如何处理完成处理程序块中取消的NSURLSessionTask？

Question

如果我创建了一个NSURLSessionDownloadTask，并且稍后在完成之前取消它，那么完成块看起来仍然会触发。

let downloadTask = session.downloadTaskWithURL(URL, completionHandler: { location, response, error in 
    ... 
} 

如何检查下载任务是否被这个区块内取消，让我不要尝试对所得到的下载操作时，没有一个？

Answer 1

对于下载任务，完成处理器将用于locationnil值和NSError对象的code值被称为将是NSURLErrorCancelled。在斯威夫特3：

let task = session.downloadTask(with: url) { location, response, error in 
    if let error = error as? NSError { 
     if error.code == NSURLErrorCancelled { 
      // canceled 
     } else { 
      // some other error 
     } 
     return 
    } 

    // proceed to move file at `location` to somewhere more permanent 
} 
task.resume() 

还是在斯威夫特2：

let task = session.downloadTaskWithURL(url) { location, response, error in 
    if let error = error { 
     if error.code == NSURLErrorCancelled { 
      // canceled 
     } else { 
      // some other error 
     } 
     return 
    } 

    // proceed to move file at `location` to somewhere more permanent 
} 
task.resume() 

---

同样，对于数据的任务，完成处理器将与Error/NSError指示是否被取消调用。在斯威夫特3：

let task = session.dataTask(with: url) { data, response, error in 
    if let error = error as? NSError { 
     if error.code == NSURLErrorCancelled { 
      // canceled 
     } else { 
      // some other error 
     } 
     return 
    } 

    // proceed to move file at `location` to somewhere more permanent 
} 
task.resume() 

还是在斯威夫特2：

let task = session.dataTaskWithURL(url) { data, response, error in 
    if let error = error { 
     if error.code == NSURLErrorCancelled { 
      // canceled 
     } else { 
      // some other error 
     } 
     return 
    } 

    // otherwise handler data here 
} 
task.resume()