es6/observable/BoundNodeCallbackObservable.js
import { Observable } from '../Observable';
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 extends Observable {
constructor(callbackFunc, selector, args, context, scheduler) {
super();
this.callbackFunc = callbackFunc;
this.selector = selector;
this.args = args;
this.context = context;
this.scheduler = scheduler;
}
/* 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(func, selector = undefined, scheduler) {
return function (...args) {
return new BoundNodeCallbackObservable(func, selector, args, this, scheduler);
};
}
_subscribe(subscriber) {
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();
const handler = function handlerFn(...innerArgs) {
const source = 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.
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 });
}
}
}
function dispatch(state) {
const self = this;
const { source, subscriber, context } = state;
// XXX: cast to `any` to access to the private field in `source`.
const { callbackFunc, args, scheduler } = source;
let subject = source.subject;
if (!subject) {
subject = source.subject = new AsyncSubject();
const handler = function handlerFn(...innerArgs) {
const source = 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
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));
}
function dispatchNext(arg) {
const { value, subject } = arg;
subject.next(value);
subject.complete();
}
function dispatchError(arg) {
const { err, subject } = arg;
subject.error(err);
}
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