Alamofire源码分析(2)——DataRequest

上次分析了SessionManager层，这次往下走分析request层。先看例子：

SessionManager.default  //直接跳过Alamofire层调用也可以
.request(url, method: .get, parameters: parameters)
.response { (result) in
    print(result)
}

1. 直接找到源码位置，可以看到这一层返回DataRequest，这个不是请求的URLRequest，但请求的URLRequest在里面创建了：

open class SessionManager {
    ...
    @discardableResult
    open func request(
        _ url: URLConvertible,
        method: HTTPMethod = .get,  /*默认为get方法*/
        parameters: Parameters? = nil,
        encoding: ParameterEncoding = URLEncoding.default,
        headers: HTTPHeaders? = nil)
        -> DataRequest
    {
        var originalRequest: URLRequest?
        //创建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)
        }
    }
    ...
}

2. 下一步则是encoding.encode(originalRequest!, with: parameters)对参数进行处理，还会判别请求方法分开处理：

public struct URLEncoding: ParameterEncoding {
    ...
    public func encode(_ urlRequest: URLRequestConvertible, with parameters: Parameters?) throws -> URLRequest {
        var urlRequest = try urlRequest.asURLRequest()//跟asURL一个意思

        guard let parameters = parameters else { return urlRequest }
        //判别请求方法分开处理
        if let method = HTTPMethod(rawValue: urlRequest.httpMethod ?? "GET"), encodesParametersInURL(with: method) {
            ...//get
        } else {
            ...//post
        }

        return urlRequest
    }
    ...
}

3. 先看对get请求的处理：

public struct URLEncoding: ParameterEncoding {
    ...
    public func encode(_ urlRequest: URLRequestConvertible, with parameters: Parameters?) throws -> URLRequest {
        var urlRequest = try urlRequest.asURLRequest()

        guard let parameters = parameters else { return urlRequest }
        //判别请求方法分开处理
        if let method = HTTPMethod(rawValue: urlRequest.httpMethod ?? "GET"), encodesParametersInURL(with: method) {
            guard let url = urlRequest.url else {
                throw AFError.parameterEncodingFailed(reason: .missingURL)
            }
            //拼接url后面的参数
            if var urlComponents = URLComponents(url: url, resolvingAgainstBaseURL: false), !parameters.isEmpty {
                let percentEncodedQuery = (urlComponents.percentEncodedQuery.map { $0 + "&" } ?? "") + query(parameters)
                urlComponents.percentEncodedQuery = percentEncodedQuery
                urlRequest.url = urlComponents.url
            }
        } else {
            ...//post
        }

        return urlRequest
    }
    ...
}

从不显眼的query函数发现，对请求参数进行了排序和符号拼接：

public struct URLEncoding: ParameterEncoding {
    ...
    private func query(_ parameters: [String: Any]) -> String {
        var components: [(String, String)] = []
        //排序
        for key in parameters.keys.sorted(by: <) {
            let value = parameters[key]!
            components += queryComponents(fromKey: key, value: value)
        }
        return components.map { "\($0)=\($1)" }.joined(separator: "&")//符号拼接
    }
    ...
}

• 这里便是对get请求参数的主要处理。

4. 回到第2步，另一种处理方式便是对post请求的处理，先对请求头做了处理，后面同样用query进行参数排序和符号拼接：

public struct URLEncoding: ParameterEncoding {
    ...
    public func encode(_ urlRequest: URLRequestConvertible, with parameters: Parameters?) throws -> URLRequest {
        var urlRequest = try urlRequest.asURLRequest()

        guard let parameters = parameters else { return urlRequest }
        //判别请求方法分开处理
        if let method = HTTPMethod(rawValue: urlRequest.httpMethod ?? "GET"), encodesParametersInURL(with: method) {
            ...//get
        } else {
            //请求头
            if urlRequest.value(forHTTPHeaderField: "Content-Type") == nil {
                urlRequest.setValue("application/x-www-form-urlencoded; charset=utf-8", forHTTPHeaderField: "Content-Type")
            }
            //请求体
            urlRequest.httpBody = query(parameters).data(using: .utf8, allowLossyConversion: false)
        }

        return urlRequest
    }
    ...
}

5. 之后便回到第1步，创建DataRequest返回，到这里DataRequest这一层就完成了。

这一层先到这里，下一层分析task。
不过下一篇是：链式函数。