How can I create a variable like newValue (used in setter) in swift 3

Question

All,

I have created a method which takes closure like this,

extension NSManagedObject{
    class func performOnWorkerContext(_ block:@escaping (_ context:NSManagedObjectContext?) -> ()){
        //Create context, Call block() and save context
    }
}

I use this method like this,('Request' is of type NSManagedObject). The class method will be modified to:

extension NSManagedObject{
    class func performOnWorkerContext(_ block: @escaping () ->()) {
        //Create context, Call block() and save context
    }
}

Request.performAndWaitOnWorkerContext { context in
    //Use the 'context' here    
}

Now, my question is how can I use this method like this,

Request.performAndWaitOnWorkerContext {    
    //Use 'context' here    
}

Here I want to use the variable 'context' (I dont know how to, which is the question). I have seen the similar implementation when we use setter in swift

Ex. If I use

var temp: Int {
    set { 
        print(newValue) // Here 'newValue' is automatically available
    }
    get {}
}

Something like this I want to implement, please suggest if its doable or not or how setters in swift do it?

The motivation behind this is it looks more elegant and we dont have to remember obvious variables accessible in this closure.

Answer 1

newValue is the default name of the parameter in the setter of a computed property, the willSet property observer, and the setter of a subscript method. It is built into the language.

In a closure, you can name the parameter

Request.performAndWaitOnWorkerContext { context in
   // Use `context` here
}

or use shorthand parameter names:

Request.performAndWaitOnWorkerContext {
   // Use `$0` here
}

but you cannot define an implicit parameter name.