Swift override instance variables

Question

I know this answer has already been posted in some other form here, but I'd like to understand more about overriding instance variables in swift.

Let's say I have this code

class BaseView:UIView{  let someVariable:Int = 1  // do some work with someVariable }  class ExtendedView:BaseView{  let someVariable:Int = 2 } 

Ok. From what I read, the constant requires an override prefix. Other answers said that I should declare the setter and getter? Why? I really don't care about those two. I just need the value replaced. I can't really use the init override because I'm inheriting from UIView and this might be quite dangerous (i think).

Any suggestions are welcomed.

Answer 1

As you say, you cannot simply redefine a constant in a subclass (it is a constant, after all). The error you get is "Cannot override with a stored property". It does appear to be possible to override a var, however, when I change the let someVariable to var someVariable I get "ambiguous use of 'someVariable'" when I access it in the subclass (note - same thing happens whether I use override or not).

The simplest solution is to use a getter. This is really a function, so you can happily override it, the backing variable will be managed for you, and if you don't supply a setter ... it will be constant for each class:

class BaseView: UIView {     var someVariable: Int { get { return 1 } }     // do some work with someVariable }  class ExtendedView: BaseView {     override var someVariable: Int { get { return 2 } } }  let a = BaseView() a.someVariable // 1 let b = ExtendedView() b.someVariable // 2 

As commentator @user3633673 points out, if you only have a getter (and not a setter), you can drop the get, but I left it in for clarity of the principle. Here's the same without it...

class BaseView: UIView {     var someVariable: Int { return 1 }     // do some work with someVariable }  class ExtendedView: BaseView {     override var someVariable: Int { return 2 } }  let a = BaseView() a.someVariable // 1 let b = ExtendedView() b.someVariable // 2 

... and, of course, in Swift 5, you can drop the return:

class BaseView: UIView {     var someVariable: Int { 1 } }  class ExtendedView: BaseView {     override var someVariable: Int { 2 } }