Limitations of bubbling events: When can bubbling not be implemented?

Limitations of bubbling events: Under what circumstances can bubbling not be achieved?

In front-end development, we often use event bubbling to handle events of DOM elements. However, sometimes bubbling is not a panacea, and there are some cases where bubbling cannot achieve our needs. This article will discuss some situations where bubbling is not possible and provide specific code examples.

1. Prevent bubbling
Normally, we use the Event.stopPropagation() method to prevent the event from bubbling. However, sometimes preventing bubbling does not achieve the desired effect.

For example, suppose we have a parent element and a child element. When the child element is clicked, we want the event handler function of the child element to be executed before the event handler function of the parent element is executed. We may try to use event.stopPropagation() in the event handler of the child element to prevent bubbling:

<div id="parent">
  <div id="child"></div>
</div>

<script>
document.getElementById('child').addEventListener('click', function(event) {
  event.stopPropagation();
  console.log('子元素点击事件');
});

document.getElementById('parent').addEventListener('click', function() {
  console.log('父元素点击事件');
});
</script>

However, the above code cannot achieve our needs, click on the child element Only the click event handler of the child element will be executed, and the click event handler of the parent element will not be executed. This is because preventing bubbling will only prevent the event from being passed to the parent element, but it will not cause the parent element's event handler to be executed after the child element's event handler is executed.

2. Event delegation
Event delegation uses the principle of event bubbling to bind the event to the parent element and trigger the corresponding processing function by judging the event source. However, sometimes there are limitations when using event delegation.

For example, suppose we have a ul element that contains multiple li elements. We hope that when any li element is clicked, the text content of the element will be output. We may try to use event delegation to achieve:

<ul id="list">
  <li>1</li>
  <li>2</li>
  <li>3</li>
</ul>

<script>
document.getElementById('list').addEventListener('click', function(event) {
  if (event.target.tagName.toLowerCase() === 'li') {
    console.log(event.target.textContent);
  }
});
</script>

The above code can achieve our needs. After clicking any li element, the text content of the element will be output. But if we add an a tag to the li element, and the bubbling is prevented when the a tag is clicked, the event delegation will not work properly:

<ul id="list">
  <li>1 <a href="#" onclick="event.stopPropagation();">阻止冒泡</a></li>
  <li>2</li>
  <li>3</li>
</ul>

<script>
document.getElementById('list').addEventListener('click', function(event) {
  if (event.target.tagName.toLowerCase() === 'li') {
    console.log(event.target.textContent);
  }
});
</script>

In the above code, click the link "Prevent Bubbling" , the event delegate will be invalid and nothing will be output. This is because preventing bubbling prevents events from bubbling to the ul element, so the event delegate cannot detect the corresponding event source.

3. Asynchronous event processing
In some cases, we may need to process events asynchronously, such as making network requests or performing other time-consuming operations. However, the bubbling mechanism cannot directly meet the needs of asynchronous event processing.

For example, suppose we have a button element. When the button is clicked, an asynchronous request needs to be sent to obtain data, and then the page is updated based on the data. We may try to perform asynchronous operations in the button's click event handler:

<button id="btn">点击</button>

<script>
document.getElementById('btn').addEventListener('click', function() {
  setTimeout(function() {
    console.log('异步操作完成');
  }, 1000);
});
</script>

The above code will output "Asynchronous operation completed" one second after the button is clicked. However, if we have a click event handler on the parent element of the button and want to execute the event handler after the asynchronous operation completes, the bubbling mechanism cannot achieve this requirement.

To sum up, although bubbling is a very common and powerful mechanism in front-end development, it also has some limitations in some cases. In these cases, we need to find other solutions to meet our needs.

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