NSFastEnumeration in Swift - ios

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
}