Type Properties & Type Methods.swift

class Foo {
    var name: String?           // instance property
    
    /**
    You define type properties with the static keyword. 
    For computed type properties for class types, 
    you can use the class keyword instead to allow subclasses to override the superclass’s implementation. 
    The example below shows the syntax for stored and computed type properties
    */
    
    static var all = [Foo]()    // static type property
    
    /*
     For type property
     you must always give stored type properties a default value.
     This is because the type itself does not have an initializer that 
     can assign a value to a stored type property at initialization time.
     
     Stored type properties are lazily initialized on their first access.
     They are guaranteed to be initialized only once, even when accessed by multiple threads simultaneously, 
     and they do not need to be marked with the lazy modifier.
     
    */
    
    class var comp: Int {       // computed type property , always need default Value
        return 22
    }

    class func alert() {        // type method
        print("There are \(all.count) foos")
    }
}

Foo.alert()       // There are 0 foos
let f = Foo()
Foo.all.append(f)
Foo.alert()       // There are 1 foos