Type alias example

playground.swift

//: Playground - noun: a place where people can play

struct Foo<T> {

    typealias FooHandler = (data: T) -> Void

    let bar: T
    var handler: FooHandler?

    init(bar: T) {
        self.bar = bar
    }

    func doSomething() {
        handler?(data: bar)
    }
}

var baz = Foo(bar: 1)
baz.handler = { data in
    print(data) // Prints 1
}
baz.doSomething()

var foobarbaz = Foo(bar: "🍌")
foobarbaz.handler = { data in
    print(data) // Prints "banana"
}
foobarbaz.doSomething()