In j**ascript, event bubbling and event capture are the two mechanisms used to describe event propagation. Event bubbling is when an event is triggered from the most specific element to the parent element, until it reaches the root element. Event capture, on the other hand, means that events are triggered from the root element to the most specific element.
Event bubbling mechanism.
Event bubbling is the most common event propagation mechanism. When an event occurs on an element (e.g. a click event), it triggers the event handler on that element, and then the parent element's event handler up the ladder until it reaches the root element. This process is like bubbles coming out of the bottom of the water, so it is called event bubbling.
The mechanism of event bubbling allows us to bind the same event handler on multiple elements in a page without having to write a separate event handler for each element. When an event is triggered, the event bubbles up to the parent element in turn, allowing for reusability and simplicity.
For example, we bind a handler for a click event to a button, and when the button is clicked, the event fires from the button, then the parent of the button, then the parent of the parent, and then the root element.
Event Capture Mechanism.
In contrast to event bubbling, event capture starts at the root element and progresses down the level to trigger the event handler. That is, events propagate from the outermost element, and then fire down the ladder until they are triggered to the most concrete element.
During the event capture phase, the event handler is triggered in order from the outside to the inside. This process is a bit like capture, so it's called event capture.
The event capture mechanism is less commonly used in real-world development because it requires an explicit specification of event capture when binding events. Most of the time, we use event bubbling to handle events more often.
Control event propagation.
In j**ascript, you can use the addeventlistener method to bind the event handler and control how the event is propagated with a third parameter. The third parameter can be a boolean or an object.
In the case of Boolean values, true means event capture is used, and false is used event bubbling.
In the case of an object, you can use the capture property to specify whether or not to use event capture.
Event delegation. Event delegate is a technique based on the event bubbling mechanism, which allows us to bind event handlers to parent elements instead of binding them individually on each child element. This reduces the number of event handlers and improves performance.
With event delegation, we can listen to the events of the child element on the parent element and perform the corresponding actions based on the target element of the event. This way, even dynamically added child elements can be automatically bound to event handlers.
The implementation principle of event delegation is to take advantage of the event bubbling mechanism. When an event on a child element is fired, the event bubbles up all the way to the parent element, triggering the event handler bound on the parent element. By judging the target element of the event, we can determine which child element triggered the event.
Application scenarios. Event bubbling and event capture mechanisms have different application scenarios in actual development.
Event bubbling is often used to handle events on specific elements, such as button clicks, link clicks, and so on. By binding event handlers on the parent element, you can easily manage and control the events of multiple child elements.
Event capture is less commonly used, but it still has a place in certain scenarios. For example, we can use the event capture mechanism when we need to bind an event handler on a specific element and we want to process the event during the event capture phase.
Event delegation is a commonly used optimization technique. By binding event handlers to parent elements, you can reduce the number of event handlers and improve performance. It is especially useful in cases where elements are added dynamically or a large number of elements.
In j**ascript, event bubbling and event capture are the two mechanisms used to describe event propagation. Event bubbling is triggered from the most specific element to the parent element, while event capture is triggered from the root element to the most specific element. With the third argument of the AddEventListener method, we can control how the event is propagated. Understanding the event bubbling and event capture mechanisms can help us better understand and handle the order in which event handlers are executed, so that we can better write jascript. At the same time, event delegation is a commonly used optimization technique that can reduce the number of event handlers and improve performance.