Alamofire的流程（一）

普通请求

在官方demo中，alamofire给出了示例代码。

Alamofire.request("https://httpbin.org/get").responseJSON { response in
    print("Request: \(String(describing: response.request))")   // original url request
    print("Response: \(String(describing: response.response))") // http url response
    print("Result: \(response.result)")                         // response serialization result

    if let json = response.result.value {
        print("JSON: \(json)") // serialized json response
    }

    if let data = response.data, let utf8Text = String(data: data, encoding: .utf8) {
        print("Data: \(utf8Text)") // original server data as UTF8 string
    }
}

在实际源码中，到底是怎么工作的呢？

request函数

/// Creates a `DataRequest` using the default `SessionManager` to retrieve the contents of the specified `url`,
/// `method`, `parameters`, `encoding` and `headers`.
@discardableResult
public func request(
    _ url: URLConvertible,
    method: HTTPMethod = .get,
    parameters: Parameters? = nil,
    encoding: ParameterEncoding = URLEncoding.default,
    headers: HTTPHeaders? = nil)
    -> DataRequest
{
    return SessionManager.default.request(
        url,
        method: method,
        parameters: parameters,
        encoding: encoding,
        headers: headers
    )
}

URLConvertible

/// Types adopting the `URLConvertible` protocol can be used to construct URLs, which are then used to construct
/// URL requests.
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
}

extension String: URLConvertible {
    /// Returns a URL if `self` represents a valid URL string that conforms to RFC 2396 or throws an `AFError`.
    ///
    /// - 如果`self`不是一个有效的URL字符串， 就会抛出`AFError.invalidURL` 异常。
    /// - returns: A URL or throws an `AFError`.
    public func asURL() throws -> URL {
        guard let url = URL(string: self) else { throw AFError.invalidURL(url: self) }
        return url
    }
}

URLConvertible协议是为了将String类型的字符串快速转化为URL类型。

SessionManager

SessionManager是用来负责创建和管理请求的对象，就如同底层的NSURLSession。

default

/// 默认的`SessionManager`实例, 由Alamofire顶层的请求方法使用，直接适用于任何特定请求.
    public static let `default`: SessionManager = {
        let configuration = URLSessionConfiguration.default
        configuration.httpAdditionalHeaders = SessionManager.defaultHTTPHeaders

        return SessionManager(configuration: configuration)
    }()

request(1)

@discardableResult
    open func request(
        _ url: URLConvertible,
        method: HTTPMethod = .get,
        parameters: Parameters? = nil,
        encoding: ParameterEncoding = URLEncoding.default,
        headers: HTTPHeaders? = nil)
        -> DataRequest
    {
        var originalRequest: URLRequest?

        do {
            originalRequest = try URLRequest(url: url, method: method, headers: headers)
            let encodedURLRequest = try encoding.encode(originalRequest!, with: parameters)
            return request(encodedURLRequest)
        } catch {
            return request(originalRequest, failedWith: error)
        }
    }

通过URL Method Header生成一个URLRequest， 并通过制定的ParameterEncoding方法，将参数合并到URLRequest中，根据是否成功，执行下一个request方法。

request(2)

    /// Creates a `DataRequest` to retrieve the contents of a URL based on the specified `urlRequest`.
    ///
    /// If `startRequestsImmediately` is `true`, the request will have `resume()` called before being returned.
    ///
    /// - parameter urlRequest: The URL request.
    ///
    /// - returns: The created `DataRequest`.
    @discardableResult
    open func request(_ urlRequest: URLRequestConvertible) -> DataRequest {
        var originalRequest: URLRequest?

        do {
            originalRequest = try urlRequest.asURLRequest()
            let originalTask = DataRequest.Requestable(urlRequest: originalRequest!)

            let task = try originalTask.task(session: session, adapter: adapter, queue: queue)
            let request = DataRequest(session: session, requestTask: .data(originalTask, task))

            delegate[task] = request

            if startRequestsImmediately { request.resume() }

            return request
        } catch {
            return request(originalRequest, failedWith: error)
        }
    }

request(3)

    // MARK: Private - Request Implementation

    private func request(_ urlRequest: URLRequest?, failedWith error: Error) -> DataRequest {
        var requestTask: Request.RequestTask = .data(nil, nil)

        if let urlRequest = urlRequest {
            let originalTask = DataRequest.Requestable(urlRequest: urlRequest)
            requestTask = .data(originalTask, nil)
        }

        let underlyingError = error.underlyingAdaptError ?? error
        let request = DataRequest(session: session, requestTask: requestTask, error: underlyingError)

        if let retrier = retrier, error is AdaptError {
            allowRetrier(retrier, toRetry: request, with: underlyingError)
        } else {
            if startRequestsImmediately { request.resume() }
        }

        return request
    }