Generic assignment in Swift

Question

I have this method:

class BottomPickerPanelView<T: Equatable>: UIView, UIPickerViewDelegate, UIPickerViewDataSource {

private var data: [String: T]? = nil

public static func create<T: Equatable>(controller: UIViewController, values: [String: T]?) -> BottomPickerPanelView {

                let view = Bundle.main.loadNibNamed("BottomPickerPanelView", owner: controller, options: nil)!.first as! BottomPickerPanelView

                view.isHidden = true

                view.pickerView.delegate = view
                view.pickerView.dataSource = view

                view.parentViewController = controller
                view.data = values

                return view
            }
    }

where 'view.data' is of type [String: T]? and T is, of course, a generic type. As you can see 'values' and 'data' are of the same type, but I get the following error: Cannot assign value of type '[String : T]?' to type '[String : _]?'.

Answer 1

When you use BottomPickerPanelView without <T>, the compiler can't know what the generic type is. Add <T> to the return type and the cast and the assignment works:

class BottomPickerPanelView<T: Equatable>: UIView, UIPickerViewDelegate, UIPickerViewDataSource {
    private var data: [String: T]? = nil

    public static func create<T: Equatable>(controller: UIViewController, values: [String: T]?) 
        -> BottomPickerPanelView<T> {
        let view = Bundle.main.loadNibNamed("BottomPickerPanelView", owner: controller, options: nil)!.first 
            as! BottomPickerPanelView<T>

        view.isHidden = true

        view.pickerView.delegate = view
        view.pickerView.dataSource = view

        view.parentViewController = controller
        view.data = values

        return view
    }
}