Detailed example of vue's $on method

Vue.js is a progressive front-end framework that provides developers with rich APIs and tools to easily build complex user interfaces and single-page applications. One of the very useful methods is the $on method, which will be described and demonstrated in detail in this article.

$on method is an instance method of the Vue object. It is mainly used to listen to a custom event in the Vue instance. When a custom event is triggered, a callback function can be executed to respond to the event. The syntax of the $on method is as follows:

vm.$on(event, callback)

Among them, vm represents the Vue instance, event represents the custom event name being monitored, and callback represents the callback function to be executed when the event is triggered.

For example, we create a Vue instance, and then listen to a custom event named "test" on the instance, as shown below:

var vm = new Vue();
vm.$on("test", function () {
  console.log("test事件被触发了！");
});

In this example, we Use the $on method in the Vue instance to listen to the "test" custom event and use an anonymous function as the callback function. This function will be executed when the "test" event is triggered.

Now let’s take a look at how to trigger a custom event. We can use the $emit method to trigger a custom event as follows:

vm.$emit("test");

In this example, we use the $emit method to trigger a custom event named "test". When we call the $emit method, the callback function defined in the $on method will be executed and the message "test event was triggered!" will be output.

In addition to the $emit method, there are some other methods that can be used to trigger custom events, such as the $dispatch and $broadcast methods. No narrative is given here.

It should be noted that the callback function registered in the $on method will not be executed immediately. When this event is triggered, the Vue instance will call all registered callback functions in order. If the $emit method is called immediately after the $on method, the callback function may not be executed because the event has not been listened to.

When using the $on method, you can also pass an optional parameter to specify the execution context of the callback function, as shown below:

vm.$on("test", function () {
  console.log(this.message);
}, { message: "Hello World!" });

In this example, we use an object to pass Execution context parameters. Obviously, in the callback function, this.message will point to the "Hello World!" string. This feature makes callback functions more flexible and adaptable.

Finally, it should be noted that the $on method only monitors custom events on the current Vue instance and will not affect other instances. If you want to share custom events between multiple instances, consider using the Vue instance's event bus mechanism.

In short, the $on method is a very important and practical method in the Vue.js framework. By using this method, we can easily implement monitoring and response to various custom events.

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