Cancel method is not propagated to then promises

[zakjan]

I'm submitting a feature request

• Library Version:
  1.2.1

Current behavior:
cancel method exists only on the promise returned by send method. If then is called on this promise, the returned promise doesn't have cancel method.

@autoinject
export class BackendRepository {
    constructor(private httpClient: HttpClient) {}

    getItems(): Promise<Item[]> {
        return this.httpClient.createRequest("/items").asGet().send()
            .then((responseData: any) => responseData.data);
    }
}

In this example, HTTP response needs to be unwrapped from a response envelope to return proper types to TS app. But it can't be cancelled in upper layers of the TS app, where the request was triggered.

Expected/desired behavior:

• What is the expected behavior?
  The promise returned by then should have cancel method.

• What is the motivation / use case for changing the behavior?
  To allow cancelling promises after callbacks are applied.

[zakjan]

My current solution looks like this:

export interface CancellablePromise<T> extends Promise<T> {
    cancel: () => void;
    abort: () => void;
}

export interface ThenableCancellablePromise<T> extends CancellablePromise<T> {
    // copied from lib.es6.d.ts
    then<TResult1 = T, TResult2 = never>(onfulfilled?: ((value: T) => TResult1 | PromiseLike<TResult1>) | undefined | null, onrejected?: ((reason: any) => TResult2 | PromiseLike<TResult2>) | undefined | null): ThenableCancellablePromise<TResult1 | TResult2>;
    _oldThen<TResult1 = T, TResult2 = never>(onfulfilled?: ((value: T) => TResult1 | PromiseLike<TResult1>) | undefined | null, onrejected?: ((reason: any) => TResult2 | PromiseLike<TResult2>) | undefined | null): CancellablePromise<TResult1 | TResult2>;
}

function thenableCancellablePromise<T>(promise: CancellablePromise<T>): ThenableCancellablePromise<T> {
    const newPromise = promise as any as ThenableCancellablePromise<T>;
    if (newPromise._oldThen) {
        return newPromise;
    }

    newPromise._oldThen = newPromise.then;
    newPromise.then = <TResult1 = T, TResult2 = never>(onfulfilled?: ((value: T) => TResult1 | PromiseLike<TResult1>) | undefined | null, onrejected?: ((reason: any) => TResult2 | PromiseLike<TResult2>) | undefined | null) => {
        const resultPromise = newPromise._oldThen(onfulfilled, onrejected);

        resultPromise.cancel = newPromise.cancel;
        resultPromise.abort = newPromise.abort;

        return thenableCancellablePromise(resultPromise);
    };

    return newPromise;
}

@autoinject
export class BackendRepository {
    constructor(private httpClient: HttpClient) {}

    getItems(): ThenableCancellablePromise<Item[]> {
        return thenableCancellablePromise(this.httpClient.createRequest("/items").asGet().send() as any as CancellablePromise<any>)
            .then((responseData: any) => responseData.data);
    }
}

I'm not sure if I can make a PR, since there is #120 opened for a long time with some activity, it could conflict.