CODEWITHSUNDEEP
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Evgeniy Kleban
Evgeniy Kleban

Reputation: 6965

Swift reverse guard issues

I have closure that may return an error. It look like:

session.dataTask(with: url) { (data, response, error) in

Next I want to handle error. I want to use guard statement for unwrap error, and if it exist, that show alert and return function. But how could I perform something like 

guard !(let error = error) else { return }

I can't. I can check for nil and then unwrap error again, but that look kind of ugly. Or I can use if let unwrapping, but for error checking I prefer guard. So, is there any way to nicely write following? 

guard !(let error = error) else { return }

Upvotes: 4

Views: 2966

Answers (3)

Dare
Dare

Reputation: 2587

guard error == nil else {
    print(error!)
    return
}

You're safe to force unwrap error in the body since you know it's isn't a null pointer.

You're better off with 

if let error = error {
    //handle error
    return
}

Upvotes: 4

vadian
vadian

Reputation: 285200

Don't fight the framework.

  • If you want to use the unwrapped optional within the braces use if let

    if let error = error {
   print(error)
   return
}

  • If you want to use the unwrapped optional after the braces use guard let

    guard let error = error else {
   return
}
print(error)

  • If the (unwrapped) content of error doesn't matter just check for nil

    guard error == nil else { ... 
if error == nil { ...

Upvotes: 9

Martin R
Martin R

Reputation: 539975

The documentation states that when the closure is called:

  • either data has a value and error is nil (if the request was successful),
  • or data is nil and error has a value (if the request failed).

What you are really interested in is the data, and in the success case you have to (somehow) unwrap that anyway.

Therefore I would check if data has a value. If that fails, you have an error. To avoid forced unwrapping use the nil-coalescing operator:

session.dataTask(with: url) { (data, response, error) in

    guard let data = data else {
        // The request failed.
        let message = error?.localizedDescription ?? "Unknown error"
        // Present message ...
        return
    }

    // The request was successful. 
    // Process data ...
}

Upvotes: 5

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