Handle uncaught exception valueForUndefinedKey in Swift

Question

I have a third party library class whose source is not open to me. For example like this:

class MyClass: NSObject {
    let myProperty = 10
}

I can get myProperty value like this.

let test = MyClass().value(forKey: "myProperty")  // 10

I want to check whether myProperty exists in MyClass. Reason is I don't want my app to crash if the 3rd party class implementation changes in future.

For testing I've tried

guard let test = MyClass().value(forKey: "myProperty1") else { return }  // crash

if let test = MyClass().value(forKey: "myProperty1") { }  // crash

do {
    let test = try MyClass().value(forKey: "myProperty1")  // crash
} catch { }

In every way I get crash.

*** Terminating app due to uncaught exception 'NSUnknownKeyException', reason: '[<__lldb_expr_80.MyClass 0x608000223a00> valueForUndefinedKey:]: this class is not key value coding-compliant for the key myProperty1.'

Dont think there's anyway to prevent this, you must update your app or ask the lib's dev to change to more convenience way of accessing property — Tj3n, Dec 26 '16 at 07:51

Hejazi · Answer 1 · 2020-04-15T16:04:13.633

When valueForKey: is invoked for an undefined key, the runtime forwards the call to valueForUndefinedKey:. The default implementation of the later simply raises NSUndefinedKeyException.

We can't catch the exception from Swift code, but we can override the method valueForUndefinedKey: in an extension and return some error:

extension NSObject {
    @objc
    func value(forUndefinedKey key: String) -> Any {
        NSError(domain: "Dynamic", code: 404, userInfo: [NSLocalizedDescriptionKey: "Accessing undefined key: '\(key)'"])
    }
}

score 1 · Accepted Answer · answered Dec 26 '16 at 08:12

You must find a better solution by talking to lib dev

You can use Mirror reflection and check its objects label

let myClass = Mirror(reflecting: MyClass())
for (_, attr) in myClass.children.enumerated() {
  if let propertyName = attr.label, propertyName == "myProperty"{
   print(propertyName, attr.value)
  }
}

Handle uncaught exception valueForUndefinedKey in Swift

2 Answers2