How to do it...

To implement a closure in Dart, follow these steps:

1. To add a closure to a function, you have to essentially define another function signature inside a function:
   

void callbackExample(void callback(String value)) {
  callback('Hello Callback');
}

2. Defining closures inline can get quite verbose. To simplify this, Dart uses the typedef keyword to create a custom type alias that will represent the closure. Let's create a typedef called NumberGetter, which will be a function that returns an integer:

typedef NumberGetter = int Function();

3. The following function will take in a NumberGetter as its parameter and invoke it in its function:

int powerOfTwo(NumberGetter getter) {
 return getter() * getter();
}

4. Let's put this all together with a function that will use all these closure examples:

void consumeClosure() {
 final getFour = () => 4;
 final squared = powerOfTwo(getFour);
 print(squared);

 callbackExample((result) {
 print(result);
 });
}

5. Finally, add an invocation to consumeClosure at the top of the playground method or in your main method:

  consumeClosure();