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Subscription Settings

Introduction

This example shows how to implement a subscription settings icon dropdown. This lets the user to change their subscription settings without navigating away from the email. It also works for websites.

Setup

We use an amp-list component to query the user's current subscription setting on email or page load from the server.

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

We'll use amp-mustache to render the dropdown icon linked to the server responded subscription state.

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

We'll also use amp-form to update the user's subscription setting on the server when they select a new setting from the icon dropdown.

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

Lastly, we use amp-bind to update the dropdown's state in response to events.

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

Implementation

Server

The server responds to GET and POST requests at https://amp.dev/documentation/examples/interactivity-dynamic-content/subscription_settings/subscription.

It responds to GET requests with a JSON object like the following:

{
  "currentSubscription": "only-mentions",
  "options": [
    {
      "value": "watching",
      "isSelected": false,
      "text": "Watching"
    },
    {
      "value": "only-mentions",
      "isSelected": true,
      "text": "Only mentions"
    },
    {
      "value": "ignoring",
      "isSelected": false,
      "text": "Ignoring"
    }
  ]
}

currentSubscription and isSelected vary depending on the user's current subscription setting. isSelected is only true for one of the objects in options at a time.

The server expects POST requests to specify one of the subscription settings above (e.g. ignoring) for a nextSubscription field in its form data. It then updates the user's current subscription setting to the specified value.

AMP

Step 1: Basic Dropdown

We start with a native select element for our dropdown and fill it with our supported subscription settings. The select element is a great choice because it is highly accessible on all platforms.

<select>
  <option value="watching">Watching</option>
  <option value="only-mentions">Only mentions</option>
  <option value="ignoring">Ignoring</option>
</select>
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Step 2: Fetching the Current Subscription Setting

We wrap the select element in an amp-list and set its src to the URL our server is listening for GET requests. This queries the user's current subscription setting.

We use an amp-mustache template to render the select element and ensure that the option element corresponding to the user's subscription setting has the selected attribute. To understand why the code is duplicative, consider that the selected attribute is a boolean attribute and review the amp-mustache restrictions.

Finally, we add a placeholder and fallback to keep the user informed about the state of the user interface and gracefully handle any issues.

Loading...
Something went wrong. Please refresh
<amp-list src="https://amp.dev/documentation/examples/interactivity-dynamic-content/subscription_settings/subscription" binding="refresh" items="." single-item layout="fill">
  <template type="amp-mustache">
    <select>
      {{#options}}
        {{#isSelected}}
          <option value="{{value}}" selected>
            {{text}}
          </option>
        {{/isSelected}}
        {{^isSelected}}
          <option value="{{value}}">
            {{text}}
          </option>
        {{/isSelected}}
      {{/options}}
    </select>
  </template>
  <div placeholder>Loading...</div>
  <div fallback>Something went wrong. Please refresh</div>
</amp-list>
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Note that the amp-list is annotated with single-item because our server responds to GET requests with a single logical value (the subscription setting) and we only want the template to be invoked once for the response. Setting items="." ensures that the template can access the response object's root fields.

Step 3: Updating the Subscription Setting on Selection

To update the user's subscription setting on the server when they select a new setting, we start by wrapping the select element in an AMP form element, setting its method to post, and setting its action-xhr attribute to the URL where our server is listening for POST requests.

The name attribute of the select element is set to nextSubscription. nextSubscription is the name of the form data field where the server expects the user's new subscription setting to be in the POST request (see the Server section) and the select element contains the user's selected subscription setting.

Finally, we trigger the form submission when the select element's value changes. We do so by using the on attribute to attach an event handler to the select element's change event, invoking the form element's submit action. Note that we gave the form element an id so that we could refer to it in the event handler.

You can test the code by selecting a new subscription setting and refreshing the page. The select element's subscription setting on page load is now the one you selected!

Loading...
Something went wrong. Please refresh
<amp-list src="https://amp.dev/documentation/examples/interactivity-dynamic-content/subscription_settings/subscription" binding="refresh" items="." single-item layout="fill">
  <template type="amp-mustache">
    <form action-xhr="https://amp.dev/documentation/examples/interactivity-dynamic-content/subscription_settings/subscription" method="post" id="form1">
      <select name="nextSubscription" on="change: form1.submit;">
        {{#options}}
          {{#isSelected}}
            <option value="{{value}}" selected>
              {{text}}
            </option>
          {{/isSelected}}
          {{^isSelected}}
            <option value="{{value}}">
              {{text}}
            </option>
          {{/isSelected}}
        {{/options}}
      </select>
    </form>
  </template>
  <div placeholder>Loading...</div>
  <div fallback>Something went wrong. Please refresh</div>
</amp-list>
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Step 4: Disabling While Submitting

After selecting a new subscription setting from the dropdown, it is unclear when the form is submitting or submitted. Furthermore, nothing prevents the user from attempting to select a new subscription setting while the form is still submitting.

Ideally the select element would be disabled while the form is submitting. However, there's a problem with this approach: values of disabled inputs are not submitted with a form. Under different circumstances we might consider using the readonly attribute instead of the disabled attribute to avoid this problem, but the select element does not support the readonly attribute. Fortunately, we can solve this problem by submitting the select element's value through a new hidden input element, which doesn't need to be disabled because users cannot interact with or see it, that we ensure always contains the same value as the select element.

We start by adding a hidden input element, and moving the name of the select element to the hidden input element so that its value submits. To keep the hidden input element's value in sync with the select element's value, we update a new nextSubscription state variable using AMP.setState whenever the select element's value changes and bind the hidden input element's value attribute to the state variable.

To disable the select element we need a state variable that represents whether the form is currently submitting that we can use in the select element's disabled attribute binding. We reuse the nextSubscription state variable by ensuring it is null when the form is not submitting by updating nextSubscription to null on the submit-success and submit-error events of the form element. Finally, we bind the select element's disabled attribute to !!nextSubscription because the state variable is truthy when the form is submitting. We do the same for each of its option elements to prevent the user from using the keyboard to switch between the values while the form is submitting.

Loading...
Something went wrong. Please refresh
<amp-list src="https://amp.dev/documentation/examples/interactivity-dynamic-content/subscription_settings/subscription" binding="refresh" items="." single-item layout="fill">
  <template type="amp-mustache">
    <form action-xhr="https://amp.dev/documentation/examples/interactivity-dynamic-content/subscription_settings/subscription" method="post" id="form2" on="submit-success: AMP.setState({ nextSubscription: null });
        submit-error: AMP.setState({ nextSubscription: null });">
      <input type="hidden" name="nextSubscription" [value]="nextSubscription">
      <select on="change: AMP.setState({ nextSubscription: event.value }), form2.submit;" [disabled]="!!nextSubscription">
        {{#options}}
          {{#isSelected}}
            <option value="{{value}}" [disabled]="!!nextSubscription" selected>
              {{text}}
            </option>
          {{/isSelected}}
          {{^isSelected}}
            <option value="{{value}}" [disabled]="!!nextSubscription">
              {{text}}
            </option>
          {{/isSelected}}
        {{/options}}
      </select>
    </form>
  </template>
  <div placeholder>Loading...</div>
  <div fallback>Something went wrong. Please refresh</div>
</amp-list>
Apri questo frammento in playground

Note that despite the disabled attribute being a boolean attribute, we can still effectively bind it because amp-bind will remove the attribute when the binding expression is false.

Step 5: Handling Form Submission Failure

We need to display an error message and revert the select element to its previous value if the form fails to submit.

To display an error message when the form fails to submit, we add element containing our error message to the form element's children. We annotate the child element with the submit-error attribute. This attribute ensures that the element is only visible after a failed form submission.

Reverting the select element's value to its previous value requires binding the selected attribute of each option element. This requires maintaining the select element's current value in a currentSubscription state variable and the element's previous value in a previousSubscription state variable.

When the select element's value changes, we set currentSubscription to the new value and previousSubscription to the element's previous value. We store these in the currentSubscription state variable, if this is not the first time the element's value has changed. Otherwise, it's stored in the currentSubscription field of the amp-list response.

We ensure the select element's selected option stays in sync with currentSubscription by binding the selected attribute of each option. Finally, we revert the select element to its previous value on form submission failure by setting currentSubscription to previousSubscription in the form element's submit-error event.

The following code snippet is configured to fail form submissions. Test out the new behavior!

Loading...
Something went wrong. Please refresh
<amp-list src="https://amp.dev/documentation/examples/interactivity-dynamic-content/subscription_settings/subscription" binding="refresh" items="." single-item layout="fill">
  <template type="amp-mustache">
    <form action-xhr="https://amp.dev/documentation/examples/interactivity-dynamic-content/subscription_settings/subscription?fail=true" method="post" id="form3" on="submit-success: AMP.setState({ nextSubscription: null });
        submit-error:
          AMP.setState({
            currentSubscription: previousSubscription,
            nextSubscription: null
          });">
      <input type="hidden" name="nextSubscription" [value]="nextSubscription">
      <select on="change:
            AMP.setState({
              previousSubscription: currentSubscription || '{{currentSubscription}}',
              currentSubscription: event.value,
              nextSubscription: event.value
            }),
            form3.submit;" [disabled]="!!nextSubscription">
        {{#options}}
          {{#isSelected}}
            <option value="{{value}}" [disabled]="!!nextSubscription" selected [selected]="(currentSubscription || '{{currentSubscription}}') == '{{value}}'">
              {{text}}
            </option>
          {{/isSelected}}
          {{^isSelected}}
            <option value="{{value}}" [disabled]="!!nextSubscription" [selected]="(currentSubscription || '{{currentSubscription}}') == '{{value}}'">
              {{text}}
            </option>
          {{/isSelected}}
        {{/options}}
      </select>
      <div submit-error>Something went wrong. Please try again</div>
    </form>
  </template>
  <div placeholder>Loading...</div>
  <div fallback>Something went wrong. Please refresh</div>
</amp-list>
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Step 6: Icon Dropdown

Styling the select element itself to look like an icon using only CSS supported by email clients isn't feasible. Instead, we'll hide the select element's persistent user interface (the box showing the current value), and show a different icon for each dropdown state in its place. However, we'll keep the dropdown shown on click.

Fortunately, styling select affects the persistent user interface while styling option affects the dropdown. We can use this fact to hide the former, but keep the latter. Of the three common choices for hiding an element using CSS (display: none;, visibility: hidden;, and opacity: 0;), opacity: 0; is most suited because we want to keep the element in the tab order as well as keep receiving its click events:

.subscription-select {
  opacity: 0;
  cursor: pointer;
}

We also set cursor: pointer; for the select element because it looks like a button now.

The next step is to create a new icon-based user interface. We start with a div element containing an amp-img component for each subscription setting. The div element is annotated with the following icon CSS class, which hides all of the images by default:

.icon > * {
  visibility: hidden;
}

The amp-img component corresponding to the current state of the select element is made visible using the following visible CSS class:

.visible {
  visibility: visible;
}

If the select element's value has not changed, then we determine the current state of the select element from the isSelected fields in the amp-list response. Otherwise, we determine the current state from the currentSubscription state variable.

Although at most one amp-img component is visible at a time, each component still takes up space on the page. This means that the components will be positioned one below the other by default. Using the display property instead of the visibility property would fix this issue, but it would cause the icon to flicker between subscription setting changes because AMP does not preload amp-img components that start out as display: none;. Instead, we use the following relative and absolute CSS classes to stack the amp-img component on top of each other on the z axis:

.relative {
  position: relative;
}

.absolute {
  position: absolute;
  top: 0;
  right: 0;
  bottom: 0;
  left: 0;
}

Note that the stacking order of the amp-img components relative to each other doesn't matter because at most one is visible at a time.

We also use these CSS classes to position the icons behind the invisible select user interface so that when a user tries to click on one of our icons they'll actually be clicking on the invisible select element and triggering its events! We keep our new icons behind the select element by placing the icon's code before the select element's code (see Stacking without the z-index property).

We ensure the select element's click target bounding box is the same size as the icons using the following icon-sized CSS class:

.icon-sized {
  width: 30px;
  height: 30px;
}

Screen readers can get all of the information they need from the select element so we make the icons invisible to screen readers using the aria-hidden attribute.

Finally, we display an outline around the icons when the select element is focused using the :focus-within pseudo selector:

.icon-container:focus-within {
  outline-offset: 2px;
  outline: 1px solid black;
  outline: -webkit-focus-ring-color auto 1px;
}

And we're done!

Loading...
Something went wrong. Please refresh
<amp-list src="https://amp.dev/documentation/examples/interactivity-dynamic-content/subscription_settings/subscription" binding="refresh" items="." single-item layout="fill">
  <template type="amp-mustache">
    <form action-xhr="https://amp.dev/documentation/examples/interactivity-dynamic-content/subscription_settings/subscription" method="post" id="form4" on="submit-success: AMP.setState({ nextSubscription: null });
        submit-error:
          AMP.setState({
            currentSubscription: previousSubscription,
            nextSubscription: null
          });">
      <div class="icon-container relative icon-sized" [class]="(nextSubscription ? 'disabled ' : ' ') + 'icon-container relative icon-sized'">
        <div class="icon icon-sized" aria-hidden>
          {{#options}}
            <amp-img width="30" height="30" src="/static/samples/img/{{value}}.png" class="{{#isSelected}}visible {{/isSelected}}absolute" [class]="((currentSubscription || '{{currentSubscription}}') == '{{value}}' ? 'visible ' : ' ') + 'absolute'">
            </amp-img>
          {{/options}}
        </div>
        <input type="hidden" name="nextSubscription" [value]="nextSubscription">
        <select class="subscription-select absolute icon-sized" on="change:
              AMP.setState({
                previousSubscription: currentSubscription || '{{currentSubscription}}',
                currentSubscription: event.value,
                nextSubscription: event.value
              }),
              form4.submit;" [disabled]="!!nextSubscription">
          {{#options}}
            {{#isSelected}}
              <option value="{{value}}" [disabled]="!!nextSubscription" selected [selected]="(currentSubscription || '{{currentSubscription}}') == '{{value}}'">
                {{text}}
              </option>
            {{/isSelected}}
            {{^isSelected}}
              <option value="{{value}}" [disabled]="!!nextSubscription" [selected]="(currentSubscription || '{{currentSubscription}}') == '{{value}}'">
                {{text}}
              </option>
            {{/isSelected}}
          {{/options}}
        </select>
      </div>
      <div submit-error>Something went wrong. Please try again</div>
    </form>
  </template>
  <div placeholder>Loading...</div>
  <div fallback>Something went wrong. Please refresh</div>
</amp-list>
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