Extension for ensuring one instance of a class is live at any one point

NSObject+MemoryLeak.swift

extension NSObject {
  
  // Call this when the class being debugged is initialized
  func debug_ensureOnlyOneInstanceSetup() {
    let type = self.dynamicType
    let currentCreatedCount = createdCount(type)
    assert(currentCreatedCount == 0, "More that 1 \(self) created, potential memory leak")
    setCreateCount(currentCreatedCount + 1, classType: type)
  }
  
  // Call this when the class being debugged is deinitialized
  func debug_ensureOnlyOneInstanceTeardown() {
    let type = self.dynamicType
    setCreateCount(0, classType: type)
  }
  
  private struct AssociatedKey {
    static var key = "key"
  }
  
  private func setCreateCount(count: Int, classType: AnyObject) {
    let intNumber = NSNumber(integer: count)
    objc_setAssociatedObject(classType, &AssociatedKey.key, intNumber,  objc_AssociationPolicy.OBJC_ASSOCIATION_RETAIN_NONATOMIC)
  }
  
  private func createdCount(classType: AnyObject) -> Int {
    let intNumber =  objc_getAssociatedObject(classType, &AssociatedKey.key) as? NSNumber
    return intNumber?.integerValue ?? 0
  }
  
}