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pofat/Describer.swift

Last active Aug 21, 2019
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weak self podcast episode 1 附錄: 用 generic struct 取代 Protocol
/*
* Standard library 有個 protocol `CustomStringConvertible`,需要實作 `description: String` 來將一個物件轉化成 description string
* 但是若你對同一個物件想要有兩種描述方式,比如
* 想要用 JSON 的方式印出來 model 的所有 property,且希望有的時候是 pretty printed 有的時候是 sorted key 怎麼辦?
* 你會需要有兩個物件來遵守 CustomStringConvertible,因為一個 protocol 在一個物件裡只能有一種實作
* 然而用 generic struct 的話就能做到這件事
* 下面的實例中你可以宣告兩個不同的 instance ,`prettyPrintedDescribing` 和 `sortedKeyDescribing`
* 想用哪個就用哪個,方便多了,也讓 compiler 有更多參與最佳化的空間
*
*/
import Foundation
struct Describer<T> {
var describe: (T) -> String
}
let prettyPrintedDesc = Describer<[String: String]> { dict in
guard let data = try? JSONSerialization.data(withJSONObject: dict, options: .prettyPrinted),
let string = String(data: data, encoding: .utf8) else {
return ""
}
return string
}
let sortedKeyDesc = Describer<[String: String]> { dict in
guard let data = try? JSONSerialization.data(withJSONObject: dict, options: .sortedKeys),
let string = String(data: data, encoding: .utf8) else {
return ""
}
return string
}
let albumDict = [
"title" : "Slippery When Wet",
"author" : "Bon Jovi",
"date" : "August 18, 1986",
"priceTag" : "$20.00"
]
print(prettyPrintedDesc.describe(albumDict))
print(sortedKeyDesc.describe(albumDict))
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