es6/observable/BoundCallbackObservable.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 BoundCallbackObservable 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 callback API to a function that returns an Observable.
*
* <span class="informal">Give it a function `f` of type `f(x, callback)` and
* it will return a function `g` that when called as `g(x)` will output an
* Observable.</span>
*
* `bindCallback` 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 output of `bindCallback` 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` function
* calls its callback with one argument, the Observable will emit that value.
* If on the other hand callback is called with multiple values, resulting
* Observable will emit an array with these arguments.
*
* It is very important to remember, that input function `func` is not called
* when output function is, but rather when Observable returned by output
* function is subscribed. This means if `func` makes AJAX request, that request
* will be made every time someone subscribes to resulting Observable, but not before.
*
* Optionally, selector function can be passed to `bindObservable`. That function
* takes the same arguments as callback, and returns value
* that will be emitted by Observable instead of callback parameters themselves.
* Even though by default multiple arguments passed to callback appear in the stream as array,
* selector function will be called with arguments directly, just as callback would.
* This means you can imagine default selector (when one is not provided explicitly)
* as function that aggregates all its arguments into array, or simply returns first argument,
* if there is only one.
*
* Last optional parameter - {@link Scheduler} - can be used to control when call
* to `func` happens after someone subscribes to Observable, as well as when results
* passed to callback will be emitted. By default subscription to Observable calls `func`
* synchronously, but using `Scheduler.async` as last parameter will defer call to input function,
* just like wrapping that call in `setTimeout` with time `0` would. So if you use async Scheduler
* and call `subscribe` on output Observable, all function calls that are currently executing,
* will end before `func` is invoked.
*
* When it comes to emitting results passed to callback, by default they are emitted
* immediately after `func` invokes callback. In particular, if callback is called synchronously,
* then subscription to resulting Observable will call `next` function synchronously as well.
* If you want to defer that call, using `Scheduler.async` will, again, do the job.
* This means that by using `Scheduler.async` you can, in a sense, ensure that `func`
* always calls its callback asynchronously, thus avoiding terrifying Zalgo.
*
* Note that Observable created by output function will always emit only one value
* and then complete right after. Even if `func` calls callback multiple times, values from
* second and following calls will never appear in the stream. If you need to
* listen for multiple calls, you probably want to use {@link fromEvent} or
* {@link fromEventPattern} instead.
*
* If `func` depends on some context (`this` property), that context will be set
* to the same context that output function has at call time. In particular, if `func`
* is called as method of some object, in order to preserve proper behaviour,
* it is recommended to set context of output function to that object as well,
* provided `func` is not already bound.
*
* If input function calls its callback in "node style" (i.e. first argument to callback is
* optional error parameter signaling whether call failed or not), {@link bindNodeCallback}
* provides convenient error handling and probably is a better choice.
* `bindCallback` will treat such functions without any difference and error parameter
* (whether passed or not) will always be interpreted as regular callback argument.
*
*
* @example <caption>Convert jQuery's getJSON to an Observable API</caption>
* // Suppose we have jQuery.getJSON('/my/url', callback)
* var getJSONAsObservable = Rx.Observable.bindCallback(jQuery.getJSON);
* var result = getJSONAsObservable('/my/url');
* result.subscribe(x => console.log(x), e => console.error(e));
*
*
* @example <caption>Receive array of arguments passed to callback</caption>
* someFunction((a, b, c) => {
* console.log(a); // 5
* console.log(b); // 'some string'
* console.log(c); // {someProperty: 'someValue'}
* });
*
* const boundSomeFunction = Rx.Observable.bindCallback(someFunction);
* boundSomeFunction().subscribe(values => {
* console.log(values) // [5, 'some string', {someProperty: 'someValue'}]
* });
*
*
* @example <caption>Use bindCallback with selector function</caption>
* someFunction((a, b, c) => {
* console.log(a); // 'a'
* console.log(b); // 'b'
* console.log(c); // 'c'
* });
*
* const boundSomeFunction = Rx.Observable.bindCallback(someFunction, (a, b, c) => a + b + c);
* boundSomeFunction().subscribe(value => {
* console.log(value) // 'abc'
* });
*
*
* @example <caption>Compare behaviour with and without async Scheduler</caption>
* function iCallMyCallbackSynchronously(cb) {
* cb();
* }
*
* const boundSyncFn = Rx.Observable.bindCallback(iCallMyCallbackSynchronously);
* const boundAsyncFn = Rx.Observable.bindCallback(iCallMyCallbackSynchronously, null, Rx.Scheduler.async);
*
* boundSyncFn().subscribe(() => console.log('I was sync!'));
* boundAsyncFn().subscribe(() => console.log('I was async!'));
* console.log('This happened...');
*
* // Logs:
* // I was sync!
* // This happened...
* // I was async!
*
*
* @example <caption>Use bindCallback on object method</caption>
* const boundMethod = Rx.Observable.bindCallback(someObject.methodWithCallback);
* boundMethod.call(someObject) // make sure methodWithCallback has access to someObject
* .subscribe(subscriber);
*
*
* @see {@link bindNodeCallback}
* @see {@link from}
* @see {@link fromPromise}
*
* @param {function} func Function with a 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 callback would deliver.
* @static true
* @name bindCallback
* @owner Observable
*/
static create(func, selector = undefined, scheduler) {
return function (...args) {
return new BoundCallbackObservable(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;
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(BoundCallbackObservable.dispatch, 0, { source: this, subscriber, context: this.context });
}
}
static dispatch(state) {
const self = this;
const { source, subscriber, context } = state;
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;
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) {
subject.error(errorObject.e);
}
}
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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