Alamofire之Request(一)

先上代码,再来看我们需要了解的是啥.

import UIKit
import Alamofire
class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        
        request("https://www.baidu.com", method: .get, parameters: ["username":"lb", "password":"123456"]).response { (response) in
            print("response === \(response)")
        }
        
    }

}

这里直接写 request 还是写 SessionManager.default.request 或是 Alamofire.request 都是一样的, 都来到 SessionManager.default 的 request 方法里.

该方法除了 url 是必传,其他都有默认值,可以不传.

@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)
    }
}

该方法我们一步一步来分析.

一 : 参数encode准备工作

① : 先创建了一个原生URLRequest. 这里没啥好说的.
② : 对参数 encoding 进行 encode .

查看encode方法

public func encode(_ urlRequest: URLRequestConvertible, with parameters: Parameters?) throws -> URLRequest {
    var urlRequest = try urlRequest.asURLRequest()
    guard let parameters = parameters else { return urlRequest }

    if let method = HTTPMethod(rawValue: urlRequest.httpMethod ?? "GET"), encodesParametersInURL(with: method) {
        guard let url = urlRequest.url else {
            throw AFError.parameterEncodingFailed(reason: .missingURL)
        }

        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 {
        if urlRequest.value(forHTTPHeaderField: "Content-Type") == nil {
            urlRequest.setValue("application/x-www-form-urlencoded; charset=utf-8", forHTTPHeaderField: "Content-Type")
        }

        urlRequest.httpBody = query(parameters).data(using: .utf8, allowLossyConversion: false)
    }
    return urlRequest
}

先判断 method 是不是 .get, .head, .delete (具体实现可以点击 encodesParametersInURL 进去查看)

是 .get, .head, .delete 则把参数直接拼接到 URL 后面.

其他方式则把参数放到 httpBody 中编码.

具体的参数拼接方式不具体讲述了, 总结一下:
1 : 由于请求是通过 ASCII编码的,所以要进行百分号编码，第一步就是对当前请求的所有路由百分号编码.
2 : query 方法

private func query(_ parameters: [String: Any]) -> String {
    var components: [(String, String)] = []

    for key in parameters.keys.sorted(by: <) {
        let value = parameters[key]!
        components += queryComponents(fromKey: key, value: value)
    }
    return components.map { "\($0)=\($1)" }.joined(separator: "&")
}

通过 ASCII 有小到大进行排序.

queryComponents 把 key 和 value 取出 , 进行参数递归,直到基础数据类型,如 bool, string ，然后进行了百分号编码, 之后添加到元祖返回。

拿到queryComponents返回的元祖数据,添加到数组中.

将保存了元祖的数组映射 ,元祖中数据变成 $1 = $2 形式 , 然后新生成的数组按 & 拼接成字符串.

二 : 开始请求 request(encodedURLRequest)

先点进去查看该方法:

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)
    }
}

1. 首先 DataRequest.Requestable 方法

DataRequest.Requestable(urlRequest: originalRequest!)

Requestable 点进去

struct Requestable: TaskConvertible {
    let urlRequest: URLRequest

    func task(session: URLSession, adapter: RequestAdapter?, queue: DispatchQueue) throws -> URLSessionTask {
        do {
            let urlRequest = try self.urlRequest.adapt(using: adapter)
            return queue.sync { session.dataTask(with: urlRequest) }
        } catch {
            throw AdaptError(error: error)
        }
    }
}

创建了一个结构体.并且初始化时就持有了 urlRequest

2. originalTask.task
调用该结构体的 task 方法,同步创建了一个 URLSessionTask. ( 源码在上面结构体源码中.)

3. DataRequest(session: , requestTask: )
创建一个 DataRequest , 自己没有 init 方法.查找其父类 Request

init(session: URLSession, requestTask: RequestTask, error: Error? = nil) {
    self.session = session

    switch requestTask {
    case .data(let originalTask, let task):
        taskDelegate = DataTaskDelegate(task: task)
        self.originalTask = originalTask
    case .download(let originalTask, let task):
        taskDelegate = DownloadTaskDelegate(task: task)
        self.originalTask = originalTask
    case .upload(let originalTask, let task):
        taskDelegate = UploadTaskDelegate(task: task)
        self.originalTask = originalTask
    case .stream(let originalTask, let task):
        taskDelegate = TaskDelegate(task: task)
        self.originalTask = originalTask
    }

    delegate.error = error
    delegate.queue.addOperation { self.endTime = CFAbsoluteTimeGetCurrent() }
}

这里根据不同的 requestTask 生成了不同的子类 TaskDelegate.
DataTaskDelegate / DownloadTaskDelegate 等等. 其实就是实际去干活的人. 它们与 SessionDelegate 也就是整体与局部的关系.

4. delegate[task] = request

这个 delegate 是我们的 SessionDelegate .
绑定 task 和 request , 方便在 SessionDelegate 下发任务，task 直接检索，方便直接获取 request.
实现方式点进原码找到如下:

var requests: [Int: Request] = [:]
private let lock = NSLock()

/// Access the task delegate for the specified task in a thread-safe manner.
open subscript(task: URLSessionTask) -> Request? {
    get {
        lock.lock() ; defer { lock.unlock() }
        return requests[task.taskIdentifier]
    }
    set {
        lock.lock() ; defer { lock.unlock() }
        requests[task.taskIdentifier] = newValue
    }
}

通过一个 requests: [Int: Request] 属性和开放出去的 subscript 下标查找方法以此实现 task 和 request 的绑定以及 delegate[task] 的 set 和 get 的实现.

5. request.resume()
直接 request.resume() 启动任务.

下篇博客会继续探讨 request 层级结构和具体流程.之后会统一进行总结.