extension URLRequest {
    public init(url: URLConvertible, method: HTTPMethod, headers: HTTPHeaders? = nil) throws {
        let url = try url.asURL()

        self.init(url: url)

        httpMethod = method.rawValue

        if let headers = headers {
            for (headerField, headerValue) in headers {
                setValue(headerValue, forHTTPHeaderField: headerField)
            }
        }
    }

    func adapt(using adapter: RequestAdapter?) throws -> URLRequest {
        guard let adapter = adapter else { return self }
        return try adapter.adapt(self)
    }
}

url是繼承了URLConvertible協議的類型

method是HTTPMethod枚舉類型

public enum HTTPMethod: String {
case get = "GET"
case head = "HEAD"
case post = "POST"
...
}

headers是HTTPHeaders類型，其實就是通過類型重命名typealias關鍵字，將key－value都為String類型的字典重命名為HTTPHeaders

//A dictionary of headers to apply to a URLRequest.
public typealias HTTPHeaders = [String: String]

并且可以看出，url和method都是必須要有的，headers是可選的

let url = try url.asURL()

這里用try，是因為asURL()定義在類型轉換時出現錯誤throws異常

self.init(url: url)

這里調用URLRequest的初始化方法
//Creates and initializes a URLRequest with the given URL and cache policy.
public init ( url: URL, cachePolicy: URLRequest.CachePolicy = default, timeoutInterval: TimeInterval = default )
這里的CachePolicy其實是NSURLRequest，默認是：.useProtocolCachePolicy
public typealias CachePolicy = NSURLRequest.CachePolicy

httpMethod = method.rawValue

舉個官方的??，貌似挺好理解

enum PaperSize: String {
    case A4, A5, Letter, Legal
}
let selectedSize = PaperSize.Letter
print(selectedSize.rawValue)
// Prints "Letter"
print(selectedSize == PaperSize(rawValue: selectedSize.rawValue)!)
// Prints "true"

if let headers = headers {
            for (headerField, headerValue) in headers {
                setValue(headerValue, forHTTPHeaderField: headerField)
            }
        }

for循環的這種方式是由于headers是元組類型

當看到using adapter: RequestAdapter?的時候，我已經知道這是個協議了，而且有throws，說明我們在return的時候需要try。
使用guard來判斷是否有值，比if更加清晰

public protocol RequestAdapter {
    func adapt(_ urlRequest: URLRequest) throws -> URLRequest
}