Xcode playgrounds can't access swift files in Sources folder

You have to add public access attribute to your classes, methods and properties in source folder to make them accessible from main playground file as they treated as separate module by compiler

Playgrounds are good for running tests. Put all your code in the Sources directory and have one publicly accessible 'test' class, for each test. Then run the publicly accessible tests from the playground.

playground

Test1.run()
Testx.run()
...

Sources/Test1.swift

public class Test1 {      
  public static func run() {
    let my_class = MyClass()
    let result = my_class.do_something()
    print(result)
  }
}

Sources/MyClass.swift

class MyClass {
  func do_something() -> String {
    return "lol"
  }
}

As mentioned when you create .swift files in Source folder they are automatically available to your playground code. To control the access of different parts of this file you can use Access level modifiers which are: public, internal & private.

According to Swift programming language access control

The default access level in most cases is internal which accessible inside the module, but not the outside.

In other words, if you declare a class without access modifier you can access it from another file in Source folder but NOT in your playground's main file. in other hand if you declare a class with public modifier you can access it in both cases.

for practical usage: let's make an Singleton implementation First: I create a new file in Source folder named 'Singy.swift' with following code:

public class Singy {
    public var name = ""
    private static var instance: Singy?
    private init() {}

    public static func getSingy() -> Singy {
        if Singy.instance == nil {
            Singy.instance = Singy()
        }
        return Singy.instance!
    }
}

Second: from my playground

var s1 = Singy.getSingy()
var s2 = Singy.getSingy()
s1.name = "One"
print(s2.name)

Both s1 and s2 reference the same instance, but it created only within the class