Favorite Button

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Introduction

This sample demonstrates how to implement a favorite/like/bookmark button in AMP. Our implementation:

shows the correct icon based on whether the user already liked an item or not. This works if the AMP is served from an AMP Cache or the original origin.
shows a placeholder while the current state is loaded asynchronously.
falls back to the original state and displays an error message if the request fails, for example when the user is offline.

Setup

We use the amp-list component to dynamically render the initial state of the favorite button.

<script async custom-element="amp-list" src="https://cdn.ampproject.org/v0/amp-list-0.1.js"></script>

The mustache component is required by amp-list.

<script async custom-template="amp-mustache" src="https://cdn.ampproject.org/v0/amp-mustache-0.2.js"></script>

We need amp-form to submit the favorite request.

<script async custom-element="amp-form" src="https://cdn.ampproject.org/v0/amp-form-0.1.js"></script>

amp-bind enables us to dynamically change the button state when we submit the form.

<script async custom-element="amp-bind" src="https://cdn.ampproject.org/v0/amp-bind-0.1.js"></script>

Managing state

We initialize the button state from a JSON endpoint using amp-state. As we use cookies to identify the user, we need to add the credentials="include" attribute.

<amp-state id="favorite" credentials="include" src="https://amp.dev/documentation/examples/interactivity-dynamic-content/favorite_button/favorite">
</amp-state>

A simple favorite button

The button is embedded inside an amp-list, which enables us to dynamically render the button based on whether the user already liked something or not. Inside the template we use mustache's implicit iterator . to access the boolean value that is returned by our /favorite endpoint: {{#.}}heart-fill{{/.}}.

We also declare a placeholder icon inside the amp-list using the placeholder attribute which is shown while the amp-list is loading.

There are a few things that are happening when the user presses the favorite button:

The form sends a toggle request when the user presses the button.
We implement an optimistic UX that will instantly update the button state when the button is pressed.
If the form submission fails (submit-error:...), we revert the favorite state to the original version and show an error message (favorite-failed-message.show).
We hide any existing error messages (favorite-failed-message.hide).

<form class="favorite-button" method="post" action-xhr="https://amp.dev/documentation/examples/interactivity-dynamic-content/favorite_button/favorite" target="_top" on="submit:AMP.setState({
                    favorite: !favorite
                 }),
                 favorite-failed-message.hide;
          submit-error:AMP.setState({
                    favorite: !favorite
                 }),
                 favorite-failed-message.show">
  <amp-list width="56" height="56" credentials="include" items="." single-item src="https://amp.dev/documentation/examples/interactivity-dynamic-content/favorite_button/favorite" binding="always">
    <template type="amp-mustache">
      <input type="submit" class="{{#.}}heart-fill{{/.}}{{^.}}heart-border{{/.}}" [class]="favorite ? 'heart-fill' : 'heart-border'" value aria-label="Favorite Toggle">
    </template>
    <div placeholder>
      <input type="submit" disabled class="heart-loading" value aria-label="favorite placeholder">
    </div>
  </amp-list>
</form>

A simple snackbar that we show when the form submission fails.

<div id="favorite-failed-message" hidden>Error: Could not favorite.
  <div on="tap:favorite-failed-message.hide" tabindex="0" role="button">CLOSE</div>
</div>

A favorite button with counter

This is a more sophisticated version of the previous sample that also includes the number of favorites. Our JSON endpoint returns two values: value and count.

<amp-state id="favoriteWithCount" credentials="include" src="https://amp.dev/documentation/examples/interactivity-dynamic-content/favorite_button/favorite-with-count">
</amp-state>

The implementation is similar to the previous sample, but also updates the count when the button is clicked

AMP.setState({
  ...,
  count: favoriteWithCount.count + (favoriteWithCount.value ? -1 : 1)
})

We use a temporary variable previousFavoriteWithCount to store the previous value in order to be able to revert the button state in case the form submission fails.

<form class="favorite-button" method="post" action-xhr="https://amp.dev/documentation/examples/interactivity-dynamic-content/favorite_button/favorite-with-count" target="_top" on="submit:AMP.setState({
               previousFavoriteWithCount: favoriteWithCount,
               favoriteWithCount: {
                 value: !favoriteWithCount.value,
                 count: favoriteWithCount.count + (favoriteWithCount.value ? -1 : 1),
               }
             }),
             favorite-failed-message.hide;
         submit-error:AMP.setState({
               value: !favoriteWithCount.value,
               favoriteWithCount: previousFavoriteWithCount.count
             }),
             favorite-failed-message.show">
  <amp-list width="200" height="56" credentials="include" items="." single-item noloading src="https://amp.dev/documentation/examples/interactivity-dynamic-content/favorite_button/favorite-with-count" binding="always">
    <template type="amp-mustache">
      <div class="favorite-container">
        <input type="submit" class="{{#value}}heart-fill{{/value}}{{^value}}heart-border{{/value}}" [class]="favoriteWithCount.value ? 'heart-fill' : 'heart-border'" value aria-label="Favorite Toggle">
        <div class="favorite-count" [text]="favoriteWithCount.count ? favoriteWithCount.count : ''">{{count}}</div>
      </div>
    </template>
    <div placeholder>
      <div class="favorite-container">
        <input type="submit" disabled class="heart-loading" value aria-label="favorite placeholder">
        <div class="favorite-count loading">0</div>
      </div>
    </div>
  </amp-list>
</form>

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Written by @sebastianbenz