Full implementation

AssociatedObjects.swift

private var AssociatedKey: UInt = 0

private final class AssociatedObjectBox<T> {
    let value: T
    
    init(_ x: T) {
        value = x
    }
}

private func lift<T>(x: T) -> AssociatedObjectBox<T> {
    return AssociatedObjectBox(x)
}

private func setAssociatedObject<T>(object: AnyObject, value: T, key: UnsafePointer<Void>, policy: objc_AssociationPolicy) {
    if let v: AnyObject = value as? AnyObject {
        objc_setAssociatedObject(object, key, v,  policy)
    } else {
        objc_setAssociatedObject(object, key, lift(value), policy)
    }
}

private func getAssociatedObject<T>(object: AnyObject, key: UnsafePointer<Void>) -> T? {
    if let v = objc_getAssociatedObject(object, key) as? T {
        return v
    } else if let v = objc_getAssociatedObject(object, key) as? AssociatedObjectBox<T> {
        return v.value
    } else { 
        return nil 
    }
}

MyViewController.swift

public final class MyViewController: UIViewController {
    // Your implementation
}

extension MyViewController: ViewModelBindable {
    
    public func bindViewModel(viewModel: MyViewModel) {
        // binding logic
    }

}

ViewModelBindable.swift

public protocol ViewModelBindable: class {

    typealias ViewModel

    var viewModel: ViewModel? { get set }

    func bindViewModel(viewModel: ViewModel)

    func unbindViewModel(viewModel: ViewModel)

    func shouldBindViewModel() -> Bool

}

ViewModelBindableExtension.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)
    }
    
}

I have problems implementing it