Building a dynamic image grid with Svelte: implementing flip card transitions

Creating engaging user interfaces often requires a delicate balance between functionality and visual appeal. In this article, we'll explore how to build a dynamic image grid component using Svelte that not only manages state efficiently but also provides smooth, eye-catching transitions as images swap in and out.

The Vision

Imagine a grid of images that periodically refreshes itself, with individual cards smoothly flipping to reveal new images.

This creates an engaging display perfect for showcasing team members, product catalogs, or any collection of images that's larger than what can be shown at once.

This is what I had to build this for an image grid widget showcasing a members list. The members images come from an API and grow over time.

I decided to build this with Svelte because, why not ?!

More seriously, I wanted something that will be compiled to just the necessary amount of code required and have a really small footprint on the website.
Based on that I had two options:

• Build it with vanilla javascript
• Use a javascript library that is going to produce a really small bundle especially given that the project is very small too.

Furthermore, I find the svelte model to be simpler and more intuitive so given the choice, especially on a small project like this one, that's what I'll default to.

As you will see a little further, svelte makes dealing with a lot of small and intricate state changes really simple compared to other solutions (again, personal taste).
There are usually less ways to mess things up.

Core Components

Our implementation consists of two main Svelte components:

1. App.svelte - The main component that manages the grid and orchestrates image swapping
2. MemberImageCard.svelte - Individual cards that handle the flip animation and image display

State Management: The Brain Behind the Grid

The heart of our widget lies in its state management. We need to track several pieces of information:

let allImages: Image[]; // All available images
let imagesToUse: Image[] = []; // Initial grid images
let imagesInUse: Image[] = []; // Current grid state
let remainingImages: Image[] = []; // Pool of unused images
let imagesSwapMap = new Map<number, Image>(); // Tracks pending swaps

Why Track Current State Separately?

You might wonder why we maintain imagesInUse separately from imagesToUse. This separation serves several crucial purposes:

1. It provides a single source of truth for the current grid state
2. It helps prevent duplicate images from appearing in the grid
3. It enables efficient updates without full grid re-renders
4. It maintains grid integrity during swapping operations

The Swap Choreography: A Detailed Look

The image swapping process is a carefully orchestrated sequence that ensures smooth transitions while maintaining grid integrity. Let's break down the switchImages function step by step:

let allImages: Image[]; // All available images
let imagesToUse: Image[] = []; // Initial grid images
let imagesInUse: Image[] = []; // Current grid state
let remainingImages: Image[] = []; // Pool of unused images
let imagesSwapMap = new Map<number, Image>(); // Tracks pending swaps

1. Selecting Images from the Pool

First, we need to determine which images from our remaining pool will be used for swapping:

const switchImages = () => {
  let newImagesSwapMap = new Map<number, Image>()
  let remainingImagesToUse
  let newRemainingImages: Image[]

This code handles two scenarios:

• If we're running low on remaining images, we use all of them
• Otherwise, we take the last N images from our pool, where N is NUMBER_OF_IMAGES_TO_SWITCH

2. Choosing Grid Positions

Next, we randomly select positions in the grid where we'll swap images:

if (remainingImages.length <= NUMBER_OF_IMAGES_TO_SWITCH) {
 // If we have fewer remaining images than needed, use all of them
 remainingImagesToUse = remainingImages.slice(0);
 newRemainingImages = [];
} else {
 // Take the last N images from the remaining pool
 remainingImagesToUse = remainingImages.slice(-NUMBER_OF_IMAGES_TO_SWITCH);
 // Keep the rest for future swaps
 newRemainingImages = remainingImages.slice(0, -NUMBER_OF_IMAGES_TO_SWITCH);
}

This creates an array of random indexes within our grid size. For example, if NUMBER_OF_IMAGES_TO_SWITCH is 1 and NUMBER_OF_IMAGES_TO_USE is 16, we might get [7], indicating we'll swap the image at position 7 in the grid.

3. Preventing Duplicates

Before performing any swap, we check if the new image is already displayed:

indexesToSwap = Array(NUMBER_OF_IMAGES_TO_SWITCH)
 .fill(null)
 .map(() => Math.floor(Math.random() * NUMBER_OF_IMAGES_TO_USE));

This function prevents the same image from appearing multiple times in our grid.

4. The Swap Operation

Now comes the core swapping logic:

const imageIsInUse = (image: Image) => {
 const inUse = imagesInUse.find((img: Image) => image.picture_url === img.picture_url);
 return inUse;
};

Let's break down what happens in each swap:

1. We get the randomly selected position (index)
2. We identify the current image at that position (imageToSwap)
3. We take a new image from our pool (imageToSwapWith)
4. If the new image is valid and not already displayed:
   • We record the swap in imagesSwapMap
   • We update the grid state in imagesInUse
   • We add the old image back to the pool at the beginning

5. Finalizing the State

After performing all swaps, we update our state:

for (let i = 0; i < indexesToSwap.length; i++) {
 let index = indexesToSwap[i];
 let imageToSwap = imagesInUse[index]; // Current image in the grid
 let imageToSwapWith = remainingImagesToUse.pop(); // New image to display

 if (imageToSwapWith && !imageIsInUse(imageToSwapWith)) {
  // Record the swap in our map
  newImagesSwapMap.set(index, imageToSwapWith);
  // Update the swap map to trigger component updates
  imagesSwapMap = newImagesSwapMap;
  // Update the grid state
  imagesInUse[index] = imageToSwapWith;
  // Add the old image back to the pool
  newRemainingImages.unshift(imageToSwap);
 } else {
  return; // Skip if the image is already in use
 }
}

6. Triggering the Animation

The imagesSwapMap is the key to triggering animations. When it updates, the relevant MemberImageCard components react:

remainingImages = newRemainingImages;
imagesInUse = imagesInUse;

This reactive statement in MemberImageCard:

1. Detects when its position is involved in a swap
2. Loads the new image on the opposite face of the card
3. Triggers the flip animation by changing faceOnDisplay
4. Resets image loading states for smooth transitions

The beauty of this system is that it maintains a smooth user experience while ensuring:

• No duplicate images appear in the grid
• Images cycle through efficiently
• The grid always maintains its structure
• Animations occur smoothly and predictably
• Failed swaps (due to duplicates) are handled gracefully

The Flip Animation: Making It Smooth

Each MemberImageCard component manages its own flip animation using CSS transforms and transitions. The magic happens through a combination of state tracking and CSS:

let allImages: Image[]; // All available images
let imagesToUse: Image[] = []; // Initial grid images
let imagesInUse: Image[] = []; // Current grid state
let remainingImages: Image[] = []; // Pool of unused images
let imagesSwapMap = new Map<number, Image>(); // Tracks pending swaps

const switchImages = () => {
  let newImagesSwapMap = new Map<number, Image>()
  let remainingImagesToUse
  let newRemainingImages: Image[]

When an image needs to swap, we:

1. Load the new image on the reverse side
2. Trigger the flip animation
3. Clean up the old image once the flip is complete

Progressive Loading for Better UX

To enhance the user experience, we implemented a progressive loading effect:

if (remainingImages.length <= NUMBER_OF_IMAGES_TO_SWITCH) {
 // If we have fewer remaining images than needed, use all of them
 remainingImagesToUse = remainingImages.slice(0);
 newRemainingImages = [];
} else {
 // Take the last N images from the remaining pool
 remainingImagesToUse = remainingImages.slice(-NUMBER_OF_IMAGES_TO_SWITCH);
 // Keep the rest for future swaps
 newRemainingImages = remainingImages.slice(0, -NUMBER_OF_IMAGES_TO_SWITCH);
}

Images start blurred and fade in smoothly once loaded, providing a polished look and feel.

Scheduling the Dance

The regular image swaps are scheduled using Svelte's onMount lifecycle function:

indexesToSwap = Array(NUMBER_OF_IMAGES_TO_SWITCH)
 .fill(null)
 .map(() => Math.floor(Math.random() * NUMBER_OF_IMAGES_TO_USE));

Conclusion

This implementation showcases the power of Svelte's reactive capabilities combined with modern CSS transforms to create a dynamic, engaging UI component.

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