Swift Set Union

[Swift Set operations]

a.union(b) - a ∪ b - the result set contains all elements from a and b

• union - immutable function
  
• unionInPlace(up to Swift v3) => formUnion - mutable function

[Mutable vs Immutable]

Read more here

With Swift 5.1, you can use one of the three following examples in order to solve your problem.

#1. Using Array's map(_:) method and Set's init(_:) initializer

In the simplest case, you can map you initial array to an array of urls (String) then create a set from that array. The Playground below code shows how to do it:

struct MyObject {
    let url: String
}

let objectArray = [
    MyObject(url: "mozilla.org"),
    MyObject(url: "gnu.org"),
    MyObject(url: "git-scm.com")
]

let urlArray = objectArray.map({ $0.url })
let urlSet = Set(urlArray)
dump(urlSet)
// ▿ 3 members
//   - "git-scm.com"
//   - "mozilla.org"
//   - "gnu.org"

#2. Using Array's reduce(into:_:) method

struct MyObject {
    let url: String
}

let objectArray = [
    MyObject(url: "mozilla.org"),
    MyObject(url: "gnu.org"),
    MyObject(url: "git-scm.com")
]

let urlSet = objectArray.reduce(into: Set<String>(), { (urls, object) in
    urls.insert(object.url)
})
dump(urlSet)
// ▿ 3 members
//   - "git-scm.com"
//   - "mozilla.org"
//   - "gnu.org"

As an alternative, you can use Array's reduce(_:_:) method:

struct MyObject {
    let url: String
}

let objectArray = [
    MyObject(url: "mozilla.org"),
    MyObject(url: "gnu.org"),
    MyObject(url: "git-scm.com")
]

let urlSet = objectArray.reduce(Set<String>(), { (partialSet, object) in
    var urls = partialSet
    urls.insert(object.url)
    return urls
})
dump(urlSet)
// ▿ 3 members
//   - "git-scm.com"
//   - "mozilla.org"
//   - "gnu.org"

#3. Using an Array extension

If necessary, you can create a mapToSet method for Array that takes a transform closure parameter and returns a Set. The Playground below code shows how to use it:

extension Array {

    func mapToSet<T: Hashable>(_ transform: (Element) -> T) -> Set<T> {
        var result = Set<T>()
        for item in self {
            result.insert(transform(item))
        }
        return result
    }

}

struct MyObject {
    let url: String
}

let objectArray = [
    MyObject(url: "mozilla.org"),
    MyObject(url: "gnu.org"),
    MyObject(url: "git-scm.com")
]

let urlSet = objectArray.mapToSet({ $0.url })
dump(urlSet)
// ▿ 3 members
//   - "git-scm.com"
//   - "mozilla.org"
//   - "gnu.org"

import Foundation

let nsarr: NSArray = NSArray(array: [1,2,3,4,5])
var set: Set<Int>
guard let arr = nsarr as? Array<Int>  else {
    exit(-1)
}
set = Set(arr)
print(set.dynamicType)
dump(set)
/*
Set<Int>
▿ 5 members
  - [0]: 5
  - [1]: 2
  - [2]: 3
  - [3]: 1
  - [4]: 4
*/

with help of free bridging, it should be easy ...