# Effect observation

Aurelia provides a higher-level API for simplifying some common tasks to handle a common reactivity intent in any application: run a function again when any of its dependencies have been changed.

This function is called an effect, and the dependencies are typically tracked when they are accessed (read) inside this effect function. The builtin `@observable` decorator from Aurelia enables this track-on-read capability by default.

Aurelia provides a few ways to declare a dependency for an effect function. The most common one is the track "on read" of a reactive property.

**In the following example:**

```typescript
class MouseTracker {
  @observable
  coord = [0, 0];
}
```

The property `coord` of a `MouseTracker` instance will be turned into a reactive property and is also aware of effect function dependency tracking.

{% hint style="info" %}
Properties decorated with `@observable` and any proxy based property accesses will be tracked as dependencies of the effect
{% endhint %}

The effect APIs are provided via the default implementation of the interface `IObservation`, which can be retrieved like one of the following examples:

* **Getting from a container directly**:

```typescript
import { IObservation } from 'aurelia';

...
const observation = someContainer.get(IObservation);
```

* **Getting through injection**:

```typescript
import { inject, IObservation } from 'aurelia';

@inject(IObservation)
class MyElement {
  constructor(observation) {
    // ...
  }
}
```

Or

```typescript
import { resolve, IObservation } from 'aurelia';

class MyElement {
  constructor(readonly observation = resolve(IObservation)) {
    // ...
  }
}
```

After getting the observation object, there are two APIs that can be used to created effects as described in the following sections:

## Run effect

Run effects describe a function to be called repeatedly whenever any dependency tracked inside it changes.

### Creating an Effect

After getting an `IObservation` instance, a run effect can be created via the method `run` of it:

```typescript
const effect = observation.run(() => {
  // code here
});
```

Note that the effect function will be run immediately.

By default, a effect is independent of any application lifecycle, which means it does not stop when the application that owns the `observation` instance has stopped. To stop/destroy an effect, call the method `stop()` on the effect object:

```typescript
const effect = observation.run(() => {
  // code here
});

// stop the effect like this
effect.stop();
```

## Cleaning up effect

Sometimes it's desirable to cleanup an effect, i.e we only want to send a request to track the last mouse position within 100ms, not all the intermediate movements. Setting a timeout in effect body and removing the timeout in the cleanup function is a simple way to achieve this, like the following example:

```ts
const effect = observation.run(() => {
  // assume mouse tracker is an observable property
  const { x, y } = app.mouseTracker;
  const id = setTimeout(() => {
    fetch(`/user/mousemove?x=${x}&y=${y}`);
  }, 100);
  return () => clearTimeout(id);
})
```

## Watching a value with a getter

Watch is a way to describe a getter based observation of an object. The below example demos how to create a watch effect:

```typescript
import { inject, IObservation } from 'aurelia';

@inject(IObservation)
class PersonalInfo {
  constructor(observation) {
    const effect = observation.watch(this.primaryInfo, (primaryInfo) => primaryInfo.name, function nameChanged(newName, oldName) {
      // do something with name
    });

    // effect.stop() later when necessary
  }
}
```

Note that the effect function will be run immediately. If you do not want to run the callback immediately, pass an option `immediate: false` as the 4th parameter:

```typescript
observation.watch(obj, getter, callback, { immediate: false });
```

By default, a watch effect is independent of any application lifecycle, which means it does not stop when the application that owns the `observation` instance has stopped. To stop an effect, call the method `stop()` on the effect object.

## Watching a value with a string as expression

Instead of having getter as a watch expression, a string can also be used, like the following example:

```typescript
import { inject, IObservation } from 'aurelia';

@inject(IObservation)
class PersonalInfo {
  constructor(observation) {
    this.person = { name: 'bob', address: { code: '1234' } };
    const effect = observation.watch(this.person, 'address.code', function nameChanged(newCode, oldCode) {
      // do something with code
    });

    // effect.stop() later when necessary
  }
}
```

## Effect examples

The following section gives some examples of what it looks like when combining `@observable` and run effect.

### Creating a run effect that logs the user mouse movement on the document

```typescript
import { resolve, IObservation, observable } from 'aurelia'

class MouseTracker {
  @observable coord = [0, 0]; // x: 0, y: 0 is the default value
}

// Inside an application:
class App {
  constructor() {
    const observation = resolve(IObservation);
    const mouseTracker = new MouseTracker();

    document.addEventListener('mousemove', (e) => {
      mouseTracker.coord = [e.pageX, e.pageY];
    });

    observation.run(() => {
      console.log(mouseTracker.coord);
    });
  }
}
```

Now whenever the user moves the mouse around, a log will be added to the console with the coordinate of the mouse.

### Creating a run effect that sends a request whenever user focus/unfocus the browser tab

```typescript
import { resolve, IObservation, observable } from 'aurelia'

class PageActivity {
  @observable active = false
}

class App {
  constructor() {
    const observation = resolve(IObservation)
    const pageActivity = new PageActivity();

    document.addEventListener(visibilityChange, (e) => {
      pageActivity.active = !document.hidden;
    });

    observation.run(() => {
      fetch('my-game/user-activity', { body: JSON.stringify({ active: pageActivity.active }) })
    });
  }
}
```


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