Effect observation

React to derived state changes with Aurelia's effect observation APIs.

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:

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.

Properties decorated with @observable and any proxy based property accesses will be tracked as dependencies of the effect

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:

import { IObservation } from 'aurelia';

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

• Getting through injection:

import { inject, IObservation } from 'aurelia';

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

Or

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:

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:

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:

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:

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:

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:

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

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

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 }) })
    });
  }
}