ViewModelBindable.swift

extension ViewModelBindable where Self: AnyObject {
    
    public var viewModel: ViewModel? {

        get {
            return getAssociatedObject(self, key: &AssociatedKey)
        }

        set(newViewModel) {
            if let viewModel = newViewModel {
               setAssociatedObject(self, value: viewModel, key:
               &AssociatedKey, policy:
           objc_AssociationPolicy.OBJC_ASSOCIATION_RETAIN_NONATOMIC)

            if shouldBindViewModel() {
                    registerBinding(viewModel)
                }
            }
        }
    }
    
    private func registerUnbinding(viewModel: ViewModel) {
        unbindViewModel(viewModel)
    }

    private func registerBinding(viewModel: ViewModel) {
        bindViewModel(viewModel)
    }

}