Problem assigning a Swift function to a function variable

Question

Here is a trivialized example of my problem:

class Test: NSObject {
    func metaHandler(name elementName: String, attributes attr : NSDictionary) -> NSNumber
    {
        return NSNumber.init()
    }
    var metaHandlerFunc: ((String, NSDictionary) -> NSNumber) = metaHandler
}

I get a compiler error when I assign the function to the var:

Cannot convert value of type '(Test) -> (String, NSDictionary) -> NSNumber' to specified type '(String, NSDictionary) -> NSNumber'

My example seems to be exactly what I have seen illustrated elsewhere. Any suggestions?

I'm running Xcode 10.3.

Martin R · Accepted Answer · 2020-07-04 18:20:25Z

1

You have defined metaHandler as an instance method, i.e. as a method which is applied to a concrete instance of Test. As such it has the signature

(Test) -> (String, NSDictionary) -> NSNumber

see for example Instance Methods are “Curried” Functions in Swift.

Defining it as a “type method” (with class or static) solves the problem:

static func metaHandler(...)

Now it has the signature (String, NSDictionary) -> NSNumber and can be assigned to var metaHandlerFunc.

Choose class func if overriding the method in a subclass should be allowed.

answered Jul 4, 2020 at 18:20

Martin R

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2 Comments

Alladinian Over a year ago

While this is true, the problem (at least when I am running the provided example) reported is that "property initializers run before 'self' is available". So a lazy var seems to solve the problem. Am I missing something?

Martin R Over a year ago

@Alladinian: You are right, that would solve the compiler error as well (feel free to post as an answer).

Alladinian · Accepted Answer · 2020-07-04 18:49:57Z

0

The problem is that you're trying to assign an instance function to an instance variable, which means that the instance should be initialized before assignment.

Btw the error I get (Swift 5.2.4) is:

cannot use instance member 'metaHandler' within property initializer; property initializers run before 'self' is available

which is a lot more helpful and can be solved by making metaHandlerFunc lazy:

lazy var metaHandlerFunc: ((String, NSDictionary) -> NSNumber) = metaHandler

alternatively, you can make the function static as stated in Martin's answer.

answered Jul 4, 2020 at 18:49

Alladinian

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