Infinite loop didSet

didSetLoop.swift

protocol AProtocol {
    var someString: String { get }
}

class A: AProtocol {
    var someString: String
    init() {
        someString = "A"
    }
}

class B {
    var myProperty: AProtocol! {
        didSet {
            self.doSomething()
        }
    }

    func doSomething() {
        NSLog(self.myProperty.someString)
    }
}

let myB = B()
myB.myProperty = A() // infinite recursion


protocol AProtocol: class {
    var someString: String { get }
}

class A: AProtocol {
    var someString: String
    init() {
        someString = "A"
    }
}

class B {
    var myProperty: AProtocol! {
        didSet {
            self.doSomething()
        }
    }

    func doSomething() {
        NSLog(self.myProperty.someString)
    }
}

let myB = B()
myB.myProperty = A() // now it works

Why there is no issue like infinite loop in didSet in my code?
My Xcode version is Version 7.2 (7C68)