What are the three stages of event capture?

The three stages of event capture are the capture stage, the target element and the bubbling stage. In the capture phase, events are passed down layer by layer starting from the outermost element. Developers can perform some preprocessing operations in this stage and prevent further transmission of events; in the target element stage, the event reaches the target element and is triggered. According to the corresponding event handler, developers can perform some specific operations in this stage; in the bubbling stage, events are passed upward layer by layer starting from the target element, and developers can perform some post-processing operations in this stage and Prevent further transmission of events.

The operating system for this tutorial: Windows 10 system, DELL G3 computer.

Event capture means that when the browser processes an event generated when a user interacts with an element in a web page, it searches for the elements associated with the event layer by layer starting from the outermost element until the target element is found. process till now. Event capture is divided into three stages: Capturing, Target, and Bubbling.

Capturing phase (Capturing):

The capturing phase is the first stage in the event processing process. When an event occurs, the browser will start from the outermost element and pass the event down layer by layer through event delegation. This delivery process is the so-called "event flow". In the capture phase, the event will start from the outermost element and be passed down layer by layer through event delegation until the target element is found. In this process, each layer of elements will trigger the corresponding event handler. These event handlers can be defined by the developer themselves, or provided by the browser by default.

In the capture phase, event handlers are executed sequentially from the outermost element to the target element. This order is determined by the direction of event flow. Developers can perform some preprocessing operations during the capture phase, such as obtaining contextual information when an event occurs, performing some necessary verification, etc. If you need to prevent further delivery of the event at this stage, you can call the event.stopPropagation() method to cancel the default behavior of the event and prevent the event from continuing to pass down.

Target element (Target):

After the capture phase, the event is delivered to the target element. The target element refers to the specific element associated with the event, such as a button, link, etc. clicked by the user. When an event reaches the target element, the event handler bound to the target element is triggered. This handler is usually defined by the developer himself and is used to handle events related to the target element.

In the event handler of the target element, developers can perform some specific operations, such as modifying the properties of the target element, calling specific functions, etc. This stage is one of the most important stages in the event handling process because it is the stage that directly handles user interaction. Developers can add custom logic in the event handler of the target element as needed to meet actual needs.

Bubbling phase (Bubbling):

The bubbling phase is the last stage in the event processing process. When the target element's event handler completes execution, the event will begin to bubble upward, passing layer by layer until the outermost element. During this process, each layer of elements will trigger the corresponding event handler again. These event handlers are passed in reverse order from the capture phase, starting with the target element and working their way up.

The bubbling stage is usually used to perform some post-processing operations, such as animation effects, notification of other elements, etc. If you need to prevent further delivery of the event during this phase, you can call the event.stopImmediatePropagation() method to cancel further bubbling of the event on the current element and prevent other event handlers from executing.

In short, the three stages of event capture are the capture stage, the target element and the bubbling stage. In the capture phase, events are passed down layer by layer starting from the outermost element. Developers can perform some preprocessing operations in this stage and prevent further transmission of events; in the target element stage, the event reaches the target element and is triggered. According to the corresponding event handler, developers can perform some specific operations in this stage; in the bubbling stage, events are passed upward layer by layer starting from the target element, and developers can perform some post-processing operations in this stage and Prevent further transmission of events.

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