根据Swift中的用户输入取消URLSession

Question

我将应用程序的API调用集中在一个名为APIService的类中。来电如下：

// GET: Attempts getconversations API call. Returns Array of Conversation objects or Error
func getConversations(searchString: String = "", completion: @escaping(Result<[Conversation], APIError>) -> Void) {
{...} //setting up URLRequest
let dataTask = URLSession.shared.dataTask(with: request) { data, response, error in
guard let httpResponse = response as? HTTPURLResponse, httpResponse.statusCode == 200, let _ = data
else {
print("ERROR: ", error ?? "unknown error")
completion(.failure(.responseError))
return
}
do {
{...} //define custom decoding strategy
}
let result = try decoder.decode(ResponseMultipleElements<[Conversation]>.self, from: data!)
completion(.success(result.detailresponse.element))
}catch {
completion(.failure(.decodingError))
}
}
dataTask.resume()
}

我正在从应用程序中的任何位置执行API调用，如：

func searchConversations(searchString: String) {
self.apiService.getConversations(searchString: searchString, completion: {result in
switch result {
case .success(let conversations):
DispatchQueue.main.async {
{...} // do stuff 
}
case .failure(let error):
print("An error occured (error.localizedDescription)")
}
})
}

我现在想实现的是，在输入searchString时，对用户敲击的每个字符执行func searchConversations。这将很容易，只需根据正在激发的UIPressesEvent调用func searchConversations。像这样：

override func pressesEnded(_ presses: Set<UIPress>, with event: UIPressesEvent?) {
guard let key = presses.first?.key else { return }
switch key.keyCode {
{...} // handle special cases
default:
super.pressesEnded(presses, with: event)
searchConversations(searchString: SearchText.text)
}
}

我现在的问题是：每当输入一个新角色时，我都想取消上一个URLSession并启动一个新的URLSession。如何在UIPressesEvent处理程序内部执行此操作？

Answer 1

基本思想是确保API返回一个稍后可以取消的对象(如果需要(，然后修改搜索例程以确保取消任何挂起的请求(如果有(：

1. 首先，让API调用返回URLSessionTask对象：

   @discardableResult
   func getConversations(searchString: String = "", completion: @escaping(Result<[Conversation], APIError>) -> Void) -> URLSessionTask {
   ...
   let dataTask = URLSession.shared.dataTask(with: request) { data, response, error in
   ...
   }
   dataTask.resume()
   return dataTask
   }
   

2. 让你的搜索程序跟踪最后一项任务，如果需要，可以取消它：

   private weak var previousTask: URLSessionTask?
   func searchConversations(searchString: String) {
   previousTask?.cancel()
   previousTask = apiService.getConversations(searchString: searchString) { result in
   ...
   }
   }
   

3. 我们经常添加一个微小的延迟，这样如果用户快速输入，我们就可以避免许多不必要的网络请求：

   private weak var previousTask: URLSessionTask?
   private weak var delayTimer: Timer?
   func searchConversations(searchString: String) {
   previousTask?.cancel()
   delayTimer?.invalidate()
   delayTimer = Timer.scheduledTimer(withTimeInterval: 0.25, repeats: false) { [weak self] _ in
   guard let self = self else { return }
   self.previousTask = self.apiService.getConversations(searchString: searchString) {result in
   ...
   }
   }
   }
   

4. 唯一的另一件事是，你可能想更改你的网络请求错误处理程序，这样请求的"取消"就不会像处理错误一样处理了。从URLSession的角度来看，取消是一个错误，但从我们的应用程序的角度来说，取消不是一个错误条件，而是一个预期的流程。

Answer 2

您可以通过使用计时器来实现这一点

1( 定义计时器变量

var requestTimer: Timer?

2( 更新searchConversations功能

@objc func searchConversations() {
self.apiService.getConversations(searchString: SearchText.text, completion: {result in
switch result {
case .success(let conversations):
DispatchQueue.main.async {
{...} // do stuff
}
case .failure(let error):
print("An error occured (error.localizedDescription)")
}
})
}

3( 更新按结束

override func pressesEnded(_ presses: Set<UIPress>, with event: UIPressesEvent?) {
guard let key = presses.first?.key else { return }
switch key.keyCode {
{...} // handle special cases
default:
super.pressesEnded(presses, with: event)
self.requestTimer?.invalidate()
self.requestTimer = Timer.scheduledTimer(timeInterval: 0.5, target: self, selector: #selector(searchConversations), userInfo: nil, repeats: false)
}
}