rxjs/src/observable/BoundNodeCallbackObservable.ts

Reactive Extensions for modern JavaScript

import { Observable } from '../Observable';
import { Subscriber } from '../Subscriber';
import { Subscription } from '../Subscription';
import { IScheduler } from '../Scheduler';
import { Action } from '../scheduler/Action';
import { tryCatch } from '../util/tryCatch';
import { errorObject } from '../util/errorObject';
import { AsyncSubject } from '../AsyncSubject';

/**
 * We need this JSDoc comment for affecting ESDoc.
 * @extends {Ignored}
 * @hide true
 */
export class BoundNodeCallbackObservable<T> extends Observable<T> {
  subject: AsyncSubject<T>;

  /* tslint:disable:max-line-length */
  static create<R>(callbackFunc: (callback: (err: any, result: R) => any) => any, selector?: void, scheduler?: IScheduler): () => Observable<R>;
  static create<T, R>(callbackFunc: (v1: T, callback: (err: any, result: R) => any) => any, selector?: void, scheduler?: IScheduler): (v1: T) => Observable<R>;
  static create<T, T2, R>(callbackFunc: (v1: T, v2: T2, callback: (err: any, result: R) => any) => any, selector?: void, scheduler?: IScheduler): (v1: T, v2: T2) => Observable<R>;
  static create<T, T2, T3, R>(callbackFunc: (v1: T, v2: T2, v3: T3, callback: (err: any, result: R) => any) => any, selector?: void, scheduler?: IScheduler): (v1: T, v2: T2, v3: T3) => Observable<R>;
  static create<T, T2, T3, T4, R>(callbackFunc: (v1: T, v2: T2, v3: T3, v4: T4, callback: (err: any, result: R) => any) => any, selector?: void, scheduler?: IScheduler): (v1: T, v2: T2, v3: T3, v4: T4) => Observable<R>;
  static create<T, T2, T3, T4, T5, R>(callbackFunc: (v1: T, v2: T2, v3: T3, v4: T4, v5: T5, callback: (err: any, result: R) => any) => any, selector?: void, scheduler?: IScheduler): (v1: T, v2: T2, v3: T3, v4: T4, v5: T5) => Observable<R>;
  static create<T, T2, T3, T4, T5, T6, R>(callbackFunc: (v1: T, v2: T2, v3: T3, v4: T4, v5: T5, v6: T6, callback: (err: any, result: R) => any) => any, selector?: void, scheduler?: IScheduler): (v1: T, v2: T2, v3: T3, v4: T4, v5: T5, v6: T6) => Observable<R>;
  static create<T>(callbackFunc: Function, selector?: void, scheduler?: IScheduler): (...args: any[]) => Observable<T>;
  static create<T>(callbackFunc: Function, selector?: (...args: any[]) => T, scheduler?: IScheduler): (...args: any[]) => Observable<T>;
  /* tslint:enable:max-line-length */

  /**
   * Converts a Node.js-style callback API to a function that returns an
   * Observable.
   *
   * <span class="informal">It's just like {@link bindCallback}, but the
   * callback is expected to be of type `callback(error, result)`.</span>
   *
   * `bindNodeCallback` is not an operator because its input and output are not
   * Observables. The input is a function `func` with some parameters, but the
   * last parameter must be a callback function that `func` calls when it is
   * done. The callback function is expected to follow Node.js conventions,
   * where the first argument to the callback is an error object, signaling
   * whether call was successful. If that object is passed to callback, it means
   * something went wrong.
   *
   * The output of `bindNodeCallback` is a function that takes the same
   * parameters as `func`, except the last one (the callback). When the output
   * function is called with arguments, it will return an Observable.
   * If `func` calls its callback with error parameter present, Observable will
   * error with that value as well. If error parameter is not passed, Observable will emit
   * second parameter. If there are more parameters (third and so on),
   * Observable will emit an array with all arguments, except first error argument.
   *
   * Optionally `bindNodeCallback` accepts selector function, which allows you to
   * make resulting Observable emit value computed by selector, instead of regular
   * callback arguments. It works similarly to {@link bindCallback} selector, but
   * Node.js-style error argument will never be passed to that function.
   *
   * Note that `func` will not be called at the same time output function is,
   * but rather whenever resulting Observable is subscribed. By default call to
   * `func` will happen synchronously after subscription, but that can be changed
   * with proper {@link Scheduler} provided as optional third parameter. Scheduler
   * can also control when values from callback will be emitted by Observable.
   * To find out more, check out documentation for {@link bindCallback}, where
   * Scheduler works exactly the same.
   *
   * As in {@link bindCallback}, context (`this` property) of input function will be set to context
   * of returned function, when it is called.
   *
   * After Observable emits value, it will complete immediately. This means
   * even if `func` calls callback again, values from second and consecutive
   * calls will never appear on the stream. If you need to handle functions
   * that call callbacks multiple times, check out {@link fromEvent} or
   * {@link fromEventPattern} instead.
   *
   * Note that `bindNodeCallback` can be used in non-Node.js environments as well.
   * "Node.js-style" callbacks are just a convention, so if you write for
   * browsers or any other environment and API you use implements that callback style,
   * `bindNodeCallback` can be safely used on that API functions as well.
   *
   * Remember that Error object passed to callback does not have to be an instance
   * of JavaScript built-in `Error` object. In fact, it does not even have to an object.
   * Error parameter of callback function is interpreted as "present", when value
   * of that parameter is truthy. It could be, for example, non-zero number, non-empty
   * string or boolean `true`. In all of these cases resulting Observable would error
   * with that value. This means usually regular style callbacks will fail very often when
   * `bindNodeCallback` is used. If your Observable errors much more often then you
   * would expect, check if callback really is called in Node.js-style and, if not,
   * switch to {@link bindCallback} instead.
   *
   * Note that even if error parameter is technically present in callback, but its value
   * is falsy, it still won't appear in array emitted by Observable or in selector function.
   *
   *
   * @example <caption>Read a file from the filesystem and get the data as an Observable</caption>
   * import * as fs from 'fs';
   * var readFileAsObservable = Rx.Observable.bindNodeCallback(fs.readFile);
   * var result = readFileAsObservable('./roadNames.txt', 'utf8');
   * result.subscribe(x => console.log(x), e => console.error(e));
   *
   *
   * @example <caption>Use on function calling callback with multiple arguments</caption>
   * someFunction((err, a, b) => {
   *   console.log(err); // null
   *   console.log(a); // 5
   *   console.log(b); // "some string"
   * });
   * var boundSomeFunction = Rx.Observable.bindNodeCallback(someFunction);
   * boundSomeFunction()
   * .subscribe(value => {
   *   console.log(value); // [5, "some string"]
   * });
   *
   *
   * @example <caption>Use with selector function</caption>
   * someFunction((err, a, b) => {
   *   console.log(err); // undefined
   *   console.log(a); // "abc"
   *   console.log(b); // "DEF"
   * });
   * var boundSomeFunction = Rx.Observable.bindNodeCallback(someFunction, (a, b) => a + b);
   * boundSomeFunction()
   * .subscribe(value => {
   *   console.log(value); // "abcDEF"
   * });
   *
   *
   * @example <caption>Use on function calling callback in regular style</caption>
   * someFunction(a => {
   *   console.log(a); // 5
   * });
   * var boundSomeFunction = Rx.Observable.bindNodeCallback(someFunction);
   * boundSomeFunction()
   * .subscribe(
   *   value => {}             // never gets called
   *   err => console.log(err) // 5
   *);
   *
   *
   * @see {@link bindCallback}
   * @see {@link from}
   * @see {@link fromPromise}
   *
   * @param {function} func Function with a Node.js-style callback as the last parameter.
   * @param {function} [selector] A function which takes the arguments from the
   * callback and maps those to a value to emit on the output Observable.
   * @param {Scheduler} [scheduler] The scheduler on which to schedule the
   * callbacks.
   * @return {function(...params: *): Observable} A function which returns the
   * Observable that delivers the same values the Node.js callback would
   * deliver.
   * @static true
   * @name bindNodeCallback
   * @owner Observable
   */
  static create<T>(func: Function,
                   selector: Function | void = undefined,
                   scheduler?: IScheduler): (...args: any[]) => Observable<T> {
    return function(this: any, ...args: any[]): Observable<T> {
      return new BoundNodeCallbackObservable<T>(func, <any>selector, args, this, scheduler);
    };
  }

  constructor(private callbackFunc: Function,
              private selector: Function,
              private args: any[],
              private context: any,
              public scheduler: IScheduler) {
    super();
  }

  protected _subscribe(subscriber: Subscriber<T | T[]>): Subscription {
    const callbackFunc = this.callbackFunc;
    const args = this.args;
    const scheduler = this.scheduler;
    let subject = this.subject;

    if (!scheduler) {
      if (!subject) {
        subject = this.subject = new AsyncSubject<T>();
        const handler = function handlerFn(this: any, ...innerArgs: any[]) {
          const source = (<any>handlerFn).source;
          const { selector, subject } = source;
          const err = innerArgs.shift();

          if (err) {
            subject.error(err);
          } else if (selector) {
            const result = tryCatch(selector).apply(this, innerArgs);
            if (result === errorObject) {
              subject.error(errorObject.e);
            } else {
              subject.next(result);
              subject.complete();
            }
          } else {
            subject.next(innerArgs.length <= 1 ? innerArgs[0] : innerArgs);
            subject.complete();
          }
        };
        // use named function instance to avoid closure.
        (<any>handler).source = this;

        const result = tryCatch(callbackFunc).apply(this.context, args.concat(handler));
        if (result === errorObject) {
          subject.error(errorObject.e);
        }
      }
      return subject.subscribe(subscriber);
    } else {
      return scheduler.schedule(dispatch, 0, { source: this, subscriber, context: this.context });
    }
  }
}

interface DispatchState<T> {
  source: BoundNodeCallbackObservable<T>;
  subscriber: Subscriber<T>;
  context: any;
}

function dispatch<T>(this: Action<DispatchState<T>>, state: DispatchState<T>) {
  const self = (<Subscription> this);
  const { source, subscriber, context } = state;
  // XXX: cast to `any` to access to the private field in `source`.
  const { callbackFunc, args, scheduler } = source as any;
  let subject = source.subject;

  if (!subject) {
    subject = source.subject = new AsyncSubject<T>();

    const handler = function handlerFn(this: any, ...innerArgs: any[]) {
      const source = (<any>handlerFn).source;
      const { selector, subject } = source;
      const err = innerArgs.shift();

      if (err) {
        self.add(scheduler.schedule(dispatchError, 0, { err, subject }));
      } else if (selector) {
        const result = tryCatch(selector).apply(this, innerArgs);
        if (result === errorObject) {
          self.add(scheduler.schedule(dispatchError, 0, { err: errorObject.e, subject }));
        } else {
          self.add(scheduler.schedule(dispatchNext, 0, { value: result, subject }));
        }
      } else {
        const value = innerArgs.length <= 1 ? innerArgs[0] : innerArgs;
        self.add(scheduler.schedule(dispatchNext, 0, { value, subject }));
      }
    };
    // use named function to pass values in without closure
    (<any>handler).source = source;

    const result = tryCatch(callbackFunc).apply(context, args.concat(handler));
    if (result === errorObject) {
      self.add(scheduler.schedule(dispatchError, 0, { err: errorObject.e, subject }));
    }
  }

  self.add(subject.subscribe(subscriber));
}

interface DispatchNextArg<T> {
  subject: AsyncSubject<T>;
  value: T;
}
function dispatchNext<T>(arg: DispatchNextArg<T>) {
  const { value, subject } = arg;
  subject.next(value);
  subject.complete();
}

interface DispatchErrorArg<T> {
  subject: AsyncSubject<T>;
  err: any;
}
function dispatchError<T>(arg: DispatchErrorArg<T>) {
  const { err, subject } = arg;
  subject.error(err);
}