How to change/update CSS class dynamically using amp-bind in Google AMP ?

Last Updated : 25 Oct, 2020
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Sometimes you want to add custom interactivity to your AMP pages to make your pages look more user-friendly and user calling. Although AMP's pre-built components are limited, so amp-bind is made to overcome this problem. It helps the developer to add custom interactivity to the pages without using AMP's pre-built components. You can use amp-bind to change the text dynamically on user interaction with the page.

Setup: To use amp-bind in your page you have to import its script in the header of the document.

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

The amp-bind of Google AMP comprises three main concepts:

  1. State: State variables are responsible for the update on the page on the basis of user actions. It is very important to define a state variable.
  2. Expression: They are like JavaScript expressions used to refer to the state.
  3. Binding: They are a special attribute that is used to link an element's property to a state via an expression.

To change the image's class we will make use of [class] attribute, it helps to change the class dynamically.

Example:

HTML 
<!doctype html>
<html amp>

<head>
    <meta charset="utf-8">
    <title>Google AMP amp-bind</title>

    <link rel="canonical" href=
"https://amp.dev/documentation/examples/components/amp-bind/index.html">

    <meta name="viewport" content=
"width=device-width,minimum-scale=1,initial-scale=1">

    <script async src=
        "https://cdn.ampproject.org/v0.js">
    </script>

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

    <style amp-boilerplate>
        body {
            -webkit-animation: -amp-start 8s 
                steps(1, end) 0s 1 normal both;
            
            -moz-animation: -amp-start 8s 
                steps(1, end) 0s 1 normal both;
            
            -ms-animation: -amp-start 8s 
                steps(1, end) 0s 1 normal both;
            
            animation: -amp-start 8s 
                steps(1, end) 0s 1 normal both;
        }

        @-webkit-keyframes -amp-start {
            from {
                visibility: hidden
            }

            to {
                visibility: visible
            }
        }

        @-moz-keyframes -amp-start {
            from {
                visibility: hidden
            }

            to {
                visibility: visible
            }
        }

        @-ms-keyframes -amp-start {
            from {
                visibility: hidden
            }

            to {
                visibility: visible
            }
        }

        @-o-keyframes -amp-start {
            from {
                visibility: hidden
            }

            to {
                visibility: visible
            }
        }

        @keyframes -amp-start {
            from {
                visibility: hidden
            }

            to {
                visibility: visible
            }
        }
    </style>
    <noscript>
        <style amp-boilerplate>
            body {
                -webkit-animation: none;
                -moz-animation: none;
                -ms-animation: none;
                animation: none
            }
        </style>
    </noscript>
    <style amp-custom>
        h1 {
            color: forestgreen;
        }

        .gfg {
            border: 5px solid crimson;
        }
    </style>
</head>

<body>
    <center>
        <h1>
            Geeks For Geeks
        </h1>

        <div style="padding: 1em;">

            <amp-img src=
"https://media.geeksforgeeks.org/wp-content/cdn-uploads/gfg_200x200-min.png"
                width="150"
                height="150" [class]="border">
            </amp-img>
            <br>
            <button on="tap:AMP.setState({
                        border: 'gfg'
                      })">
                Change Class
            </button>
        </div>
    </center>
</body>

</html>

Output:

Initially, the image has no class but when the button is hit, its class is updated to gfg which has a border attribute thus a border appears on the screen around the image.


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