Support React Native EventEmitter type with fromEvent

[ajcrites]

Feature Request

React Native defines its own EventEmitter type that it claims is a subset of the Node.js EventEmitter type, but it is a bit different: https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/react-native/index.d.ts#L143

Specifically, removeListener returns void. The return type of addListener and parameters are also different, but this doesn't cause a typing problem.

Is your feature request related to a problem? Please describe.
If you attempt to use fromEvent with a React Native EventEmitter implementation such as https://github.com/urbanairship/react-native-module you will get a type error:

fromEvent(UrbanAirship, 'pushReceived');

Argument of type 'typeof UrbanAirship' is not assignable to parameter of type 'FromEventTarget<{}>'.
  Type 'typeof UrbanAirship' is not assignable to type 'ArrayLike<EventTargetLike<{}>>'.
    Index signature is missing in type 'typeof UrbanAirship'.

Describe the solution you'd like
There are at least two solutions:

1. Add a ReactNativeEventEmitter type.
2. Allow addListener and removeListener for NodeStyleEventEmitter to return any.

The isNodeStyleEventEmitter check can be used for both, but unfortunately I don't think there is a way to know whether the provided EventEmitter is Node.js style or React Native except based on type. However, the return types for addListener and removeListener are not used, so being able to distinguish between them at runtime won't matter at this time.

Describe alternatives you've considered
You can of course do fromEvent(UrbanAirship as any... or use fromEventPattern, but these are more verbose and you lose type safety you could get from the former.

[cartant]

I would like to see the Node-style declarations relaxed, too.

But rather than any, I'd like to see the return values declared as void | {}. As @ajcrites has pointed out, the return values are not used in RxJS, so this should not be a problem.

Did you want to submit a PR for this, Andrew, or do you want me to do it?

[ajcrites]

I'll be happy to -- other than changing the return type to void | {}, React Native's EventEmitter.addListener handler returns EmitterSubscription, so it is incompatible with NodeEventHandler. Can we change that return type to void | {} as well?

[cartant]

I don't think NodeEventHandler needs to change, but I do think symbol needs to be removed from the first parameter's union type:

interface NodeStyleEventEmitter {
    addListener: (eventName: string | symbol, handler: NodeEventHandler) => this;
    removeListener: (eventName: string | symbol, handler: NodeEventHandler) => this;
}
declare type NodeEventHandler = (...args: any[]) => void;

interface RelaxedNodeStyleEventEmitter {
    addListener: (eventName: string, handler: NodeEventHandler) => void | {};
    removeListener: (eventName: string, handler: NodeEventHandler) => void | {};
}

interface EmitterSubscription {
    context: any;
}
interface EventEmitterListener {
    addListener(eventType: string, listener: (...args: any[]) => any, context?: any): EmitterSubscription;
}
interface EventEmitter extends EventEmitterListener {
    removeListener(eventType: string, listener: (...args: any[]) => any): void;
}

declare const nodeEmitter: NodeStyleEventEmitter;
declare const reactNativeEmitter: EventEmitter;

const e1: NodeStyleEventEmitter = reactNativeEmitter; // Error
const e2: RelaxedNodeStyleEventEmitter = reactNativeEmitter; // OK
const e3: RelaxedNodeStyleEventEmitter = nodeEmitter; // OK

When you submit a PR, could you please add some type tests to make sure that actual Node-style event emitters and React Native event emitters play nice with fromEvent. Just use interface declarations, like those above, within each type test.

See here for an example.