【Swift】配列の重複を取り除く

Swiftで配列の重複を取り除く方法です。
structの特定の値をkeyとして取り除く方法を今回はご紹介します。

extension Array {
    func unique<T>(by keyPath: KeyPath<Element, T>) -> Array where T: Hashable {
        var dictionary: [T: Element] = [:]
        forEach { value in
            dictionary[value[keyPath: keyPath]] = value
        }
        return dictionary.map(\.value)
    }
}

使い方

struct Item {
  var title: String
  var value: Int
}

let items = [
    Item(title: "a", value: 1),
    Item(title: "b", value: 2),
    Item(title: "b", value: 3),
    Item(title: "c", value: 4),
]

/*
 Result
 [
     Item(title: "a", value: 1),
     Item(title: "b", value: 3),
     Item(title: "c", value: 4),
 ]
 */
let filteredItems = items.unique(by: \.title)