is the screen shot

Answer 1

After a while poking around the swift framework files, I finally found this nice class called NSFastGenerator. NSSet and friends seem to be using the same Generator.

For ZBarSymbolSet, here's how you'd extend it to support for-in loops:

extension ZBarSymbolSet: SequenceType {
    public func generate() -> NSFastGenerator {
        return NSFastGenerator(self)
    }
}

Update: Looks like Swift 2.0's protocol extensions fixed this for us!

Answer 2

Your defined class ZBarSymbolSet needs to implement the Swift SequenceType interface in order to be usable in for <identifier> in <sequence> syntax. The SequenceType interface is

protocol SequenceType : _Sequence_Type {
    typealias Generator : GeneratorType
    func generate() -> Generator
}

and thus you see the mention of Generator as reported in your error message.

Also in the syntax:

for <identifier> in <sequence> {
  <statements>
}

the <identifer> is only in scope for <statements>. Thus your second use of symbol in the if will be out of scope and an error. One proper idiom would be:

var symbolFound : ZBarSymbol?

for symbol in result {
  symbolFound = symbol
  break
}

if symbolFound ...

If course, but the time ZBarSymbolSet implements SequenceType it would also implement CollectionType with subscript and thus the whole 'find the first element' code would be var symbol = result[0]

Answer 3

Here is John Estropia's answer for Swift 3:

extension ZBarSymbolSet: Sequence {

    public typealias Iterator = NSFastEnumerationIterator

    public func makeIterator() -> NSFastEnumerationIterator {
        return NSFastEnumerationIterator(self)
    }

}

Then you for-in loop would look like this:

for element in results {
    let symbol = element as! ZBarSymbol
    // ...
}

This answer could be improved by also adopting the IteratorProtocol so you can specify the element associated type as ZBarSymbol. I have not figured out how to do this yet.

Answer 4

Step1: 
extension ZBarSymbolSet: SequenceType {
    public func generate() -> NSFastGenerator {
        return NSFastGenerator(self)
    }
}

Step2:
var results: NSFastEnumeration = info.objectForKey(ZBarReaderControllerResults) as NSFastEnumeration

    var symbolFound : ZBarSymbol?

    for symbol in results as ZBarSymbolSet {
        symbolFound = symbol as? ZBarSymbol
        break
    }
    resultString = NSString(string: symbolFound!.data)

Answer 5

Here's a way to do it without writing an extension

    var iterator = NSFastEnumerationIterator(collection)
    while let element = iterator.next() {
        // use element
    }

Answer 6

Also, if you know that all objects in your ZBarSymbolSet are ZBarSymbol objects (since ObjC doesn't enforce all objects being ZBarSymbol objects), you can do the following:

extension ZBarSymbolSet {

    public struct ZBarSymbolSetIterator {
        public typealias Element = ZBarSymbol
        private let enumerator: NSFastEnumerationIterator

        init(_ symbols: ZBarSymbolSet) {
            self.enumerator = NSFastEnumerationIterator(symbols)
        }

        public mutating func next() -> ZBarSymbol {
            if let object = self.enumerator.next() {
                return object as? ZBarSymbol
            }
            else { return nil }
        }
    }

    public func makeIterator() -> ZBarSymbolSetIterator {
        return ZBarSymbolSetIterator(self)
    }
}

Now your for-loop will look like this:

for element in results {
    // element is a ZBarSymbol
}