Assign value to optional dictionary in Swift

Question

I\'m finding some surprising behavior with optional dictionaries in Swift.

var foo:Dictionary?

if (foo == nil) {
    foo = [\"bar\": \         


        

Answer 1

This is because your Dictionary is optional. If it's nil, you won't add an entry to it.

You can do this way:

var dict: [String : String]?
if let dict = dict {
  dict["key"] = "value" // add a value to an existing dictionary
} else {
  dict = ["key" : "value"] // create a dictionary with this value in it
}

Or, if you are given an optional dictionary, for example HTTPHeaders - which in AlamoFire is a [String : String] dictionary - and you want to either add a value if it's non-nil, or create it with this value if it's nil, you could do like so:

let headers: HTTPHeaders? // this is an input parameter in a function for example

var customHeaders: HTTPHeaders = headers ?? [:] // nil coalescing
customHeaders["key"] = "value"