如何使用 jQuery 非同步載入映像？

Asynchronously Loading Images with jQuery

Asynchronous loading techniques are crucial for enhancing the performance of web pages by preventing blocking requests that can slow down page rendering. This technique becomes particularly useful when loading external images that may take time to load, potentially leading to a choppy user experience.

One common approach to asynchronous image loading is using the $.ajax() method. However, it's worth noting that this method may not be suitable for loading images, as it's designed for retrieving data and not specifically optimized for image handling.

Alternative Approach: Using Image Element

A more efficient way to asynchronously load images with jQuery is by creating a new image element dynamically. Here's how to achieve this:

var img = $("<img />").attr('src', 'http://somedomain.com/image.jpg')
    .on('load', function() {
        if (!this.complete || typeof this.naturalWidth == "undefined" || this.naturalWidth == 0) {
            alert('broken image!');
        } else {
            $("#something").append(img);
        }
    });

This code accomplishes the following:

1. Creates a new HTML image element ($("")) and sets the src attribute to the desired image URL.
2. Adds an event listener for the 'load' event, which is triggered when the image has finished loading.
3. Within the load handler, checks if the image has fully loaded and its dimensions are valid.
4. If the image has loaded successfully, it appends the image element to a DOM element with an ID of "something."

Custom Timeout Implementation

This approach does not include a timeout setting, which is essential for handling possible 404 errors. To implement a custom timeout, you can incorporate the following code:

var timeout = setTimeout(function() {
    alert('Image timed out');
}, 5000); // 5 seconds timeout

img.on('load', function() {
    clearTimeout(timeout); // Clear the timeout once the image loads
});

Adding this code to the previous snippet ensures that if the image doesn't load within a specified timeout (5 seconds in this case), it displays an alert indicating a timeout.

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