Using closures as callbacks

Functions and closures can capture the current scope, which means all of the declared variables outside of the function or closure definition, such as local variables or self. In the case of self, you can inadvertently extended the lifetime of your objects and leak memory:

class MyClass {
    var running = false
    func run() {
        running = true
        DispatchQueue.main.asyncAfter(deadline: .now() + 10) {
            self.running = false
        }
    }
}

var instance: MyClass? = MyClass()
instance?.run()
instance = nil

Can you spot the potential issue in this code?

Depending on the use case, you may want instance to be destroyed when it is not referenced by any owner. In our case, we'll probably cause a memory leak, as the dispatch block is referencing self without any memory management qualifier.