---
title: Effect API docs
description: All the API docs of Effect
---

# Effect API docs

Signals are trackable values, but they are only one half of the equation.
To complement those are observers that can be updated by those trackable values. An effect is one such observer; it runs a side effect that depends on signals.

An effect can be created by using `Effect`.
The effect subscribes automatically to any signal used in the callback and reruns when any of them change.

## Constructor

```dart
Effect(
  void Function(DisposeEffect dispose) callback, {
    ErrorCallback? onError,
    EffectOptions options = const EffectOptions(),
  }
);
```

`callback` is the function used to run the effect and tracks all the signals used.

`onError` is an optional callback that is called when the effect throws an error.
By default the effect error is thrown but with `onError` you can gracefully handle it.

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

```dart
final count = Signal(1);
Effect((_) {
  print('The count is ${count.value}');
});
// prints "The count is 1"
count.value++;
// prints "The count is 2"
```

<Info>An effect runs immediately when created.</Info>

## `bool get disposed`

Returns `true` if the effect has been disposed.

## `void dispose()`

Disposes the effect.
If the effect has already been disposed, this method does nothing.

Removes the subscriber from all the tracked signals.

```dart
final count = Signal(0);
final effect = Effect((_) {
  print("The count is ${count.value}");
});
effect.dispose(); // disposes the effect
```

## `void call()`

Disposes the effect, like above.

```dart
final count = Signal(0);
final disposeEffect = Effect((_) {
  print("The count is ${count.value}");
});
disposeEffect(); // disposes the effect
```

---

> An`Effect` only tracks the __current__ dependencies.

```dart
final name = Signal("John");
final lastName = Signal("Doe");
final showFullName = Signal(false);

Effect((_) {
  if (showFullName()) {
    print("${name.value} ${lastName.value}");
  } else {
    print(name.value);
  }
});
```

In the case above the Effect prints `John` because `showFullName` is `false`.
If you try to change the `lastName` to `Smith`, the effect won't react because is not tracking `lastName`.

If you set `showFullName` to `true`, the effect will print `John Doe` and will track all the signals: `showFullName`, `name` and `lastName`.

<Warning>Use conditional logic carefully when using `Effect`.</Warning>