CODEWITHSUNDEEP
genericsswift3
koen
koen

Reputation: 5729

Generic dictionary in Swift

I have several structs Foo, Bar, Baz, and I use the function below to read a plist into a [String : Foo] dictionary. And I have a method that creates a [String : Bar] dictionary, etc. This all works well.

func loadFoo() -> [String : Foo]? {
    guard
       let path = Bundle.main.path(forResource: "foo", ofType: "plist"),
       let plistDict = NSDictionary(contentsOfFile: path) as? [String : AnyObject]
       else { return nil }

    var dict = [String : Foo]()

    for (key, f) in plistDict {
       if let info = f as? [String : AnyObject] {
           let foo = Foo(symbol: key, info: info)
           dict[key] = foo
        }
    }

    return dict
}

Of course, this screams "use generics", but I have not yet been able to get it to work. For instance, I replaced Foo with <A> or A, but get a bunch of compiler errors (I am away from my Mac right now, so cannot reproduce the errors here).

Any suggestions how to tackle this?

Upvotes: 3

Views: 7198

Answers (1)

vadian
vadian

Reputation: 285079

  • Create a protocol that all common structs must conform to by implementing the init method init(symbol:info:)

    protocol Fooable {
  init(symbol: String, info: [String : AnyObject])
}

  • Make all your structs adopt that protocol

    struct Baz : Fooable { ...

  • Now you can make the load method generic by constraining the type to Fooable

    func loadFoo<T:Fooable>() -> [String : T]? {
  guard
    let path = Bundle.main.path(forResource: "foo", ofType: "plist"),
    let plistDict = NSDictionary(contentsOfFile: path) as? [String : AnyObject]
    else { return nil }

  var dict = [String : T]()

  for (key, f) in plistDict {
    if let info = f as? [String : AnyObject] {
      let foo = T(symbol: key, info: info)
      dict[key] = foo
    }
  }

  return dict
}

  • and call it with 

    let bazObject : [String : Baz]? = loadFoo()

    This syntax to annotate the type passes Baz as the generic parameter.

Upvotes: 3

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