How to override localizedDescription for custom Error in Swift 3?

Question

Error protocol has only one property localizedDescription. I tried to create custom object inherited from NSObject and Error but I can not override localizedDescription. How can I do that?

This code does not allow me to get custom description:

class MyError: NSObject, Error {
    var desc = ""
    init(str: String) {
        desc = str
    }
    override var description: String {
        get {
            return "MyError: \(desc)"
        }
    }
   var localizedDescription: String {
        get {
            return self.description
        }
    }
}

func test_my_code() {
    let error = MyError(str: "my test string")
    let x = error as Error
    print(x.localizedDescription)
}

Calling function "test_my_code" get unexpected result: "The operation couldn’t be completed...".

What should I do to get result "MyError: my test string" ?


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| osx   | swift   | ios   | swift3   | inheritance   2016-12-26 04:12 1 Answers

Answers ( 1 )

  1. 2016-12-26 05:12

    The documentation about new Error bridging feature is not clear enough still now, so this answer may need some updates in the near future, but according to SE-0112 and the latest Swift source code, you may need to use LocalizedError rather than Error and implement errorDescription.

    class MyError: NSObject, LocalizedError {
        var desc = ""
        init(str: String) {
            desc = str
        }
        override var description: String {
            get {
                return "MyError: \(desc)"
            }
        }
        //You need to implement `errorDescription`, not `localizedDescription`.
        var errorDescription: String? {
            get {
                return self.description
            }
        }
    }
    
    func test_my_code() {
        let error = MyError(str: "my test string")
        let x = error as Error
        print(x.localizedDescription)
    }
    test_my_code() //->MyError: my test string
    

    Other than using LocalizedError, this default implementation works:

    (NSError.swift, the link shown above)

    public extension Error {
        /// Retrieve the localized description for this error.
        var localizedDescription: String {
            return NSError(domain: _domain, code: _code, userInfo: nil).localizedDescription
        }
    }
    

    It is a little complicated how Swift defines _domain or _code from arbitrary types just conforming to Error, but it seems that NSError generates "The operation couldn’t be completed..." for unknown combinations of domain and code.

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