Alamofire.swift（三）－URLRequest

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是继承了URLConvertible协议的类型

method是HTTPMethod枚举类型

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

headers是HTTPHeaders类型，其实就是通过类型重命名typealias关键字，将key－value都为String类型的字典重命名为HTTPHeaders

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

并且可以看出，url和method都是必须要有的，headers是可选的

let url = try url.asURL()

这里用try，是因为asURL()定义在类型转换时出现错误throws异常

self.init(url: url)

这里调用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 )
这里的CachePolicy其实是NSURLRequest，默认是：.useProtocolCachePolicy
public typealias CachePolicy = NSURLRequest.CachePolicy

httpMethod = method.rawValue

举个官方的🌰，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循环的这种方式是由于headers是元组类型

当看到using adapter: RequestAdapter?的时候，我已经知道这是个协议了，而且有throws，说明我们在return的时候需要try。
使用guard来判断是否有值，比if更加清晰

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