value(forKey:) for NSObject not working in Swift 4

Question

Trying out Swift 4 in Xcode 9 Beta.

For some reason I'm getting a crash when using key value accessors on an NSObject.

Any ideas?

import Cocoa

class Person: NSObject {
    var name = ""
    var age = 0
}

let alpha = Person()
alpha.name = "Robert"
alpha.age = 53

alpha.value(forKey: "name")
// error: Execution was interrupted, reason: EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0).

Related: [How can I deal with @objc inference deprecation with #selector() in Swift 4?](https://stackoverflow.com/q/44390378/2976878) — Hamish, Jun 09 '17 at 15:38
If you call `value(forKey:)` with `#keyPath(Person.name)` instead of a string then the compiler will show a proper error message and Fix-it! — Martin R, Jun 09 '17 at 15:39
Or even, now that you're in Swift 4, use Swift's key-value coding mechanism `print(alpha[keyPath: \Person.name])` :) Then you don't have to inherit from `NSObject` and can even make `Person` a `struct`. — Hamish, Jun 09 '17 at 15:50
I'm using key-value coding in order to dynamically select the property to access from a string variable. Is there any way to do this using the new Swift 4 mechanism? — closetCoder, Jun 09 '17 at 15:56
@closetCoder You can store and pass about key paths, e.g `let name = \Person.name`, but they're not strings (they're strongly typed instead). Strings are such weak types to represent key-paths, and should really be avoided unless absolutely necessary. What's your use case for using a string key path? — Hamish, Jun 09 '17 at 16:01
Data source for an NSTableView. I use the identifier of the column to select which property to return as the value. — closetCoder, Jun 09 '17 at 16:02
@closetCoder When and where do the identifiers get set? I don't have much experience with `NSTableView`, but how about just having your data source hold an array of `PartialKeyPath` (key paths that start at `Person`, but can go to any sub-property in the object graph), with each corresponding to the property to display for a given column index? Then just subscript your model with the key path for a given column index. — Hamish, Jun 09 '17 at 16:23
@Hamish The identifiers are set in Interface Builder. It is not a problem to use a switch statement. It just seems a bit inelegant to do stuff like case "name": return data.name case "age": return data.age etc. — closetCoder, Jun 09 '17 at 16:28
@closetCoder Oh I agree (you wouldn't have to switch with the strongly-typed key-paths either) – but the problem with using strings as key paths is that they're really error-prone (e.g typos, rename/remove a model property without changing the key-path) as well as the fact that they don't carry any type information, so the compiler cannot enforce you're using them correctly (e.g using them on the correct type of object). — Hamish, Jun 09 '17 at 16:34

Sulthan · Accepted Answer · 2017-06-09T15:37:23.337

25

Key-value coding requires Objective-C. In Swift 4 you have to mark members accessible from Obj-C explicitly:

@objcMembers
class Person: NSObject {

or

class Person: NSObject {
    @objc var name = ""
    var age = 0
}

See Swift Evolution 160

edited Jun 09 '17 at 15:37

answered Jun 09 '17 at 15:34

Sulthan

128,090
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score 7 · Answer 2 · answered Jun 09 '17 at 15:37

7

ObjC inference has been limited in Swift 4. You can mark your class with objcMembers for the compiler to generate ObjC-compliant accessors:

@objcMembers
class Person: NSObject {
    var name = ""
    var age = 0
}

Or get back the old behavior by setting Swift 3 Objc Inference to On in Build Settings:

answered Jun 09 '17 at 15:37

Code Different

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Note that the "Swift 3 @objc Inference" build setting is only really there to assist with the migration, and therefore isn't a permanent solution. – Hamish Jun 09 '17 at 15:41

value(forKey:) for NSObject not working in Swift 4

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