Swift 扩展的访问控制

在Swift中，我们可以使用扩展（extension）来为现有的类、结构体、枚举或协议添加新的功能。扩展是一种强大的工具，它可以让我们在不修改原始代码的情况下，为已有的类型添加新的功能或者修改现有的功能。

swift
public class MyClass {
    private var privateProperty: Int = 0
    fileprivate func fileprivateMethod() {
        print("This is a fileprivate method.")
    }
}
extension MyClass {
    public func publicMethod() {
        print("This is a public method.")
        fileprivateMethod()
    }
}

swift
internal struct MyStruct {
    var internalProperty: String = ""
}
extension MyStruct {
    internal func internalMethod() {
        print("This is an internal method.")
    }
}

swift
fileprivate extension String {
    fileprivate func fileprivateMethod() {
        print("This is a fileprivate method.")
    }
}
let myString = "Hello, World!"
myString.fileprivateMethod()

swift
private class MyClass {
    private func privateMethod() {
        print("This is a private method.")
    }
}
extension MyClass {
    private func anotherPrivateMethod() {
        print("This is another private method.")
        privateMethod()
    }
}