Why does line 19 trigger didSet on stringProto?

DeathByDidSet.swift

protocol StringProto {
    var string: String? { get set }
}

class StringClass : StringProto {
	var string : String? = ""
}

class SetterTest {
	var stringProto: StringProto? {
        didSet {
			println("didSet")
			self.initStringVal()
        }
	}
	
	func initStringVal() {
		println("initStringVal")
		test.stringProto?.string = "init value"
	}
}

var test = SetterTest()

test.stringProto = StringClass()

And the answer is to change line 2 to: var string: String? { get nonmutating set } as the Protocol doesn't know if it's dealing with a value type.

Also, can have StringProto inherit from class

protocol StringProto: inherit class {
    var string: String? { get set }
}