Swift 3 NSArray比较无

Question

I'm trying to migrate an objc project to swift3. I'm not sure how can I compare an array to nil. I have found this topic, but that was 2 years ago and the swift's syntax has changed a lot.

If I have a code like this in swift:

let variable = something as? NSArray
  if variable == nil {
    // do something
  }

It won't let me to compare this variable with nil, causing an error "comparing this variable, always returns false". I have tried comparing variable.description with " ", but does it do the same thing? By "something" i meant:

var variable = dict.object(forKey: someString) as! NSArray

The main thing I wanted to do with this was:

var variable = dict.object(forKey: someString) as! NSArray
  if variable == nil {
    //create 
  }
  else {
    // append
  }

Answer 1

That's what the optional unwrapping syntax is for. You can combine the unwrapping and cast into one if statement:

if let variable = something as? NSArray {
    // variable is not nil and is an NSArray
    // Now you can do something with it.
} else {
    // Either something is nil or it is not able to be cast as an NSArray
    // Handle this case.
}

I should also mention that if you don't need to use something in Objective-C, then you should use the Swift-native array type. This can be declared like this:

let someArray = ["string1", "string2"]

Answer 2

This line indicates that variable is and must be an NSArray. If dict.object(forKey: someString) is not an NSArray, this will cause a crash

var variable = dict.object(forKey: someString) as! NSArray
//                                               ^
// This exclamation mark means you are certain this is an NSArray
// Also, because there is no question mark after NSArray, this variable
// is not optional. It cannot be nil

However, you then use

if variable == nil {

And this is where the warning comes from. The variable can never be nil, because the variable is not optional

---

What you probably want is:

if let variable = dict.object(forKey:someString) as? NSArray    

This will return false if:

• dict.object(forKey:someString) returns a nil object
• the object returned is not an NSArray

After this variable is now a non-optional NSArray. It is guaranteed to be an NSArray and is guaranteed to not be nil . You can use it without unwrapping it. eg

if let variable = dict.object(forKey:someString) as? NSArray {
    for element in variable {
    }
}
else {
    //The dict doesn't contain the object yet. `variable` is nil
    //Create a new array and add it to dict
    let newArray = ["First Value"]
    dict[someString] = newArray
}

Answer 3

let variable = something as? NSArray

With this declaration, variable will be an optional type ( NSArray? ) and never nil. This is because casting with as? returns an optional value that either contains the successfully casted object or nothing. You can see this by alt -clicking the variable name in Xcode.

If you want to know whether it contains a value, you need to use the if let syntax:

if let variable = variable {
    // variable is guaranteed to be an NSArray here.
}

Answer 4

You can also use this format with guard-else :

guard let variable = something as? NSArray else {
    // your variable is nil. Do something if needed
}
// your variable is available in this scope. Do something when variable contains Array