Events without payloads

[dmicic]

Sometimes you just want to publish a event without any any payload. Currently it's possible to use the createEventDefinition without the type declaration. However, the event creator method still forces you to pass in some dummy payload.

const creator = createEventDefinition()("eventname");
const event = creator({});

This PR solves this problem by introducing the function createEmptyEventDefinition which sets the generic parameter to undefined. Not sure whether a generic parameter should be undefined but at least this way it's obvious that the payload is empty and also the IDE will highlight an error if you type myevent.payload.property.

const creator = createEmptyEventDefinition()("eventname");
const event = creator();

Another option would be to introduce another event type without the payload property. But that would be too much overhead to maintain...

@ryardley What's your view on this?

[codecov[bot]]

Codecov Report

Merging #24 into master will not change coverage. The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #24   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           5      5           
  Lines          56     63    +7     
  Branches       11     11           
=====================================
+ Hits           56     63    +7

Impacted Files	Coverage Δ
src/EventBus.ts	100% <100%> (ø)	:arrow_up:

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[ryardley]

Nice work on the PR. I am just wondering if the overhead of having to maintain a second function is worth it.

[ryardley]

What about if we try and merge it to a single function: UPDATE: Found solution see below

export function createEventDefinition<P=undefined>(
  options?: EventDefinitionOptions<P> | TestPredicateFn<P>
) {
  return <T extends string>(type: T) => {
    const eventCreator = (payload?: P):{type:T, payload:P} => {

      // Allow runtime payload checking for plain JavaScript usage
      if (options && payload) {
        const testFn = typeof options === "function" ? options : options.test;
        if (testFn && !testFn(payload)) {
          showWarning(
            `${JSON.stringify(payload)} does not match expected payload.`
          );
        }
      }

      return {
        type,
        payload:payload as P
      };
    };
    eventCreator.eventType = type;
    eventCreator.toString = () => type; // allow String coercion to deliver the eventType
    return eventCreator;
  };
}

const event = createEventDefinition()("foo")();
event.payload; // undefined

const event2 = createEventDefinition<{foo:string, bar:number}>()("foo")({foo:'asdas', bar:4});
event2.payload; // {foo:'asdas', bar:4}

[ryardley]

Ahh just realised the above wont work with this :( :

const event = createEventDefinition<{foo:string, bar:number}>()("foo")();

[ryardley]

so there is this option still not perfect: Found solution see below

export function createEventDefinition<P=undefined>(
  options?: EventDefinitionOptions<P> | TestPredicateFn<P>
) {
  return <T extends string>(type: T) => {

    function eventCreator(payload:void): {type:T}
    function eventCreator(payload:P): {type:T, payload:P}

    function eventCreator (payload: P | void) {

      // Allow runtime payload checking for plain JavaScript usage
      if (options && payload) {
        const testFn = typeof options === "function" ? options : options.test;
        if (testFn && !testFn(payload)) {
          showWarning(
            `${JSON.stringify(payload)} does not match expected payload.`
          );
        }
      }

      if(payload === undefined){
        return {type}
      }

      return {
        type,
        payload
      };
    };
    eventCreator.eventType = type;
    eventCreator.toString = () => type; // allow String coercion to deliver the eventType
    return eventCreator;
  };
}

const event = createEventDefinition()("foo")();
event.payload; // Property 'payload' does not exist on type '{ type: "foo"; }'

const event2 = createEventDefinition<{foo:string, bar:number}>()("foo")();
event2.payload; // Property 'payload' does not exist on type '{ type: "foo"; }'

const event3 = createEventDefinition<{foo:string, bar:number}>()("foo")({foo:'asdasd', bar: 123});
event3.payload; // {foo:'asdasd', bar: 123}

[ryardley]

So as this does look hard to achieve 100% without a separate function we could perhaps wrap the original one? Found a solution See below

export function createEmptyEventDefinition(options?: EventDefinitionOptions<undefined> | TestPredicateFn<undefined>) {
  const innerTypeCreator = createEventDefinition<undefined>(options);
  return <T extends string>(type: T) => { 
    const eventCreator = innerTypeCreator(type);
    return () => eventCreator(undefined);
  }
}

I do feel like this is another example where we need to bend to TypeScript though and it is quite frustrating!

[ryardley]

Actually if we use void it works:

export function createEventDefinition<P=void>(
  options?: EventDefinitionOptions<P> | TestPredicateFn<P>
) {
  return <T extends string>(type: T) => {

    function eventCreator (payload: P) {

      // Allow runtime payload checking for plain JavaScript usage
      if (options && payload) {
        const testFn = typeof options === "function" ? options : options.test;
        if (testFn && !testFn(payload)) {
          showWarning(
            `${JSON.stringify(payload)} does not match expected payload.`
          );
        }
      }

      return {
        type,
        payload
      };
    };
    eventCreator.eventType = type;
    eventCreator.toString = () => type; // allow String coercion to deliver the eventType
    return eventCreator;
  };
}

const event = createEventDefinition()("foo")();
event.payload; // void

const event2 = createEventDefinition<{foo:string, bar:number}>()("foo")(); // Expected 1 arguments, but got 0.

const event3 = createEventDefinition<{foo:string, bar:number}>()("foo")({foo:'asdasd', bar: 123});
event3.payload; // {foo:'asdasd', bar: 123}

[ryardley]

Closing this in favour of https://github.com/ryardley/ts-bus/pull/25