Improving the interactivity

The starter code provides a pretty bare user experience. There are a couple ways we can improve it:

  • Add an indicator that displays the current slide and total number of slides.
  • When a user selects a different shirt color, change the image carousel to show images of shirts in the selected color.

Prior to the introduction of the amp-bind component, adding features like these weren't possible. Let's get a hands-on experience with amp-bind and add these new features to our sample code!

Install the amp-bind component

amp-bind is an AMP component that affords custom interactivity via data binding and JS-like expressions. To use amp-bind, you must install it in the page.

Open the static/index.html file, and add the following script to the list of AMP components in the <head> section of the page:

<script async custom-element="amp-bind"

Add a slide indicator

amp-bind works by binding element attributes to custom expressions. These expressions can reference the "state" (mutable JSON data). We can initialize this state through the <amp-state> component included with amp-bind.

Initialize the slide state

Let's initialize a state variable to keep track of the index of the currently displayed slide in the image carousel. Open static/index.html and add the following to the top of the <body> of the page (before the <header>):

<amp-state id="selected">
  <script type="application/json">
      "slide": 0

The data within <amp-state> elements are accessible by their associated ID. For example, we can refer to this variable by the following expression fragment:

selected.slide // Evaluates to 0.

Update the slide state

Next, let's update this variable when the user changes slides on the carousel by adding the following "on" action to the existing amp-carousel element:

<amp-carousel type="slides" layout="fixed-height" height=250 id="carousel"
    on="slideChange:AMP.setState({selected: {slide: event.index}})">

Now, whenever the displayed slide for the amp-carousel changes, the action AMP.setState will be called with the following argument:

  selected: {
    slide: event.index

The event.index expression evaluates to the new slide index, and the AMP.setState() action merges this object literal into the current state. This replaces the current value of selected.slide with the value of event.index.

TIP – AMP.setState() performs a deep merge of nested object literals. For more details, see the amp-bind documentation.

Bind the indicator elements

Next, let's make use of this state variable that tracks the currently displayed slide, and create a slide indicator. Find the slide indicator element (look for <!-- TODO: "Add a slide indicator" -->) and add the following bindings to its children:

<!-- TODO: "Add a slide indicator" -->
<p class="dots">
  <!-- The <span> element corresponding to the current displayed slide
       will have the 'current' CSS class. -->
  <span [class]="selected.slide == 0 ? 'current' : ''" class="current"></span>
  <span [class]="selected.slide == 1 ? 'current' : ''"></span>
  <span [class]="selected.slide == 2 ? 'current' : ''"></span>

[class] is a binding that changes the class attribute and you can use it to add or remove CSS classes from any element.

Try it out: Refresh the page and change the slide!

By changing the slide on the carousel, it:

  1. Triggers the slideChange event ...
  2. Which calls the AMP.setState action ...
  3. Which updates the state variable selected.slide ...
  4. Which updates the [class] binding on the indicator <span> elements!

Nice! Now we have a working slide indicator.

See if you can add functionality so that when a user taps on a slide's indicator dot, it updates the image carousel with the selected item. As a hint, use the tap event and [slide] binding on amp-carousel.

It would be nice if we could see images of different shirt colors when we change the selected color. With amp-bind we can do this by binding [src] on the amp-img elements within the amp-carousel.

Initialize the SKU state

First, we need to initialize the state data with the image source URLs of each color shirt. Let's do this with a new <amp-state> element:

<!-- Available shirts. Maps unique string identifier to color and image URL string. -->
<amp-state id="shirts">
  <script type="application/json">
      "1001": {
        "color": "black",
        "image": "./shirts/black.jpg"
      "1002": {
        "color": "blue",
        "image": "./shirts/blue.jpg"
      "1010": {
        "color": "brown",
        "image": "./shirts/brown.jpg"
      "1014": {
        "color": "dark green",
        "image": "./shirts/dark-green.jpg"
      "1015": {
        "color": "gray",
        "image": "./shirts/gray.jpg"
      "1016": {
        "color": "light gray",
        "image": "./shirts/light-gray.jpg"
      "1021": {
        "color": "navy",
        "image": "./shirts/navy.jpg"
      "1030": {
        "color": "wine",
        "image": "./shirts/wine.jpg"

This <amp-state> element contains a JSON object that maps a shirt identifier string (i.e., a SKU) to the color and image URL of the corresponding shirt. A JSON array would also work here, but using an object allows us to do some more cool stuff that you'll see soon.

Now we can access the image URL via a shirt's identifier. For example, shirts['10014'].color evaluates to "dark green" and shirts['10030'].imagereturns the image URL for the "wine" shirt color.

Track the selected SKU

If we add another state variable that tracks the selected SKU, we can bind an expression to the amp-img elements to update their src attributes when the selected SKU changes. Add a new sku key to the existing amp-state#selected element's JSON:

<amp-state id="selected">
  <script type="application/json">
      "slide": 0,
      "sku": "1001"

Update the SKU state

Add an "on" action to the amp-selector that updates the selected.sku variable whenever a new color is selected:

<amp-selector name="color"
    on="select:AMP.setState({selected: {sku: event.targetOption}})">

TIP – This could also be done by adding on="tap:AMP.setState(...) actions to each amp-img child inside the amp-selector. One of the great things about amp-selector is that it simplifies markup in ways like this.

Bind the image elements

Then, add bindings to the amp-img:

<!-- Update the `src` of each <amp-img> when the `selected.sku` variable changes. -->
<amp-img width=200 height=250 src="./shirts/black.jpg"
<amp-img width=300 height=375 src="./shirts/black.jpg"
<amp-img width=400 height=500 src="./shirts/black.jpg"

NOTE – In practice, each image in the carousel would likely have a different src. This could be done by replacing the single image with an array of images. For simplicity, this tutorial uses a single image at different magnifications.

Try it out: Refresh the page and select a different color for a shirt. When you do, the carousel's images are updated to show shirts of the selected color.