ember-modifier
A library for writing Ember modifiers
Last updated 8 days ago by rwjblue .
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ember-modifier

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

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 differences, 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.

Compatibility

  • Ember.js v3.12 or above
  • Ember CLI v2.13 or above
  • Node.js v10 or above

Installation

ember install ember-modifier

Philosophy

Modifiers are a basic primitive for interacting with the DOM in Ember. For example, Ember ships with a built-in modifier, {{on}}:

<button {{on "click" @onClick}}>
  {{@text}}
</button>

All modifiers get applied to elements directly this way (if you see a similar value that isn't in an element, it is probably a helper instead), and they are passed the element when applying their effects.

Conceptually, modifiers take tracked, derived state, and turn it into some sort of side effect - usually, mutating the DOM node they are applied to in some way, but they might also trigger other types of side effects.

Woah woah woah, hold on, what's a "side effect"?

A "side effect" is something that happens in programming all the time. Here's an example of one in an Ember component that attempts to make a button like in the first example, but without modifiers:

// ???? DO NOT COPY THIS ????
import Component from '@glimmer/component';

export default class MyButton extends Component {
  get setupEventHandler() {
    document.querySelector('#my-button').addEventListener(this.args.onClick);

    return undefined;
  }
}
<button id="#my-button">
  {{this.setupEventHandler}}

  {{@text}}
</button>

We can see by looking at the setupEventListener getter that it isn't actually returning a value, instead it always returns undefined. However, it also adds the @onClick argument as an event listener to the button in the template when the getter is run, as the template is rendering, which is a side effect

  • it is an effect of running the code that doesn't have anything to do with the "main" purpose of that code, in this case to return a dynamically computed value. In fact, this code doesn't compute a value at all, so this component is misusing the getter in order to run its side effect whenever it is rendered in the template.

Side effects can make code very difficult to reason about, since any function could be updating a value elsewhere. In fact, the code above is very buggy:

  1. If the @onClick argument ever changes, it won't remove the old event listener, it'll just keep adding new ones.
  2. It won't remove the old event listener when the component is removed.
  3. It uses a document element selector that may not be unique, and it has no guarantee that the element will exist when it runs.
  4. It will run in Fastboot/Server Side Rendering, where no DOM exists at all, and it'll throw errors because of this.

However, there are lots of times where its difficult to write code that doesn't have side effects. Sometimes it would mean having to rewrite a large portion of an application. Sometimes, like in the case of modifying DOM, there isn't a clear way to do it at all with just getters and components.

This is where modifiers come in. Modifiers exist as a way to bridge the gap between derived state and side effects in way that is contained and consistent, so that users of a modifier don't have to think about them.

Managing "side effects" effectively

Let's look again at our original example:

<button {{on "click" @onClick}}>
  {{@text}}
</button>

We can see pretty clearly from this template that Ember will:

  1. Create a <button> element
  2. Append the contents of the @text argument to that button
  3. Add a click event handler to the button that runs the @onClick argument

If @text or @onClick ever change, Ember will keep everything in sync for us. We don't ever have to manually set element.textContent or update anything ourselves. In this way, we can say the template is declarative - it tells Ember what we want the output to be, and Ember handles all of the bookkeeping itself.

Here's how we could implement the {{on}} modifier so that it always keeps things in sync correctly:

import { modifier } from 'ember-modifier';

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

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

Here, we setup the event listener using the positional parameters passed to the modifier. Then, we return a destructor - a function that undoes our setup, and is effectively the opposite side effect. This way, if the @onClick handler ever changes, we first teardown the first event listener we added - leaving the element in its original state before the modifier ever ran - and then setup the new handler.

This is what allows us to treat the {{on}} modifier as if it were just like the {{@text}} value we put in the template. While it is side effecting, it knows how to setup and teardown that side effect and manage its state. The side effect is contained - it doesn't escape into the rest of our application, it doesn't cause other unrelated changes, and we can think about it as another piece of declarative, derived state. Just another part of the template!

In general, when writing modifiers, especially general purpose/reusable modifiers, they should be designed with this in mind. Which specific effects are they trying to accomplish, how to manage them effectively, and how to do it in a way that is transparent to the user of the modifier.

Should modifiers always be self-contained?

Sometimes modifiers won't be completely self-contained. For instance, the @ember/render-modifiers package provides modifiers that call component methods directly, giving the component the ability to manage the side effect. This is ok, but it limits the reusability of whatever the component is doing, so breaking those effects out into individual modifiers is generally preferable.

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 modifier(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.args.positional;

    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. (See below for a discussion of the types.)
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. Deprecated since 2.0. Prefer willDestroy().
willDestroy()
Called when the modifier itself is about to be destroyed; use for teardown code. Called after willRemove.
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

Both the functional and class APIs can be used with TypeScript!

Before checking out the Examples with Typescript below, there is an important caveat you should understand about type safety!

True type safety requires runtime checking, since templates are not currently type-checked: the arguments passed to your modifier can be anything. They’re typed as unknown by default, which means by default TypeScript will require you to work out the type passed to you at runtime. For example, with the ScrollPositionModifier shown above, you can combine TypeScript’s type narrowing with the default types for the class to provide runtime errors if the caller passes the wrong types, while providing safety throughout the rest of the body of the modifier. Here, didReceiveArguments would be guaranteed to have the correct types for this.scrollPosition and this.isRelative:

import Modifier from 'ember-modifier';

export class ScrollPositionModifier extends ClassBasedModifier {
  get scrollPosition(): number {
    const scrollValue = this.args.positional[0];
    if (typeof scrollValue !== "number") {
      throw new Error(
        `first argument to 'scroll-position' must be a number, but ${scrollValue} was ${typeof scrollValue}`
      );
    }

    return scrollValue;
  }

  get isRelative(): boolean {
    const { relative } = this.args.named;
    if (typeof relative !== "boolean") {
      throw new Error(
        `'relative' argument to 'scroll-position' must be a boolean, but ${relative} was ${typeof relative}`
      );
    }

    return relative;
  }

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

You can also avoid writing these runtime checks by extending Modifier with predefined 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 {
    return this.args.positional[0];
  }

  get isRelative(): boolean {
    return this.args.named.relative
  }

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

However, while doing so is slightly more convenient, it means you get much worse feedback in tests or at runtime if someone passes the wrong kind of arguments to your modifier.

Examples with TypeScript

Functional modifier

Let’s look at a variant of the move-randomly example from above, implemented in TypeScript, and now requiring a named argument, the maximum offset. Using the recommended runtime type-checking, it would look like this:

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

const { random, round } = Math;

export default modifier((element, _, named) => {
  if (!(element instanceof HTMLElement)) {
    throw new Error(`move-randomly can only be installed on HTML elements!`);
  }

  const { maxOffset } = named;
  if (typeof maxOffset !== "number") {
    throw new Error(
      `The 'max-offset' argument to 'move-randomly' must be a number, but was ${typeof maxOffset}`
    );
  }

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

  return () => clearInterval(id);
});

A few things to notice here:

  1. TypeScript correctly infers the types of the arguments for the function passed to the modifier; you don't need to specify what element or positional or named are.

  2. If we returned a teardown function which had the wrong type signature, that would also be an error.

    If we return a value instead of a function, for example:

    export default modifier((element, _, named) => {
      // ...
    
      return id;
    });
    

    TypeScript will report:

    Argument of type '(element: Element, _: Positional, named: Record<string, unknown>) => Timeout' is not assignable to parameter of type 'FunctionalModifier<Positional, Record<string, unknown>>'.
      Type 'Timeout' is not assignable to type 'void | Teardown'.
        Type 'Timeout' is not assignable to type 'Teardown'.
          Type 'Timeout' provides no match for the signature '(): void'.
    

    Likewise, if we return a function with the wrong signature, we will see the same kinds of errors. If we expected to receive an argument in the teardown callback, like this:

    export default modifier((element, _, named) => {
      // 
    
      return (interval: number) => clearTimeout(interval);
    });
    

    TypeScript will report:

    Argument of type '(element: Element, _: Positional, named: Record<string, unknown>) => (interval: number) => void' is not assignable to parameter of type 'FunctionalModifier<Positional, Record<string, unknown>>'.
      Type '(interval: number) => void' is not assignable to type 'void | Teardown'.
        Type '(interval: number) => void' is not assignable to type 'Teardown'.
    

Class-based

To support correctly typing args in the constructor for the case where you do runtime type checking, we supply a ModifierArgs interface import. Here’s what a fully typed modifier that alerts "This is a typesafe modifier!" an amount of time after receiving arguments that depends on the length of the first argument and an optional multiplier (a nonsensical thing to do, but one that illustrates a fully type-safe class-based modifier):

import Modifier, { ModifierArgs } from 'ember-modifier';

export default class NeatModifier extends Modifier {
  interval?: number;

  constructor(owner: unknown, args: ModifierArgs) {
    super(owner, args);
    // other setup you might do
  }

  get lengthOfInput(): number {
    if (typeof this.args.positional[0] !== 'string') {
      throw new Error(
        `positional arg must be 'string' but was ${typeof this.args.positional[0]}`
      );
    }

    return this.args.positional[0].length;
  }

  get multiplier(): number {
    if (this.args.named.multiplier === undefined) {
      return 1000;
    }

    if (typeof this.args.named.multiplier !== "number") {
      throw new Error(
        `'multiplier' arg must be a number but was ${typeof this.args.named
          .multiplier}`
      );
    }

    return this.args.named.multiplier;
  }

  didReceiveArguments() {
    this.interval = setInterval(() => {
      alert("this is a typesafe modifier!");
    }, this.multiplier * this.lengthOfInput);
  }

  willDestroy() {
    clearInterval(this.interval);
  }
}

Additional reading

See this pull request comment for background 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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