What the difference between setValue and myObject.propertyName in iOS Swift 3.1?

Question

I have object like this:

class Human: NSObject {
    var name:String!
    var age:Int!
}

And make instance from object like this:

let instance = Human()

And set my values like this:

instance.name = "Kirk"
instance.age = 87

And print it like this:

print("name: \(instance.name)") // Kirk
print("age: \(instance.age)") //87

Every things are OK. but now set my values another way like this:

instance.setValue("Kirk", forKey: "name")
instance.setValue(87, forKey: "age")

Get this error for Int dataType:

*** Terminating app due to uncaught exception 'NSUnknownKeyException', reason: '[ setValue:forUndefinedKey:]: this class is not key value coding-compliant for the key age.'

Why this happened?
And How to fix this because i need to using setValue() function.

The difference is that `instance.name` is accessing the property via native Swift mechanisms, whereas `instance.setValue("Kirk", forKey: "name")` is requires key value coding (KVC), which only works if the types can be represented in Objective-C. As others pointed out, `Int!` cannot be represented in Objective-C, so it cannot participate in KVC. Change `age` from `Int!` to `Int` or `NSNumber!`, and it will work fine. (Clearly if you change it to `Int`, you'll need to make sure it's initialized to a non-`nil` value, presumably with `init` method that takes the value as a parameter.) — Rob, May 22 '17 at 09:36
Possible duplicate of [Why can I not use KVC from an Objective-C object to a Swift Property?](http://stackoverflow.com/questions/35783888/why-can-i-not-use-kvc-from-an-objective-c-object-to-a-swift-property) — Martin R, May 22 '17 at 09:44

Sulthan · Accepted Answer · 2017-05-22T09:29:52.057

7

Key-value coding (KVC) relies on Objective-C functionality and Int! type is technically an optional value type. Optional value types cannot be represented in Objective-C and that's why the call fails.

You can see the compilation error if you add an explicit dynamic attribute:

class Human: NSObject {
    var name:String!   
    dynamic var age: Int!
}

It would work with Int.

Using implicitly unwrapped optionals in this case is a bad idea anyway. You should have the properties as non-optionals and initialize them in an initializer.

edited May 22 '17 at 09:29

answered May 22 '17 at 09:22

Sulthan

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Some optional value types (such as `String!`) can be represented in Obj-C – `Int`'s just problematic because it gets bridged to `NSInteger` and therefore has no way to represent `nil`. – Hamish May 22 '17 at 09:44
@Hamish True, there are obvious exceptions where the bridged object is a reference type (`Data`, arrays, dictionaries etc.). – Sulthan May 22 '17 at 09:54

Bilal · Answer 2 · 2017-05-22T09:56:25.503

2

Int! can't be represented int Objective-C as other mentioned. This should work

class Human: NSObject {
    var name:String!
    var age : Int;

    init(name:String?, age:Int) {
        self.age = age
        self.name = name
    }
}

edited May 22 '17 at 09:56

answered May 22 '17 at 09:43

Bilal

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Updated as suggested by @Rob – Bilal May 22 '17 at 09:59

What the difference between setValue and myObject.propertyName in iOS Swift 3.1?

2 Answers2