How do I handle task priority?

Question

minseoowen OP

Created Apr ’21

Replies 1

Boosts 0

Participants 2

This code is about getting data from a website.

Code Block var numberOfResults = 0
func getDataFrom(link:String) {
    guard let url = URL(string:link) else { return }
    let endPoint = NSMutableURLRequest(url: url)
  
    let task = URLSession.shared.dataTask(with: endPoint as URLRequest) { (data,response,error) in
        guard let data = data else {
            print("\(Fatal_Error.dataNotFound)")
            return
        }
       guard response != nil, error == nil else {
         
            print("\(Fatal_Error.noResponse)")
            return
        }
        
        var jsonData:fetchData?
     
        do {
            jsonData = try JSONDecoder().decode(fetchData.self, from: data)
            
            guard let existingData = jsonData else { return }
            numberOfResults = existingData.totalResults
            print(numberOfResults)
        } catch { jsonData = nil }
    }
        task.resume()
}
getDataFrom(link: link)
print(numberOfResults)

In the playground, when I execute all of this code at once,
numberOfResults gives me a value of zero which is a default value as shown above.

When I execute getDataFrom(link:link) first and then print(numberOfResults), it gives me the value I want which shows that print function is executed before the task in getDataFrom function is fully executed.

Is there any way to access numberOfResults variable after the task has been fully executed?

Also, I tried to handle this order problem by calling the function
in @main to be executed first when the app is launched in my project but there was a time delay to access the data (when I access the var right away, it will give me default value while it gives me the value I want only after accessing the var few second after) so I want to ask if there is a way to shorten the time taken to execute dataTask.

Answered by DTS Engineer in 671476022

URLSession is an async API. That is, you make a request and at some point in the future it calls the completion handler with the response. Code like this:

Code Block  getDataFrom(link: link)
print(numberOfResults)

won’t work because the request is still in flight at the time you print numberOfResults on line 3.

How you handle this very much depends on the context. If you’re working in a playground you might use code like that shown below. However, in a real product you’d adopt a technique that’s aligned with your overall architecture. For example:

In a toy project you might use the synthetic synchronous approach.
In a real app you might use Combine to process async data and apply it to your model. See Processing URL Session Data Task Results with Combine.

If you post some details about the context in which this code is running, I’ll see if I can come up with some more concrete advice.

Share and Enjoy
—
Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"

Code Block  import Foundation
import PlaygroundSupport
func load(url: URL, completionHandler: @escaping (Result<HTTPURLResponse, Error>) -> Void) {
    let request = URLRequest(url: url, cachePolicy: .reloadIgnoringLocalCacheData, timeoutInterval: 60.0)
    URLSession.shared.dataTask(with: request) { (data, response, error) in
        let result: Result<HTTPURLResponse, Error>
        if let error = error as NSError? {
            result = .failure(error)
        } else {
            let response = response as! HTTPURLResponse
            let data = data!
            _ = data    // ignore `data` for the moment
            result = .success(response)
        }
        DispatchQueue.main.async {
            completionHandler(result)
        }
    }.resume()
}
PlaygroundPage.current.needsIndefiniteExecution
load(url: URL(string: "https://example.com")!) { result in
    print(result)
    PlaygroundPage.current.finishExecution()
}

Boost

Answer 1

DTS Engineer OP

Apple

Apr ’21

Accepted Answer

URLSession is an async API. That is, you make a request and at some point in the future it calls the completion handler with the response. Code like this:

Code Block  getDataFrom(link: link)
print(numberOfResults)

won’t work because the request is still in flight at the time you print numberOfResults on line 3.

How you handle this very much depends on the context. If you’re working in a playground you might use code like that shown below. However, in a real product you’d adopt a technique that’s aligned with your overall architecture. For example:

In a toy project you might use the synthetic synchronous approach.
In a real app you might use Combine to process async data and apply it to your model. See Processing URL Session Data Task Results with Combine.

If you post some details about the context in which this code is running, I’ll see if I can come up with some more concrete advice.

Share and Enjoy
—
Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"

Code Block  import Foundation
import PlaygroundSupport
func load(url: URL, completionHandler: @escaping (Result<HTTPURLResponse, Error>) -> Void) {
    let request = URLRequest(url: url, cachePolicy: .reloadIgnoringLocalCacheData, timeoutInterval: 60.0)
    URLSession.shared.dataTask(with: request) { (data, response, error) in
        let result: Result<HTTPURLResponse, Error>
        if let error = error as NSError? {
            result = .failure(error)
        } else {
            let response = response as! HTTPURLResponse
            let data = data!
            _ = data    // ignore `data` for the moment
            result = .success(response)
        }
        DispatchQueue.main.async {
            completionHandler(result)
        }
    }.resume()
}
PlaygroundPage.current.needsIndefiniteExecution
load(url: URL(string: "https://example.com")!) { result in
    print(result)
    PlaygroundPage.current.finishExecution()
}

1