Computed

A computed is a signal that depends on other signals. To create a computed, you have to use the Computed class.

A Computed automatically subscribes to any signal provided and reruns when any of them change.

final count = Signal(0);
final doubleCount = Computed(() => count.value * 2);

Effect(() {
  print('The counter is ${count.value}');
  print('The double counter is ${doubleCount.value}');
});

count
  ..value = 1
  ..value = 2;

// The output will be:
// The counter is 0
// The double counter is 0
// The counter is 1
// The double counter is 2
// The counter is 2
// The double counter is 4

Or you may want to subscribe only to a sub-field of a Signal value.

// sample User class
class User {
  const User({
    required this.name,
    required this.age,
  });

  final String name;
  final int age;

  User copyWith({
    String? name,
    int? age,
  }) {
    return User(
      name: name ?? this.name,
      age: age ?? this.age,
    );
  }
}

// create a user signal
final user = Signal(const User(name: "name", age: 20));

// create a derived signal just for the age
final age = Computed(() => user().age);

// adding an effect to print the age
Effect(() {
  print('age changed from ${age.previousValue} into ${age.value}');
});

// just update the name, the effect above doesn't run because the age has not changed
user.updateValue((value) => value.copyWith(name: 'new-name'));

// just update the age, the effect above prints
user.updateValue((value) => value.copyWith(age: 21));

A derived signal is not of type Signal but is a ReadSignal. The difference with a normal Signal is that a ReadSignal doesn’t have a value setter, in other words it’s a read-only signal.

With a Computed you can also transform the value type:

final counter = Signal(0); // type: int
final isGreaterThan5 = Computed(() => counter() > 5); // type: bool

isGreaterThan5 will update only when the counter value becomes lower/greater than 5.

• If the counter value is 0, isGreaterThan5 is equal to false.
• If you update the value to 1, isGreaterThan5 doesn’t emit a new value, but still contains false.
• If you update the value to 6, isGreaterThan5 emits a new true value.