previousValue with computed select

[PaulHalliday]

Is there a way to generate a previousValue for a select statement? I'm looking to optimise for performance on an object with many properties that updates at 60hz, so would be interested in having previousValue/initialValue on select slices.

For example:

class Data {
  final int resultA;
  final int resultB;

  Data({this.resultA = 0, this.resultB = 0});

  Data copyWith({int? resultA, int? resultB}) {
    return Data(
      resultA: resultA ?? this.resultA,
      resultB: resultB ?? this.resultB,
    );
  }
}

void main() {
  final a = signal<Data>(Data(resultA: 1, resultB: 2));
  final selected = a.select((s) => s.value.resultA);
  final c = computed(() => selected.value);

  a.value = Data(resultA: 2, resultB: 3);
  a.value = a.value.copyWith(resultA: 3);

  // Not defined as it's a ReadonlySignal
  print(selected.previousValue);

  // Creating a computed for `previousValue` doesn't seem ideal, but it's null either way at this point
  print(c.previousValue);
}

Perhaps I'm thinking about this wrong/overthinking it, but the idea was to:

1. Listen to a specific slice of a signal (in this case resultA)
2. previousValue does not exist on ReadonlySignal so I have to create a computed value from that slice to access previousValue.

[rodydavis]

There is a PR I am about to merge that will fix this!

[rodydavis]

Fixed on 5.0.0!

[PaulHalliday]

🙌

Is this live in beta-3? As the select extension is typed as a ReadonlySignal which doesn't inherently have a previousValue, attempting to use the select statement on its own to resolve the above doesn't work.

/// Select a value and return a computed signal to listen for changes
///
/// ```dart
/// final a = signal({'a': 1, 'b': 2});
/// final b = a.select((val) => val()['a'] as int);
/// ```
ReadonlySignal<R> select<R>(
  R Function(ReadonlySignal<T>) selector, {
  bool autoDispose = false,
  String? debugLabel,
}) {
  return computed(
    () => selector(this),
    autoDispose: autoDispose,
    debugLabel: debugLabel,
  );
}

Would an additional extension API such as this suffice here?

/// Select a value from the `ReadonlySignal` and return a computed signal to listen for changes
///
/// ```dart
/// final a = signal({'a': 1, 'b': 2});
/// final b = a.selectWithPrevious((val) => val.value['a'] as int);
/// ```
Computed<R> selectWithPrevious<R>(
 R Function(ReadonlySignal<T>) selector, {
 bool autoDispose = false,
 String? debugLabel,
}) {
 return computed(() => select(
       selector,
       autoDispose: autoDispose,
       debugLabel: debugLabel,
     ).value);
}

Thanks for your thoughts on this.

[rodydavis]

I just added it on the latest beta version. 🚀

Didn't realize it was not on read only signal.

Extension should not be needed now!

[PaulHalliday]

Ayy! Much appreciated.

Replaced my extension with select and all my tests still pass.

Cheers!