工程日记之ChildLost(1)：URLSession

URLSession 是什么

URL Loading System提供了访问URL资源的系统，提供了访问http/https/自定义URL访问的接口。其中，URLSession实例可以创建多个URLSessionTask实例，完成不同的工作。

我们可以将URLSession类理解为会话层，用于管理网络接口的创建、维护、删除等等工作，我们要做的工作也只是会话层之后的层。

• URLSessionDataTask : 一个 数据请求 的 URLSessionTask 的子类，用于数据的请求。
• URLSessionUploadTask : 是一个 上传数据 的 URLSessionDataTask 的子类。
• URLSessionDownloadTask ： 是一个下载数据 的 URLSessionTask 的子类。
• URLSessionStreamTask : 是一个双向会话的 URLSessionTask 的子类。

基础使用

这里上传下载的都是一个字符串；实际形式是json（需要把NSData转成Json）

get

                        let url1 = "https://coconutnut.xyz:2019/tumble"
                        let sessionConfigure = URLSessionConfiguration.default
                       // sessionConfigure.httpAdditionalHeaders = ["Content-Type": "application/json"]
                       // sessionConfigure.timeoutIntervalForRequest = 30
                        sessionConfigure.requestCachePolicy = .reloadIgnoringLocalCacheData
                        let session = URLSession(configuration: sessionConfigure)
                         
                        guard let url =  URL(string: url1) else {
                               return
                        }
                        let url1Request = URLRequest(url: url)
                        let dataTask1 = session.dataTask(with: url1Request) { (data, response, error) in
                         guard data != nil else {return}
                         let str = String(data: data!, encoding: String.Encoding.utf8)
                         // 这里需要再加一个逻辑判断
                          guard let strnow = str else{return}

最后返回的strnow就是我们要获得的字符串；

post

let order = Qumot(value: "1")
    guard let uploadData = try? JSONEncoder().encode(order) else {
        return
    }
    print(uploadData)
  
    let url = URL(string: "https://coconutnut.xyz:2019/qumot")!
    var request = URLRequest(url: url)
    request.httpMethod = "POST"
    request.setValue("application/json", forHTTPHeaderField: "Content-Type")
    let task = URLSession.shared.uploadTask(with: request, from: uploadData) { data, response, error in
        if let error = error {
            print ("error: (error)")
            return
        }
        guard let response = response as? HTTPURLResponse,
            (200...299).contains(response.statusCode) else {
            print ("server error")
            return
        }
        if let mimeType = response.mimeType,
            mimeType == "application/json",
            let data = data,
            let dataString = String(data: data, encoding: .utf8) {
            print ("got data: (dataString)")
        }
    }
    task.resume()

使用Delegate

后台唤醒的地方，可以使用自己写的回调函数

可以在这些地方用到闭包

Tasks in a session also share a common delegate object. You implement this delegate to provide and obtain information when various events occur, including when:

• Authentication fails.

• Data arrives from the server.

• Data becomes available for caching.

If you don’t need the features provided by a delegate, you can use this API without providing one by passing nil when you create a session. 

 另外这是一个异步的接口，你可以一边处理UI一边访问网络

Like most networking APIs, the URLSession API is highly asynchronous. It returns data to your app in one of two ways, depending on the methods you call:

• By calling a completion handler block when a transfer finishes successfully or with an error.

• By calling methods on the session’s delegate as data arrives and when the transfer is complete.

URLSession的状态

1. default 

2. emphemaral ： 为了保证用户隐私，不将cookie或者本地cache或者证书持久化

3. background ： 把下载的任务交给System，则即使app已经被terminated此时依然可以被下载，但是当app relaunch的时候要根据identifier重拾这个下载任务并且去retrieve data

https://developer.apple.com/documentation/foundation/urlsessionconfiguration/1407496-background

 https://developer.apple.com/documentation/foundation/url_loading_system

https://developer.apple.com/documentation/foundation/url_loading_system/uploading_data_to_a_website