How can I iterate over all items in NSMapTable in Swift

Question

I'm trying something like this in Swift but not working. Error is: Type () does not conform to type BooleanType

//visibleCollectionReusableHeaderViews is of type NSMapTable!

var enumerator: NSEnumerator = visibleCollectionReusableHeaderViews.objectEnumerator()
var myValue: AnyObject!

while (( myValue = enumerator.nextObject()))
{

}

What am I doing wrong? I don't think I understand how to iterate over an NSMapTable, or even just to get the first item in it.

score 18 · Accepted Answer · answered Aug 25 '14 at 14:14

18

In Swift, this is done using conditional assignment.

let enumerator = visibleCollectionReusableHeaderViews.objectEnumerator()

while let myValue: AnyObject = enumerator.nextObject() {
    println(myValue)
}

Note the non optional type for myValue. Otherwise this loop would be infinite as myValue continued to accept nil objects.

answered Aug 25 '14 at 14:14

Mick MacCallum

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Thanks this did it! By any chance, know of any way to now update a section in a uicollectionview? myValue actually is my header reusable view UICollectionElementKindSectionHeader so i'd like to visually redraw the items in there (the manual constraints need a refresh). I can do uicollectionview.reloaddata() but it's a little slow and redraws the ENTIRE view when I just need the header. – NullHypothesis Aug 25 '14 at 14:18
@user3379785 Well, if you're okay with reloading the entire section, you could use `collectionView.reloadSections(NSIndexSet(index: someIndexInteger))`, but if you're only interested in reloading the header, then I'm not exactly sure. It does look like a few options are explored here though. http://stackoverflow.com/questions/13831035/reload-uicollectionview-header-or-footer – Mick MacCallum Aug 25 '14 at 14:23
3

You will need to put `enumator.nextObject() as? YourObject` if you want to use methods on YourObject, for example. – theblang Jun 01 '15 at 02:10

score 3 · Answer 2 · answered Jul 14 '17 at 08:25

3

Or a clearer and shorter approach (Swift 3):

for key in table.keyEnumerator() {
    print(key)
}

for object in table.objectEnumerator() ?? NSEnumerator() {
    print(object)
}

answered Jul 14 '17 at 08:25

Ian Bytchek

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score -1 · Answer 3 · answered May 25 '17 at 23:17

-1

In Swift 3 you can use for-in loop through allObjects property:

for myValue in visibleCollectionReusableHeaderViews.allObjects {
  // do stuff
}

answered May 25 '17 at 23:17

kelin

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Wouldn't this impact performance a considerable amount compared to using a `while` loop? – Zoyt Jun 13 '17 at 00:07
1

Well a `for-in` loop is faster, because it can be efficiently parallelized. So, if you want to do some really costing stuff inside the loop, this way should be preferred. But we don't know, how `allObjects` property is composed. Probably, calling this property may cause an overhead. But I think, `NSMapTable` has an array inside anyway, and when we access the `allObjects`, we really get an immutable copy. Such operation shouldn't be expensive. – kelin Jun 13 '17 at 13:49
1

`NSMapTable` has no member `allObjects` – Andreas Ley Jun 28 '17 at 20:30
@AndreasLey, really, it's a property of `NSHashTable`. – kelin Jun 28 '17 at 20:39
@kelin but `allObjects` is not a property of `NSMapTable`, which is what is being discussed in this question – gyratory circus May 11 '18 at 00:45

How can I iterate over all items in NSMapTable in Swift

3 Answers3