ios swift method parameters - ios

Question

I'm reading the swift getting started apple's guide. And they give the following code :
required init?(coder aDecoder: NSCoder)
Why are they two parameters name ? coder and aDecoder.
I don't understand why we don't use this syntax that works to :
required init?(coder: NSCoder)
Thank you.

Answer 1

coder is external parameter name. It is used when calling method:
SomeClass(coder: someCoder)
aDecoderis local parameter name. It is used inside method:
required init?(coder aDecoder: NSCoder) {
super.init(coder: aDecoder)
}
You may specify just local parameter name:
required init?(coder: NSCoder)
In this case external parameter name automatically takes the same value coder.
The main use is readability. Your example is not very obvious. Lets take a better one from the link posted by Eric D:
func sayHello(to person: String, and anotherPerson: String) -> String {
return "Hello \(person) and \(anotherPerson)!"
}
print(sayHello(to: "Bill", and: "Ted"))
This is more readable (better explains what method is doing) when calling sayHello method than
func sayHello(person: String, anotherPerson: String) -> String {
return "Hello \(person) and \(anotherPerson)!"
}
print(sayHello(person: "Bill", anotherPerson: "Ted"))
Of course you can write
func sayHello(to: String, and: String) -> String {
return "Hello \(to) and \(and)!"
}
print(sayHello(to: "Bill", and: "Ted"))
But in this case local variables are using bad naming style