Swift Protocol - Property type subclass
The problem is with the setter in the protocol.
Let's say you want to GET the panelView from LeftPanelController. That's fine, because LeftPanelView can do everything PanelView can do (and more).
If you want to SET the panelView of LeftPanelController though, you can give it any PanelView. Because you're defining the panelView variable as a LeftPanelView, the setter could sometimes fail.
To fix this, you could do the following in LeftPanelController:
var panelView: PanelView = LeftPanelView()The implication of this is that you won't be able to access any methods or properties that are specific to LeftPanelView without casting it first. If that's not an issue, then this should fix your problem!