協議和擴展

查看這段代碼先看到的是兩個protocol和實現這些協議的extension

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• URLConvertible:構造一個URL，實現的類有String，URL，URLComponents,方法具體實現都比較簡單。

 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：構造一個URLRequest,實現的類只有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的地方都用對應的Protocol表示，不直接使用對應類型，這個設計讓代碼顯得更加靈活。

extension對URLRequest添加了一個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)
    }
}

各種請求方法

Alamofire中有四大類的請求方法，Data Request，Download Request，Stream Request，每類方法中又包含若干種不同的請求方法。由于這里的請求方法并沒有真正的執行網絡請求，只是對SessionManager中的請求方法的封裝，所有都是很簡單調用SessionManager方法而已。

Data Request

平時使用得最多的請求類型，可以創建一個簡單的網絡請求

///必填參數是實現了URLConvertible的類型，比如上面介紹的
String, URL，URLComponents 。
public func request(
    _ url: URLConvertible,
    method: HTTPMethod = .get,
    parameters: Parameters? = nil,
    encoding: ParameterEncoding = URLEncoding.default,
    headers: HTTPHeaders? = nil)
    -> DataRequest

///只需要一個URLRequestConvertible
public func request(_ urlRequest: URLRequestConvertible) -> DataRequest 

Download Request

前兩個方法和上面的DataRequest的方法大同小異,第三個方法參數需要傳入一個Data,這個Data應該是用于斷點下載使用,上次任務取消后保留的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