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 The global event bus is not a native API provided by Vue. It is a clever design method summarized by developers in practice. Its principle is to add a property to the prototype of Vue. Once this property is added, Vue Instances and vue components can use this property with the help of prototypes, so as to realize the intercommunication and transfer of data between data. If we need to implement complex communication between components, it is not so convenient to use the props properties of parent and child components. We have written a lot of redundant code layer by layer. At this time, we can use the message bus writing method, which is more simple and convenient. 

 The parent component uses props to pass data (such as a method) to the child component, and the child component needs to use custom events to pass data back to the parent component! We can use v-on to bind custom events, and then pass them to subcomponents, which will call after receiving the method. Each Vue instance implements the Events interface, that is: use $on(eventName) to listen for events and $emit(eventName) to trigger events.