How to access Class's property in required convenience init?

Question

reepic OP

Created Jul ’20

Replies 3

Boosts 0

Views 4.1k

Participants 3

Hi, I'm new to Swift. I am reading Mastering macOS Programming Chapterc13, about NSCoding, here is the code:

Code Block class Person: NSObject, NSCoding {
  
	var name: String
	let kName = "Name"
  
	init(name: String) {
 		self.name = name
 		super.init()
	}
  
	required convenience init?(coder: NSCoder) {
 		guard let name = coder.decodeObject(forKey: kName) as? String else { return nil }	
		self.init(name: name)
 }
  
	func encode(with coder: NSCoder) {
		coder.encode(name, forKey: kName)
	}
}

And I got an error in line 12:
'self' used in property access 'kName' before 'self.init' call

Of course, I just find rules in the Swift Language Guide - Initialization :

Safety check 3
A convenience initializer must delegate to another initializer before assigning a value to any property (including properties defined by the same class).

Maybe I am stuck here because I should init() before I can access the property kName. But I need to get the name and assigning to the init(name:) after

Can someone tell me the best practice to solve it is?

Thanks!

Answered by OOPer in 620419022

Safety check 4
An initializer cannot call any instance methods, read the values of any instance properties, or refer to self as a value until after the first phase of initialization is complete.

You cannot access the instance propertykName before self.init(name:) is called.

One way is making it a class property.

Code Block class Person: NSObject, NSCoding {
		
		var name: String
		static let kName = "Name"
		
		init(name: String) {
				self.name = name
				super.init()
		}
		
		required convenience init?(coder: NSCoder) {
				guard let name = coder.decodeObject(forKey: Person.kName) as? String else { return nil }
				self.init(name: name)
		}
		
		func encode(with coder: NSCoder) {
				coder.encode(name, forKey: Person.kName)
		}
}

Boost

Answer 1

OOPer OP

Jul ’20

Accepted Answer

Safety check 4
An initializer cannot call any instance methods, read the values of any instance properties, or refer to self as a value until after the first phase of initialization is complete.

You cannot access the instance propertykName before self.init(name:) is called.

One way is making it a class property.

Code Block class Person: NSObject, NSCoding {
		
		var name: String
		static let kName = "Name"
		
		init(name: String) {
				self.name = name
				super.init()
		}
		
		required convenience init?(coder: NSCoder) {
				guard let name = coder.decodeObject(forKey: Person.kName) as? String else { return nil }
				self.init(name: name)
		}
		
		func encode(with coder: NSCoder) {
				coder.encode(name, forKey: Person.kName)
		}
}

1

Answer 2

Claude31 OP

Jul ’20

Or you could use the String directly

Code Block 		required convenience init?(coder: NSCoder) {
				 guard let name = coder.decodeObject(forKey: "Name") as? String else { return nil }
				self.init(name: name)
 }

1

Answer 3

reepic OP

Jul ’20

@Claude31
I have also considered this solution, but I think it's better to set the kName as a constant so that it can be reused

That's great.! So many thanks to you @OOPer

0