Declare an array of Int in Realm Swift

Lists of primitives are not supported yet unfortunately. There is issue #1120 to track adding support for that. You'll find there some ideas how you can workaround that currently.

The easiest workaround is create a object to hold int values. Then the model to have a List of the object.

class Foo: Object {
    let integerList = List<IntObject>() // Workaround
}

class IntObject: Object {
    dynamic var value = 0
}

Fortunately arrays of primitive types are now supported in Realm 3.0 and above. (Oct 31 2017)

You can now store primitive types or their nullable counterparts (more specifically: booleans, integer and floating-point number types, strings, dates, and data) directly within RLMArrays or Lists. If you want to define a list of such primitive values you no longer need to define cumbersome single-field wrapper objects. Instead, you can just store the primitive values themselves!

class MyObject : Object {
    @objc dynamic var myString: String = ""
    let myIntArray = List<Int>()
}

Source: https://realm.io/blog/realm-cocoa-reaches-3-0/

The accepted offer is very costly in term of memory. You might get a List of very big "n" of objects.

It's not a matter of right and wrong but I think it's good to write here a different workaround.

Another approach:
I decided to use a single string to represent an Int array.

In my Realm class I defined a variable:

dynamic var arrInt: String? = nil

And use it very easily:

let arrToSave = [0, 1, 33, 12232, 394]
<MY_CUSTOM_REALM_CLASS>.arrInt = arrToSave.map { String(describing: $0) }.joined(separator: ",")

And the way back:

let strFetched = <MY_CUSTOM_REALM_CLASS>.arrInt 
let intArray = strFetched.components(separatedBy: ",").flatMap { Int($0) }

Will be happy to hear your feedback, as I think this approach is better.