---
title: ReadSignal API docs
description: All the API docs of ReadSignal, a read-only signal
---

# ReadSignal API docs

A read-only __Signal__.

All [Computed](/api-docs/computed) signals are __ReadSignal__s.

If you want to convert a __Signal__ into a __ReadSignal__ use the [toReadSignal](/api-docs/signal#toreadsignal) method

## Constructor

```dart
ReadSignal(
  T initialValue, {
  SignalOptions<T>? options,
});
```

`initialValue` is the initial value of the signal.

`options` are the [options](/api-docs/signal-options) of the signal.

## `T get value`

Returns the current signal value.

For example:
```dart
final count = Signal(0);
print(count.value); // prints 0
```

## `T call()`

Returns the current signal value.

For example:
```dart
final count = Signal(0);
print(count()); // prints 0
```

## `bool get hasPreviousValue`

Returns `true` if the signal has a previous value.

For example:
```dart
final count = Signal(0);
print(count.hasPreviousValue); // prints false
count.value++;
print(count.hasPreviousValue); // prints true
```

## `T? get previousValue`

Returns the previous value, or `null` if the signal has no previous value.

For example:
```dart
final count = Signal(0);
print(count.previousValue); // prints null
count.value++;
print(count.previousValue); // prints 0
```

## `DisposeObservation observe(ObserveCallback listener, {bool fireImmediately = false})`

Observe the signal and trigger the __listener__ every time the value changes with the previous and current values.

For example:

```dart
final count = Signal(0);
count.observe((previousValue, value) {
  print("The count changed from $previousValue to $value");
});
count.value++;
// prints "The count changed from 0 to 1"
```

The observation will not be fired when created with the current value, if you want to run it at creation, set `fireImmediately` to `true`, for example:

```dart
final count = Signal(0);
count.observe((previousValue, value) {
  print("The count changed from $previousValue to $value");
}, fireImmediately: true);
// prints "The count changed from null to 0"
```

## `FutureOr<T> until(bool Function(T value) condition)`

Returns the future that completes when the __condition__ evalutes to true.
If the __condition__ is already true, it completes immediately.

For example:
```dart
final count = Signal(0);
await count.until((value) => value >= 10); // await until value >= 10
```

## `bool get disposed`

Returns `true` if the signal has been disposed.

## `void dispose()`

Disposes the signal.
If the signal has already been disposed, this method does nothing.
All observers will be unsubscribed.

## `String toString()`

Returns a string representation of the signal.
For example:
```dart
final count = Signal(0);
count.value++;
print(count.toString());
// prints "ReadSignal<int>(value: 1, previousValue: 0, options: ...)"
```

## `toValueNotifier()`

Converts this __ReadSignal__ into a __ValueNotifier__.

If you need to convert a __ValueNotifier__ into a __ReadSignal__ use `toSignal()` instead.