async-case.ts

type PromiseState<T> = {
  state: 'fulfilled', value: T
} | {state: 'pending'} | {state: 'rejected', error: Error};


let wm = new WeakMap<Promise<any>, PromiseState<any>>()

export function asyncCase<T, U>(promise: Promise<T>, handlers: {
  pending?: () => U;
  fulfilled?: (t: T) => U;
  rejected?: (e: any) => U;
}) {
  if (!wm.has(promise)) {
    let fp = observable({state: 'pending', value: null, error: null});
    wm.set(promise, fp);
    promise.then(
      val => Object.assign(fp, {state: 'fulfilled', value: val}),
      err => Object.assign(fp, {state: 'rejected', error: err}));
  }
  let promiseData = wm.get(promise) as PromiseState<T>
  switch (promiseData.state) {
    case 'pending':
        return handlers.pending && handlers.pending()
    case 'rejected':
        return handlers.rejected && handlers.rejected(promiseData.error)
    case 'fulfilled':
        return handlers.fulfilled && handlers.fulfilled(promiseData.value)
  }
}