---
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, {Duration? timeout})`

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

The optional __timeout__ parameter specifies the maximum time to wait for the condition to be met. If provided and the timeout is reached before the condition is met, the future will complete with a __TimeoutException__.

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

// With timeout - will throw TimeoutException if condition not met in 5 seconds
await count.until(
  (value) => value >= 10, 
  timeout: Duration(seconds: 5),
);
```

## `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.