Question 231706 in Html Question posted in
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Html – Lazy loading in Flask

I am learning Flask and attempting to make a website like youtube. I have some experience with JS and know that you can write a script that will load an element only if it is in the viewport of the user.

Are there any built-in tools in Flask that I can use to achieve this or do I have to do this with JS? If that is the case, could you give me the code for that?

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2

Answers


  1. 0

    Flask is primarily a web framework for Python, so it doesn’t have built-in tools specifically for handling client-side JavaScript functionality such as loading elements in the viewport.

    To achieve the desired behavior, you’ll need to utilize JavaScript in your Flask application. Below is a sample code snippet that demonstrates how you can load an element only when it is in the viewport using JavaScript:

    <!DOCTYPE html>
<html>
<head>
  <title>lazy loading Example</title>
  <style>
    .hidden {
      display: none;
    }
  </style>
</head>
<body>
  <div id="content" class="hidden">
    <!-- Your element here -->
    <img src="example-image.jpg" alt="Example Image">
  </div>

  <script>
    function isInViewport(element) {
      const rect = element.getBoundingClientRect();
      return rect.top >= 0 && rect.bottom <= (window.innerHeight || document.documentElement.clientHeight);
    }

    function loadContent() {
      const content = document.getElementById('content');
      if (isInViewport(content)) {
        content.classList.remove('hidden');
      }
    }

    // Initial check on page load
    loadContent();

    // Check on the scroll
    window.addEventListener('scroll', loadContent);
  </script>
</body>
</html>

    In this code, the isInViewport() function checks if the given element is in the viewport by using the getBoundingClientRect() method. The loadContent() function checks if the element is in the viewport, and if so, removes the hidden class to display it.

    You can integrate this JavaScript code into your Flask templates or static files to achieve lazy loading of elements in your Flask application. Remember to make necessary modifications to suit your specific use case, such as handling different elements or adding additional logic as needed.

    I hope this is helpful.

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  2. 0

    Only way to do it is through JS. I will assume you are not using any frameworks in the front-end. here is an example of vanilla JavaScript:
    html

    <body>
  <div class="image-container">
    <img class="lazy-image" data-src="image.jpg" alt="Lazy-loaded Image">
  </div>

  <script src="lazy-load.js"></script>
</body>

    JavaScript:

    const lazyImages = document.querySelectorAll('.lazy-image');

const options = {
  root: null, // Use the viewport as the root
  rootMargin: '0px', // No additional margin
  threshold: 0.2 // 20% of the image must be visible
};

const callback = (entries, observer) => {
  entries.forEach(entry => {
    if (entry.isIntersecting) {
      const image = entry.target;
      const src = image.getAttribute('data-src');

      if (src) {
        image.src = src;
        observer.unobserve(image);
      }
    }
  });
};

const observer = new IntersectionObserver(callback, options);

lazyImages.forEach(image => {
  observer.observe(image);
});
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