协议和扩展

查看这段代码先看到的是两个protocol和实现这些协议的extension

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• URLConvertible:构造一个URL，实现的类有String，URL，URLComponents,方法具体实现都比较简单。

 public protocol URLConvertible {
    /// Returns a URL that conforms to RFC 2396 or throws an `Error`.
    ///
    /// - throws: An `Error` if the type cannot be converted to a `URL`.
    ///
    /// - returns: A URL or throws an `Error`.
    func asURL() throws -> URL
}

• URLRequestConvertible：构造一个URLRequest,实现的类只有URLRequest

/// Types adopting the `URLRequestConvertible` protocol can be used to construct URL requests.
public protocol URLRequestConvertible {
    /// Returns a URL request or throws if an `Error` was encountered.
    ///
    /// - throws: An `Error` if the underlying `URLRequest` is `nil`.
    ///
    /// - returns: A URL request.
    func asURLRequest() throws -> URLRequest
}

extension URLRequestConvertible {
    /// The URL request.
    public var urlRequest: URLRequest? { return try? asURLRequest() }
}

extension URLRequest: URLRequestConvertible {
    /// Returns a URL request or throws if an `Error` was encountered.
    public func asURLRequest() throws -> URLRequest { return self }
}

在Alamofire中需要使用URL和URLRequest的地方都用对应的Protocol表示，不直接使用对应类型，这个设计让代码显得更加灵活。

extension对URLRequest添加了一个init和adapt方法。

extension URLRequest {
    /// Creates an instance with the specified `method`, `urlString` and `headers`.
    ///
    /// - parameter url:     The URL.
    /// - parameter method:  The HTTP method.
    /// - parameter headers: The HTTP headers. `nil` by default.
    ///
    /// - returns: The new `URLRequest` instance.
    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)
    }
}

各种请求方法

Alamofire中有四大类的请求方法，Data Request，Download Request，Stream Request，每类方法中又包含若干种不同的请求方法。由于这里的请求方法并没有真正的执行网络请求，只是对SessionManager中的请求方法的封装，所有都是很简单调用SessionManager方法而已。

Data Request

平时使用得最多的请求类型，可以创建一个简单的网络请求

///必填参数是实现了URLConvertible的类型，比如上面介绍的
String, URL，URLComponents 。
public func request(
    _ url: URLConvertible,
    method: HTTPMethod = .get,
    parameters: Parameters? = nil,
    encoding: ParameterEncoding = URLEncoding.default,
    headers: HTTPHeaders? = nil)
    -> DataRequest

///只需要一个URLRequestConvertible
public func request(_ urlRequest: URLRequestConvertible) -> DataRequest 

Download Request

前两个方法和上面的DataRequest的方法大同小异,第三个方法参数需要传入一个Data,这个Data应该是用于断点下载使用,上次任务取消后保留的Data。

public func download(
    _ url: URLConvertible,
    method: HTTPMethod = .get,
    parameters: Parameters? = nil,
    encoding: ParameterEncoding = URLEncoding.default,
    headers: HTTPHeaders? = nil,
    to destination: DownloadRequest.DownloadFileDestination? = nil)
    -> DownloadRequest

public func download(
    _ urlRequest: URLRequestConvertible,
    to destination: DownloadRequest.DownloadFileDestination? = nil)
    -> DownloadRequest

public func download(
    resumingWith resumeData: Data,
    to destination: DownloadRequest.DownloadFileDestination? = nil)
    -> DownloadRequest