Dart Closures

A closure is a function that captures variables from the scope where it was created. The function keeps access to those variables after the outer code has finished.

A counter factory

makeCounter builds a function that remembers count. The first counter reaches 2, and a second counter starts again at 1.

int Function() makeCounter() {
  var count = 0;
  return () {
    count++;
    return count;
  };
}

void main() {
  var next = makeCounter();
  print(next());
  print(next());
  var fresh = makeCounter();
  print(fresh());
}

Seeing a later change

The closure reads message when it runs, not when it was made. Changing message between the two calls changes the second print.

void main() {
  var message = 'start';
  var show = () {
    print(message);
  };
  show();
  message = 'updated';
  show();
}

One capture per loop turn

Each turn of the loop builds a closure around that turn value of i. The stored closures print 0, 1, and 2 in order.

void main() {
  var readers = <int Function()>[];
  for (var i = 0; i < 3; i++) {
    readers.add(() => i);
  }
  print(readers[0]());
  print(readers[1]());
  print(readers[2]());
}

Example

int Function(int) makeTotal() {
  var total = 0;
  return (int amount) {
    total += amount;
    return total;
  };
}

void main() {
  var add = makeTotal();
  print(add(5));
  print(add(3));
  print(add(10));
}

Key points

A closure remembers the variables it captures, not just copies of their values.
Each call to a factory function creates a fresh set of captured variables.
Two closures made by two calls share nothing with each other.
A change to a captured variable after creation is visible when the closure runs later.
A closure built in a loop captures the value from its own iteration.

In this example, makeTotal returns a closure that captures total. Each call adds its amount to the same captured total. The prints show the running total growing from 5 to 8 to 18.

Try it Yourself

Edit the code, then press Run.

Running is simulated · DartPad

int Function(int) makeTotal() {
  var total = 0;
  return (int amount) {
    total += amount;
    return total;
  };
}

void main() {
  var add = makeTotal();
  print(add(5));
  print(add(3));
  print(add(10));
}

Exercise

A counter factory is called twice, making counter A and counter B. A runs twice and B runs once. What does the call to B print?

3 1 2 0