Making a shared class in swift - swift

Question

In swift, is there anyway I can create a shared class? That is, say there is a class called Value:
class Value{
var a = 0
}
I want to use this class to be shared among different object instances.
For example,
class Main{
func main(){
print(Value.a)
Value.a++
}
}
class OtherClass{
func otherMain(){
print(Value.a)
Value.a++
}
}
let main = Main()
let other = OtherClass()
//I want this call to print 0
main.main()
//I want this call to print 1
other.otherMain()
I tried static on var a, but then it won't let me do arithmetics such as addition... But it is working if I change the Value into static instead of class. I thought only difference between struct and class was either variable type or reference type...
I'll appreciate to any help. Thanks

Answer 1

The best way to go about what you want to do, unfortunately, is not to do it at all. It seems that the architecture you're designing is quite fragile and two independent classes shouldn't depend on a shared object state and if they do, they should be related via class inheritance or protocols.
A better solution would be:
class MyClass {
static var a = 0
}
class SubclassA: MyClass {
func someFunc() {
print(self.dynamicType.a)
self.dynamicType.a += 1
}
}
class SubclassB: MyClass {
func otherFunc() {
print(self.dynamicType.a)
}
}
There's a few reasons you should do it this way, over your previous solution:
Your shared state can stay private to those two classes. No one outside of those classes can or should modify that value.
Singleton patterns while sometimes useful, have a bunch of problems. This article explains it better than I can.
You have to think of your application and your code architecture as a state machine, moving from one state to another. The goal is to write code that's reusable, modular, generic, and has as little state as possible. The more stateless your code, the better. That's the whole idea behind platforms like ReactJS and ReactiveCocoa; they help simplify that application state.
What you might be looking for is data persistence, something like Realm or CoreData, where you persist data to disk and then you can read it back at a later time as needed. Imagine for example that instead of an Int your shared state was an array of UIImages. With the singleton pattern, you're keeping those images in memory at all times. What if the list grows to be 200 or 300 in length? Then you have memory issues.