在swift 里引用循环的造成，主要有两个方法，

1.类之间的引用

2.clourse 闭包引用

解决办法：

weak references and unowned references. 也就是weak，unowned

weak,来修饰可选的，? 结尾的，如：var weak city: String ?

unowned 来修身 一直存在的。 var unowned city: String

• class Person {
  	letname: String
  	init(name:String) { self.name =name }
  	var apartment: Apartment?
  	deinit { println("\(name) is being deinitialized") }
  }
  class Apartment {
  	letnumber: Int
  	init(number:Int) { self.number =number }
  	weak var tenant: Person?
  	deinit { println("Apartment #\(number) is being deinitialized") }
  }

  weak 避免循环引用的例子

class Customer {
	letname: String
	var card: CreditCard?
	init(name:String) {
	self.name = name
	}
	deinit { println("\(name) is being deinitialized") }
}
class CreditCard {
	let number: UInt64
	unowned let customer: Customer
	init(number:UInt64,customer: Customer) {
		self.number = number
		self.customer = customer
	}
	deinit { println("Card #\(number) is being deinitialized") }
}

lazyvar someClosure: (Int,String) -> String = {
[unownedself,weak delegate = self.delegate!] (index:Int,stringToProcess: String) -> String in
// closure body goes here
}

详情请看apple swift 文档