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A library for writing Ember modifiers
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README.md

ember-modifier

This addon is the next iteration of both ember-class-based-modifier and ember-functional-modifiers. Some breaking changes to the APIs have been made, for a list of difference, see the API differences section.

Huge thanks to @sukima and @spencer516 for their contributions! This project is based on their work, and wouldn't have been possible without them.

This addon provides an API for authoring element modifiers in Ember. It mirrors Ember's helper API, with a form for writing simple functional modifiers, and form for writing more complicated class modifiers.

Compatibility

  • Ember.js v3.4 or above
  • Ember CLI v2.13 or above
  • Node.js v8 or above

Installation

ember install ember-modifier

Usage

This addon does not provide any modifiers out of the box; instead, this library allows you to write your own. There are two ways to write modifiers:

  1. Functional modifiers
  2. Class-based modifiers
import Modifier, { modifier } from 'ember-modifier';

These are analogous to Ember's Helper APIs, helper and Helper.

Functional Modifiers

modifier is an API for writing simple modifiers. For instance, you could implement Ember's built-in {{on}} modifier like so with modifier:

// /app/modifiers/on.js
import { modifier } from 'ember-modifier';

export default modifier((element, [eventName, handler]) => {
  element.addEventListener(eventName, handler);

  return () => {
    element.removeEventListener(eventName, handler);
  }
});

Functional modifiers consist of a function that receives:

  1. The element
  2. An array of positional arguments
  3. An object of named arguments
modifier((element, positional, named) => { /* */ });

This function runs the first time when the element the modifier was applied to is inserted into the DOM, and it autotracks while running. Any values that it accesses will be tracked, including the arguments it receives, and if any of them changes, the function will run again.

The modifier can also optionally return a destructor. The destructor function will be run just before the next update, and when the element is being removed entirely. It should generally clean up the changes that the modifier made in the first place.

Generating a Functional Modifier

To create a modifier (and a corresponding integration test), run:

ember g modifier scroll-top

Example without Cleanup

For example, if you wanted to implement your own scrollTop modifier (similar to this), you may do something like this:

// app/modifiers/scroll-top.js
import { modifier } from 'ember-modifier';

export default modifier((element, [scrollPosition]) => {
  element.scrollTop = scrollPosition;
})
<div class="scroll-container" {{scroll-top @scrollPosition}}>
  {{yield}}
</div>

Example with Cleanup

If the functionality you add in the modifier needs to be torn down when the element is removed, you can return a function for the teardown method.

For example, if you wanted to have your elements dance randomly on the page using setInterval, but you wanted to make sure that was canceled when the element was removed, you could do:

// app/modifiers/move-randomly.js
import { modifier } from 'ember-modifier';

const { random, round } = Math;

export default makeFunctionalModifier(element => {
  const id = setInterval(() => {
    const top = round(random() * 500);
    const left = round(random() * 500);
    element.style.transform = `translate(${left}px, ${top}px)`;
  }, 1000);

  return () => clearInterval(id);
});
<button {{move-randomly}}>
  {{yield}}
</button>

Class Modifiers

Sometimes you may need to do something more complicated than what can be handled by functional modifiers. For instance:

  1. You may need to inject services and access them
  2. You may need fine-grained control of updates, either for performance or convenience reasons, and don't want to teardown the state of your modifier every time only to set it up again.
  3. You may need to store some local state within your modifier.

In these cases, you can use a class modifier instead. Here's how you would implement the {{on}} modifier with a class:

import Modifier from 'ember-modifier';

export default class OnModifier extends Modifier {
  event = null;
  handler = null;

  // methods for reuse
  addEventListener() {
    let [event, handler] = this.args.positional;

    // Store the current event and handler for when we need to remove them
    this.event = event;
    this.handler = handler;

    this.element.addEventListener(event, handler);
  }

  removeEventListener() {
    let [event, handler] = this;

    if (event && handler) {
      this.element.removeEventListener(event, handler);

      this.event = null;
      this.handler = null;
    }
  }

  // lifecycle hooks
  didReceiveArguments() {
    this.removeEventListener();
    this.addEventListener();
  }

  willRemove() {
    this.removeEventListener();
  }
}

This may seem more complicated than the functional version, but that complexity comes along with much more control.

As with functional modifiers, the lifecycle hooks of class modifiers are tracked. When they run, they any values they access will be added to the modifier, and the modifier will update if any of those values change.

Generating a Class Modifier

To create a modifier (and a corresponding integration test), run:

ember g modifier scroll-top --class

Example without Cleanup

For example, let's say you want to implement your own {{scroll-position}} modifier (similar to this).

This modifier can be attached to any element and accepts a single positional argument. When the element is inserted, and whenever the argument is updated, it will set the element's scrollTop property to the value of its argument.

// app/modifiers/scroll-position.js

import Modifier from 'ember-modifier';

export default class ScrollPositionModifier extends Modifier {
  get scrollPosition() {
    // get the first positional argument passed to the modifier
    //
    // {{scoll-position @someNumber relative=@someBoolean}}
    //                  ~~~~~~~~~~~
    //
    return this.args.positional[0];
  }

  get isRelative() {
    // get the named argument "relative" passed to the modifier
    //
    // {{scoll-position @someNumber relative=@someBoolean}}
    //                                       ~~~~~~~~~~~~
    //
    return this.args.named.relative
  }

  didReceiveArguments() {
    if(this.isRelative) {
      this.element.scrollTop += this.scrollPosition;
    } else {
      this.element.scrollTop = this.scrollPosition;
    }
  }
}

Usage:

{{!-- app/components/scroll-container.hbs --}}

<div
  class="scroll-container"
  style="width: 300px; heigh: 300px; overflow-y: scroll"
  {{scroll-position this.scrollPosition relative=false}}
>
  {{yield this.scrollToTop}}
</div>
// app/components/scroll-container.js

import Component from '@glimmer/component';
import { tracked } from '@glimmer/tracking';
import { action } from '@ember/object';

export default class ScrollContainerComponent extends Component {
  @tracked scrollPosition = 0;

  @action scrollToTop() {
    this.scrollPosition = 0;
  }
}
{{!-- app/templates/application.hbs --}}

<ScrollContainer as |scroll|>
  A lot of content...

  <button {{on "click" scroll}}>Back To Top</button>
</ScrollContainer>

Example with Cleanup

If the functionality you add in the modifier needs to be torn down when the modifier is removed, you can use the willRemove hook.

For example, if you want to have your elements dance randomly on the page using setInterval, but you wanted to make sure that was canceled when the modifier was removed, you could do this:

// app/modifiers/move-randomly.js

import { action } from '@ember/object';
import Modifier from 'ember-modifier';

const { random, round } = Math;
const DEFAULT_DELAY = 1000;

export default class MoveRandomlyModifier extends Modifier {
  setIntervalId = null;

  get delay() {
    // get the named argument "delay" passed to the modifier
    //
    // {{move-randomly delay=@someNumber}}
    //                       ~~~~~~~~~~~
    //
    return this.args.named.delay || DEFAULT_DELAY;
  }

  @action moveElement() {
    let top = round(random() * 500);
    let left = round(random() * 500);
    this.element.style.transform = `translate(${left}px, ${top}px)`;
  }

  didReceiveArguments() {
    if (this.setIntervalId !== null) {
      clearInterval(this.setIntervalId);
    }

    this.setIntervalId = setInterval(this.moveElement, this.delay);
  }

  willRemove() {
    clearInterval(this.setIntervalId);
    this.setIntervalId = null;
  }
}

Usage:

<div {{move-randomly}}>
  Catch me if you can!
</div>

Example with Service Injection

You can also use services into your modifier, just like any other class in Ember.

For example, suppose you wanted to track click events with ember-metrics:

// app/modifiers/track-click.js

import { action } from '@ember/object';
import { inject as service } from '@ember/service';
import Modifier from 'ember-modifier';

export default class TrackClickModifier extends Modifier {
  @service metrics;

  get eventName() {
    // get the first positional argument passed to the modifier
    //
    // {{track-click "like-button-click" page="some page" title="some title"}}
    //               ~~~~~~~~~~~~~~~~~~~
    //
    return this.args.positional[0];
  }

  get options() {
    // get the named arguments passed to the modifier
    //
    // {{track-click "like-button-click" page="some page" title="some title"}}
    //                                   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    //
    return this.args.named;
  }

  @action onClick() {
    this.metrics.trackEvent(this.eventName, this.options);
  }

  didInstall() {
    this.element.addEventListener('click', this.onClick, true);
  }

  willRemove() {
    this.element.removeEventListener('click', this.onClick, true);
  }
}

Usage:

<button {{track-click "like-button-click" page="some page" title="some title"}}>
  Click Me!
</button>

API

element
The DOM element the modifier is attached to.
args: { positional: Array, named: Object }
The arguments passed to the modifier. args.positional is an array of positional arguments, and args.named is an object containing the named arguments.
isDestroying
true if the modifier is in the process of being destroyed, or has already been destroyed.
isDestroyed
true if the modifier has already been destroyed.
constructor(owner, args)
Constructor for the modifier. You must call super(...arguments) before performing other initialization. The element is not yet available at this point (i.e. its value is null during construction).
didReceiveArguments()
Called when the modifier is installed and anytime the arguments are updated.
didUpdateArguments()
Called anytime the arguments are updated but not on the initial install. Called before didReceiveArguments.
didInstall()
Called when the modifier is installed on the DOM element. Called after didReceiveArguments.
willRemove()
Called when the DOM element is about to be destroyed; use for removing event listeners on the element and other similar clean-up tasks.
willDestroy()
Called when the modifier itself is about to be destroyed; use for teardown code. Called after willRemove. The element is no longer available at this point (i.e. its value is null during teardown).
Lifecycle Summary
Install Update Remove this.element this.args
constructor() (1) after super()
didUpdateArguments() (1) ✔️ ✔️
didReceiveArguments() (2) (2) ✔️ ✔️
didInstall() (3) ✔️ ✔️
willRemove() (1) ✔️ ✔️
willDestroy() (2) ✔️
  • (#) Indicates the order of invocation for the lifecycle event.
  • Indicates that the method is not invoked for a given lifecycle / property is not available.
  • ✔️ Indicates that the property is available during the invocation of the given method.

TypeScript

Using the class API, you can use .ts instead of .js and it'll just work, as long as you do runtime checks to narrow the types of your args when you access them.

// app/modifiers/scroll-position.ts
import Modifier from 'ember-modifier';

export default class ScrollPositionModifier extends Modifier {
  // ...
}

But to avoid writing runtime checks, you can extend Modifier with your own args, similar to the way you would define your args for a Glimmer Component.

// app/modifiers/scroll-position.ts
import Modifier from 'ember-modifier';

interface ScrollPositionModifierArgs {
  positional: [number],
  named: {
    relative: boolean
  }
}

export default class ScrollPositionModifier extends Modifier<ScrollPositionModifierArgs> {
  get scrollPosition(): number {
    // get the first positional argument passed to the modifier
    //
    // {{scoll-position @someNumber relative=@someBoolean}}
    //                  ~~~~~~~~~~~
    //
    return this.args.positional[0];
  }

  get isRelative(): boolean {
    // get the named argument "relative" passed to the modifier
    //
    // {{scoll-position @someNumber relative=@someBoolean}}
    //                                       ~~~~~~~~~~~~
    //
    return this.args.named.relative
  }

  didReceiveArguments() {
    if(this.isRelative) {
      this.element.scrollTop += this.scrollPosition;
    } else {
      this.element.scrollTop = this.scrollPosition;
    }
  }
}

See this pull request comment for a full discussion about using TypeScript with your Modifiers.

API Differences

API differences from ember-functional-modifiers

  • Renamed package to ember-modifier
  • Renamed makeFunctionalModifier to modifier, and to a named export instead of the default
  • Removed isRemoving flag from modifier destructors. In cases where fine-grained control over the lifecycle is needed, class modifiers should be used instead.
  • Removed service injection from functional modifiers. In cases where services are needed, class modifiers should be used instead.

API differences from ember-class-based-modifier

  • Renamed package to ember-modifier
  • Removed classic API

API differences from ember-oo-modifiers

  • Renamed package to ember-modifier.
  • Removed classic API
  • No Modifier.modifier() function.
  • Arguments, both positional and named, are available on this.args.
  • Named arguments do not become properties on the modifier instance.
  • Arguments are not passed to life-cycle hooks.
  • Renamed didInsertElement to didInstall and willDestroyElement to willRemove. This is to emphasize that when the modifier is installed or removed, the underlying element may not be freshly inserted or about to go away. Therefore, it is important to perform clean-up work in the willRemove to reverse any modifications you made to the element.
  • Changed life-cycle hook order: didReceiveArguments fires before didInstall, and didUpdateArguments fires before didReceiveArguments, mirroring the classic component life-cycle hooks ordering.
  • Added willDestroy, isDestroying and isDestroyed with the same semantics as Ember objects and Glimmer components.

Contributing

See the Contributing guide for details.

License

This project is licensed under the MIT License.

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