Async

Utils for async programming.

• PromiseResolver
• AsyncOperationManager

PromiseResolver

export type PromiseResolverState = 'running' | 'resolved' | 'rejected';

export class PromiseResolver<T> {
    readonly promise: Promise<T>;
    readonly state: PromiseResolverState;

    resolve(value?: T | PromiseLike<T>): void;
    reject(reason?: any): void;
}

C#, the origin of the async/await pattern, uses TaskCompletionSource to manage Tasks manually.

V8, the most commonly used JavaScript engine, uses PromiseResolver to manage Promises manually.

But in JavaScript, we only have the Promise constructor, and there is no way to know the Promise's state.

Now you can use this PromiseResolver class to create and manage Promises more conveniently.

function delay(timeout: number): Promise<void> {
    const resolver = new PromiseResolver<void>();
    setTimeout(() => resolver.resolve(), timeout);
    return resolver.promise;
}

AsyncOperationManager

export interface AsyncOperationInfo<T> {
    id: number;
    promise: Promise<T>;
}

export default class AsyncOperationManager {
    add<T>(): AsyncOperationInfo<T>;

    resolve<T>(id: number, result: T): void;
    reject(id: number, reason: Error): void;
}

Assume you have an RPC service, every operation has an ID, and the remote will return the result with this ID.

AsyncOperationManager can help you manage the IDs and convert callbacks to Promises.

declare const MyService;
const manager = new AsyncOperationManager();

MyService.on('complete', (id: number, result: number) => {
    manager.resolve(id, result);
});

MyService.on('error', (id: number, message: string) => {
    manager.reject(id, new Error(message));
});

function callService(payload: number): Promise<number> {
    const { id, promise } = manager.add<number>();
    MyService.post({ id, payload });
    return promise;
}