The Component Lifecycle

Listed below are the component lifecycle hooks in order of execution according to render scenario.

1. init
2. didReceiveAttrs
3. willRender
4. didInsertElement
5. didRender

1. didUpdateAttrs
2. didReceiveAttrs
3. willUpdate
4. willRender
5. didUpdate
6. didRender

1. willDestroyElement
2. willClearRender
3. didDestroyElement

Below are some samples of ways to use lifecycle hooks within your components.

didUpdateAttrs runs when the attributes of a component have changed, but not when the component is re-rendered, via component.rerender, component.set, or changes in models or services used by the template.

Since didUpdateAttrs is called prior to rerender, you can use this hook to execute code when specific attributes are changed. This hook can be an effective alternative to an observer, as it will run prior to a re-render, but after an attribute has changed.

An example of this scenario in action is a profile editor component. As you are editing one user, and the user attribute is changed, you can use didUpdateAttrs to clear any error state that was built up from editing the previous user.

<ul class="errors">
  {{#each this.errors as |error|}}
    <li>{{error.message}}</li>
  {{/each}}
</ul>
<fieldset>
  {{input name="user.name" value=this.name change=(action "required")}}
  {{input name="user.department" value=this.department change=(action "required")}}
  {{input name="user.email" value=this.email change=(action "required")}}
</fieldset>

import { A } from '@ember/array';
import Component from '@ember/component';

export default Component.extend({
  init() {
    this._super(...arguments);
    this.set('errors', A([]));
  },

  didUpdateAttrs() {
    this._super(...arguments);
    this.set('errors', A([]));
  },

  actions: {
    required(event) {
      if (!event.target.value) {
        this.errors.pushObject({ message: `${event.target.name} is required`});
      }
    }
  }
});

didReceiveAttrs runs after init, and it also runs on subsequent re-renders, which is useful for logic that is the same on all renders. It does not run when the re-render has been initiated internally.

Since the didReceiveAttrs hook is called every time a component's attributes are updated whether on render or re-render, you can use the hook to effectively act as an observer, ensuring code is executed every time an attribute changes.

For example, if you have a component that renders based on a JSON configuration, but you want to provide your component with the option of taking the config as a string, you can leverage didReceiveAttrs to ensure the incoming config is always parsed.

import Component from '@ember/component';

export default Component.extend({
  didReceiveAttrs() {
    this._super(...arguments);
    const profile = this.data;
    if (typeof profile === 'string') {
      this.set('profile', JSON.parse(profile));
    } else {
      this.set('profile', profile);
    }
  }
});

Suppose you want to integrate your favorite date picker library into an Ember project. Typically, 3rd party JS/jQuery libraries require a DOM element to bind to. So, where is the best place to initialize and attach the library?

After a component successfully renders its backing HTML element into the DOM, it will trigger its didInsertElement() hook.

Ember guarantees that, by the time didInsertElement() is called:

1. The component's element has been both created and inserted into the DOM.
2. The component's element is accessible via the component's this.element property.

The element property allows you to access the component's DOM element. For example, you can set an attribute using the Element.setAttribute() method:

import Component from '@ember/component';

export default Component.extend({
  didInsertElement() {
    this._super(...arguments);
    this.element.setAttribute('contenteditable', true);
  }
});

The element property will, by default, return a DOM object for the component's root element, but you can also target child elements within the component's template by passing a selector to querySelector or querySelectorAll:

import Component from '@ember/component';

export default Component.extend({
  didInsertElement() {
    this._super(...arguments);
    let buttons = this.element.querySelectorAll('div p button');
    buttons.forEach(function(button) {
      button.classList.add('enabled');
    });
  }
});

Let's initialize our date picker by overriding the didInsertElement() method.

Date picker libraries usually attach to an <input> element, so we will use this.element.querySelector to find an appropriate input within our component's template.

import Component from '@ember/component';

export default Component.extend({
  didInsertElement() {
    this._super(...arguments);
    this.datePicker = myDatePickerLib(this.element.querySelectorAll('input.date'));
  }
});

didInsertElement() is also a good place to attach event listeners. This is particularly useful for custom events or other browser events which do not have a built-in event handler.

For example, perhaps you have some custom CSS animations trigger when the component is rendered and you want to handle some cleanup when it ends:

import Component from '@ember/component';

export default Component.extend({
  didInsertElement() {
    this._super(...arguments);
    this.element.addEventListener('animationend', () => {
      this.element.classList.remove('sliding-anim');
    });
  }
});

There are a few things to note about the didInsertElement() hook:

• It is only triggered once when the component element is first rendered.
• In cases where you have components nested inside other components, the child component will always receive the didInsertElement() call before its parent does.
• Setting properties on the component in didInsertElement() triggers a re-render, and for performance reasons, is not allowed.
• While didInsertElement() is technically an event that can be listened for using on(), it is encouraged to override the default method itself, particularly when order of execution is important.

The didRender hook is called during both render and re-render after the template has rendered and the DOM updated. You can leverage this hook to perform post-processing on the DOM of a component after it's been updated.

In this example, there is a list component that needs to scroll to a selected item when rendered. Since scrolling to a specific spot is based on positions within the DOM, we need to ensure that the list has been rendered before scrolling. We can first render this list, and then set the scroll.

The component below takes a list of items and displays them on the screen. Additionally, it takes an object representing which item is selected and will select and set the scroll top to that item.

<SelectedItemList @items={{this.items}} @selectedItem={{this.selection}} />

When rendered the component will iterate through the given list and apply a class to the one that is selected.

<div class="item-list">
  {{#each this.items as |item|}}
    <div class="list-item {{if item.isSelected 'selected-item'}}">{{item.label}}</div>
  {{/each}}
</div>

The scroll happens on didRender, where it will scroll the component's container to the element with the selected class name.

import Component from '@ember/component';

export default Component.extend({
  didReceiveAttrs() {
    this._super(...arguments);
    this.items.forEach((item) => {
      if (item.id === this.selectedItem.id) {
        item.isSelected = true;
      }
    });
  },

  didRender() {
    this._super(...arguments);
    const scrollTarget = Math.abs(this.element.getBoundingClientRect().top - this.element.querySelector('.selected-item').getBoundingClientRect().top);
    this.element.querySelector('.item-list').scrollTop = scrollTarget;
  }
});

When a component detects that it is time to remove itself from the DOM, Ember will trigger the willDestroyElement() method, allowing for any teardown logic to be performed.

Component teardown can be triggered by a number of different conditions. For instance, the user may navigate to a different route, or a conditional Handlebars block surrounding your component may change:

{{#if this.falseBool}}
  <MyComponent />
{{/if}}

Let's use this hook to cleanup our date picker and event listener from above:

import Component from '@ember/component';

export default Component.extend({
  willDestroyElement() {
    this.element.removeEventListener('animationend');
    this.datePicker.destroy();
    this._super(...arguments);
  }
});