How to load images async with RxJs and perform a m

Question

I am trying to convert my promise based code to RxJs but have a hard time to get my head around Rx especially RxJs.

I have a an array with paths.

var paths = ["imagePath1","imagePath2"];

And I like to load images in Javascript

var img = new Image();
img.src = imagePath;
image.onload // <- when this callback fires I'll add them to the images array

and when all Images are loaded I like to execute a method on.

I know there is

Rx.Observable.fromArray(imagepathes)

there is also something like

Rx.Observable.fromCallback(...)

and there is something like flatMapLatest(...) And Rx.Observable.interval or timebased scheduler

Based on my research I would assume that these would be the ingredients to solve it but I cannot get the composition to work.

So how do I load images from a array paths and when all images are loaded I execute a method based on an interval?

Thanks for any help.

Answer 1

I don't think you can do that easily with observables, as there's nothing there to indicate a finish (unless you have an initial size). Look at the other answers for the Rx version.

However, you can use an array of Promises:

/**
 * Loads an image and returns a promise
 * @param {string} url - URL of image to load
 * @return {Promise<Image>} - Promise for an image once finished loading.
 */
function loadImageAsync(url) {
    return new Promise(function(resolve, reject) {
        var img = new Image();
        img.src = imagePath;
        image.onload = function() { resolve(img); };
        image.onerror = reject;
    });
}

And with that, you can easily do something like this:

var imageUrls = ['url1', 'url2', 'url3'];
Promise.all(imageUrls.map(loadImageAsync))
    .then(function(arrayOfImageElements) {
        // All done!
    });

Answer 2

At first you need a function that will create a Observable or Promise for separate image:

function loadImage(imagePath){
   return Rx.Observable.create(function(observer){
     var img = new Image();
     img.src = imagePath;
     img.onload = function(){
       observer.onNext(img);
       observer.onCompleted();
     }
     img.onError = function(err){
       observer.onError(err);
     }
   });
}

Than you can use it to load all images

Rx.Observable
  .fromArray(imagepathes)
  .concatMap(loadImage) // or flatMap to get images in load order
  .toArray()
  .subscribe(function(images){
    // do something with loaded images
  })

Answer 3

function loadImage(url){
    var img = new Image;
    img.src = url;
    var o = new Rx.Subject();
    img.onload = function(){ o.onNext(img); o.onCompleted(); };
    img.onerror = function(e){ o.onError(e); }; // no fromEvent for err handling
    return o;
}

var imageUrls = ['url1', 'url2', 'url3'];
var joined = Rx.Observable.merge(imageUrls.map(loadImage));

// consume one by one:
joined.subscribe(function(item){
    // wait for item
});

joined.toArray().subscribe(function(arr){
    // access results array in arr
});

Or in short:

var imageUrls = ['url1', 'url2', 'url3'];
fromArray(imageUrls).map(url => {
    var img = new Image;
    img.src = url;
    return fromEvent(img, "load");
}).toArray().subscribe(function(arr){
    // access results here
});

Answer 4

Here is the Angular / Typescript version to load an Image with RxJS:

import { Observable, Observer } from "rxjs"; 

public loadImage(imagePath: string): Observable<HTMLImageElement> {
  return Observable.create((observer: Observer<HTMLImageElement>) => {
    var img = new Image();
    img.src = imagePath;
    img.onload = () => {
      observer.next(img);
      observer.complete();
    };
    img.onerror = err => {
      observer.error(err);
    };
  });
}

Answer 5

The other RX based solutions here did not really work for me. Bogdan Savluk’s version did not work at all. Benjamin Gruenbaum’s version waits until an image is loaded before starting to load the next image so it gets really slow (correct me if I am wrong) Here is my solution which just compares the total amount of images with the number of already loaded images and if they are equal, the onNext() method of the returned Observable gets called with the array of images as an argument:

var imagesLoaded = function (sources) {

  return Rx.Observable.create(function (observer) {

    var numImages = sources.length
    var loaded = 0
    var images = []

    function onComplete (img) {
      images.push(img)
      console.log('loaded: ', img)

      loaded += 1
      if (loaded === numImages) {
        observer.onNext(images)
        observer.onCompleted()
      }
    }

    sources.forEach(function (src) {
      var img = new Image()
      img.onload = function () {
        onComplete(img)
      }
      console.log('add src: ' + src)
      img.src = src
      if (img.complete) {
        img.onload = null
        onComplete(img)
      }

    })

  })

}

Usage:

console.time('load images'); // start measuring execution time

imagesLoaded(sources)
  // use flatMap to get the individual images
  // .flatMap(function (x) {
  //   return Rx.Observable.from(x)
  // })

  .subscribe(function (x) {
    console.timeEnd('load images'); // see how fast this was
    console.log(x)
  })

Answer 6

I think you don't have to create an Observable yourself for this.

import { from, fromEvent } from 'rxjs';
import { mergeMap, map, scan, filter } from 'rxjs/operators';

const paths = ["imagePath1","imagePath2"];

from(paths).pipe(
   mergeMap((path) => {
      const img = new Image();

      img.src = path;
      return fromEvent(img, 'load').pipe(
          map((e) => e.target)
      );
   }),
   scan((acc, curr) => [...acc, curr], []),
   filter((images) => images.length === paths.length)
).subscribe((images) => {
   // do what you want with images
});