Typescript：接口获取属性的默认值

Question

I am writing an event system with different types of events in angular 11
I am defining an enum for the different types of events

enum EventTypes {
  Pending,
  Active,
  Complete
}

and interfaces for the different events

interface PendingEvent { type: EventTypes.Pending }

interface ActiveEvent { type: EventTypes.Active, progress: number }

interface CompleteEvent { type: EventTypes.Complete, data: any }

I then define a type to bundle those events

type Event = PendingEvent | ActiveEvent | CompleteEvent;

But now, every time i create a new event i have to give a type.

const event: ActiveEvent = {
  progress: 83
}

I get an error saying that i am missing the 'type' property.
I expect the type to be automatically set depending on the event type i create.

Answer 1

If you use classes you might be able to achieve what you want. You can even keep the interface if you like, or simplify it to one interface.

interface EventInterface { type: EventTypes; progress?: number; data?: any}

class ActiveEvent implements EventInterface {
   constructor (progress: number) {
     this.type =  EventTypes.Active;
     this.progress = progress;
     }
}

const event = new ActiveEvent(82);

Similarly you can make classes for the other two types of Events, which constructors automatically set the type.

Of course, classes and instances of classes aren't the same as typed variables as you have now, but it sounded like this might help your (guessed by me) use case.

Information here may be helpful: https://www.typescriptlang.org/docs/handbook/interfaces.html#implementing-an-interface