swift - Is force cast really bad and should always avoid it?

Question

I started to use swiftLint and noticed one of the best practices for Swift is to avoid force cast. However I used it a lot when handling tableView, collectionView for cells :

let cell = collectionView.dequeueReusableCellWithReuseIdentifier(cellID, forIndexPath: indexPath) as! MyOffersViewCell

If this is not the best practice, what's the right way to handle this? I guess I can use if let with as?, but does that mean for else condition I will need to return an empty cell? Is that acceptable?

if let cell = collectionView.dequeueReusableCellWithReuseIdentifier(cellID, forIndexPath: indexPath) as? MyOffersViewCell {
      // code
} else {
      // code
}

Answer 1

This question is probably opinion based, so take my answer with a grain of salt, but I wouldn't say that force downcast is always bad; you just need to consider the semantics and how that applies in a given situation.

as! SomeClass is a contract, it basically says "I guarantee that this thing is an instance of SomeClass". If it turns out that it isn't SomeClass then an exception will be thrown because you violated the contract.

You need to consider the context in which you are using this contract and what appropriate action you could take if you didn't use the force downcast.

In the example you give, if dequeueReusableCellWithIdentifier doesn't give you a MyOffersViewCell then you have probably misconfigured something to do with the cell reuse identifier and an exception will help you find that issue.

If you used a conditional downcast then you are going to get nil and have to handle that somehow - Log a message? Throw an exception? It certainly represents an unrecoverable error and something that you want to find during development; you wouldn't expect to have to handle this after release. Your code isn't going to suddenly start returning different types of cells. If you just let the code crash on the force downcast it will point straight to the line where the issue occurred.

Now, consider a case where you are accessing some JSON retrieved from a web service. There could be a change in the web service that is beyond your control so handling this more gracefully might be nice. Your app may not be able to function but at least you can show an alert rather than simply crashing:

BAD - Crashes if JSON isn't an array

 let someArray=myJSON as! NSArray 
 ...

Better - Handle invalid JSON with an alert

guard let someArray=myJSON as? NSArray else {
    // Display a UIAlertController telling the user to check for an updated app..
    return
}