Please, can someone explain why I can add another eventArray if I define the instance (t2Array) in onAppear while, if I declare it as @State (t3Array) at the beginning of the struct, I get the error: Cannot use mutating member on immutable value: ‘self’ is immutable

Struct for both is:

struct T2 {
var name : String
var eventArray : [String]  }

The following works fine in “onAppear”:

 var t2Array = [T2(name: "T2", eventArray: ["event1"])]
 t2Array[0].eventArray.append("event2")
 

If I do the following, I get the error

 @State var t3Array:[T2]?

and in “onAppear” the error is at the append of event2:

  t3Array = [T2(name: "T2", eventArray: ["event1"])]
   self.t3Array[0].eventArray.append("event2"). //<<<-- error marked here