首先我們先看一下最簡單的一個(gè)請(qǐng)求方法

public static func request(_ url: URLConvertible,
                               method: HTTPMethod = .get,
                               parameters: Parameters? = nil,
                               encoding: ParameterEncoding = URLEncoding.default,
                               headers: HTTPHeaders? = nil,
                               interceptor: RequestInterceptor? = nil) -> DataRequest

解讀一下各個(gè)參數(shù)的意思：

url

我們可以看到url的參數(shù)類型為URLConvertible，點(diǎn)進(jìn)去看一下會(huì)看到這是一個(gè)協(xié)議類型求摇，定義如下

/// Types adopting the `URLConvertible` protocol can be used to construct `URL`s, which can then be used to construct
/// `URLRequests`.
public protocol URLConvertible {
    /// Returns a `URL` from the conforming instance or throws.
    ///
    /// - Returns: The `URL` created from the instance.
    /// - Throws:  Any error thrown while creating the `URL`.
    func asURL() throws -> URL
}

而swift中通過extension可以對(duì)協(xié)議進(jìn)行默認(rèn)實(shí)現(xiàn)，Alamofire中對(duì)String和URL兩個(gè)類實(shí)現(xiàn)了該協(xié)議，并添加默認(rèn)實(shí)現(xiàn)如下

extension String: URLConvertible {
    /// Returns a `URL` if `self` can be used to initialize a `URL` instance, otherwise throws.
    ///
    /// - Returns: The `URL` initialized with `self`.
    /// - Throws:  An `AFError.invalidURL` instance.
    public func asURL() throws -> URL {
        guard let url = URL(string: self) else { throw AFError.invalidURL(url: self) }

        return url
    }
}

extension URL: URLConvertible {
    /// Returns `self`.
    public func asURL() throws -> URL { return self }
}

method

請(qǐng)求類型打月，為一個(gè)enum

public enum HTTPMethod: String {
    /// `CONNECT` method.
    case connect = "CONNECT"
    /// `DELETE` method.
    case delete  = "DELETE"
    /// `GET` method.
    case get     = "GET"
    /// `HEAD` method.
    case head    = "HEAD"
    /// `OPTIONS` method.
    case options = "OPTIONS"
    /// `PATCH` method.
    case patch   = "PATCH"
    /// `POST` method.
    case post    = "POST"
    /// `PUT` method.
    case put     = "PUT"
    /// `TRACE` method.
    case trace   = "TRACE"
}

parameters 及 headers

請(qǐng)求參數(shù)及請(qǐng)求頭
Parameters我們可以看到實(shí)際上是一個(gè)別名public typealias Parameters = [String: Any]
HTTPHeaders為一個(gè)結(jié)構(gòu)體辞嗡，持有一個(gè)[HTTPHeader]的數(shù)組不狮，我們可以通過HTTPHeader的構(gòu)造方法來創(chuàng)建header參數(shù)欣除。
HTTPHeaders提供了一些默認(rèn)header

public extension HTTPHeaders {
    /// The default set of `HTTPHeaders` used by Alamofire. Includes `Accept-Encoding`, `Accept-Language`, and
    /// `User-Agent`.
    static let `default`: HTTPHeaders = [.defaultAcceptEncoding,
                                         .defaultAcceptLanguage,
                                         .defaultUserAgent]
}

interceptor

對(duì)于這個(gè)參數(shù)，我的理解是在請(qǐng)求前及請(qǐng)求失敗時(shí)候?qū)equest做統(tǒng)一處理的地方潭袱，首先我們看一下這個(gè)協(xié)議的定義

/// Type that provides both `RequestAdapter` and `RequestRetrier` functionality.
public protocol RequestInterceptor: RequestAdapter, RequestRetrier {}

extension RequestInterceptor {
    public func adapt(_ urlRequest: URLRequest, for session: Session, completion: @escaping (AFResult<URLRequest>) -> Void) {
        completion(.success(urlRequest))
    }

    public func retry(
        _ request: Request,
        for session: Session,
        dueTo error: Error,
        completion: @escaping (RetryResult) -> Void)
    {
        completion(.doNotRetry)
    }
}

第一個(gè)方法柱嫌，會(huì)在請(qǐng)求發(fā)起之前進(jìn)行調(diào)用，比如我們可以在這個(gè)地方對(duì)urlRequest進(jìn)行一些請(qǐng)求參數(shù)及請(qǐng)求頭的修改屯换；
第二個(gè)方法則會(huì)在請(qǐng)求失敗后進(jìn)行調(diào)用慎式。(有請(qǐng)求結(jié)果，例如response.statusCode = 400之類的不會(huì)調(diào)用趟径，因?yàn)檫@不算一次失敗的請(qǐng)求，當(dāng)然如果我們想讓如此之類的請(qǐng)求也走該回調(diào)方便做統(tǒng)一處理癣防，那么我們的請(qǐng)求方法需要這么寫 Alamofile.request(url).validate().response蜗巧。 中間添加一個(gè)validate)

我們看一下validate具體做了什么：
首先Validation.swift文件中有這么一個(gè)參數(shù)fileprivate var acceptableStatusCodes: Range<Int> { return 200..<300 }，也就是200..<300以外的statusCode會(huì)走上邊的retry(_request:for:dueTo:completion)方法

我們看到Validation.swift文件中對(duì)DataRequest有如下擴(kuò)展

extension DataRequest {
    /// Validates that the response has a status code in the specified sequence.
    ///
    /// If validation fails, subsequent calls to response handlers will have an associated error.
    ///
    /// - parameter range: The range of acceptable status codes.
    ///
    /// - returns: The request.
    @discardableResult
    public func validate<S: Sequence>(statusCode acceptableStatusCodes: S) -> Self where S.Iterator.Element == Int {
        return validate { [unowned self] _, response, _ in
            return self.validate(statusCode: acceptableStatusCodes, response: response)
        }
    }
}

該方法蕾盯，調(diào)用了Request.swift文件中的方法

@discardableResult
    public func validate(_ validation: @escaping Validation) -> Self {
        let validator: () -> Void = { [unowned self] in
            guard self.error == nil, let response = self.response else { return }

            let result = validation(self.request, response, self.data)

            if case .failure(let error) = result { self.error = error }

            self.eventMonitor?.request(self,
                                       didValidateRequest: self.request,
                                       response: response,
                                       data: self.data,
                                       withResult: result)
        }

        protectedValidators.append(validator)

        return self
    }

該方法把block保存在protectedValidators中幕屹，當(dāng)請(qǐng)求結(jié)束后，會(huì)對(duì)請(qǐng)求結(jié)果code進(jìn)行一個(gè)比對(duì)，如下

func didCompleteTask(_ task: URLSessionTask, with error: Error?) {
        self.error = self.error ?? error
        // 如果我們請(qǐng)求時(shí)調(diào)用了 .validate() 則這個(gè)數(shù)組中便存在對(duì)應(yīng)的 block望拖，進(jìn)行遍歷渺尘，會(huì)把 200..<300 以外的code算為 Error，然后就會(huì)走 retry(_request:for:dueTo:completion) 這個(gè)方法
        protectedValidators.directValue.forEach { $0() }

        eventMonitor?.request(self, didCompleteTask: task, with: error)

        retryOrFinish(error: self.error)
    }

    func retryOrFinish(error: Error?) {
        guard let error = error, let delegate = delegate else { finish(); return }

        delegate.retryResult(for: self, dueTo: error) { retryResult in
            switch retryResult {
            case .doNotRetry, .doNotRetryWithError:
                self.finish(error: retryResult.error)
            case .retry, .retryWithDelay:
                delegate.retryRequest(self, withDelay: retryResult.delay)
            }
        }
    }

上面的protectedValidators.directValue.forEach { $0() }這句代碼會(huì)走到該方法

public func validate(_ validation: @escaping Validation) -> Self {
        let validator: () -> Void = { [unowned self] in
            guard self.error == nil, let response = self.response else { return }

            let result = validation(self.request, response, self.data)

            if case .failure(let error) = result { self.error = error }

            self.eventMonitor?.request(self,
                                       didValidateRequest: self.request,
                                       response: response,
                                       data: self.data,
                                       withResult: result)
        }

        protectedValidators.append(validator)

        return self
    }

這個(gè)方法中的let result = validation(self.request, response, self.data)則會(huì)對(duì)code進(jìn)行對(duì)比

fileprivate func validate<S: Sequence>(
        statusCode acceptableStatusCodes: S,
        response: HTTPURLResponse)
        -> ValidationResult
        where S.Iterator.Element == Int
    {
        if acceptableStatusCodes.contains(response.statusCode) {
            return .success(Void())
        } else {
            let reason: ErrorReason = .unacceptableStatusCode(code: response.statusCode)
            return .failure(AFError.responseValidationFailed(reason: reason))
        }
    }