aladin-lite/_canvas/js/Downloader.js

/******************************************************************************
 * Aladin HTML5 project
 * 
 * File Downloader
 * Queue downloading for image elements
 * 
 * Author: Thomas Boch[CDS]
 * 
 *****************************************************************************/

Downloader = (function() {
	var NB_MAX_SIMULTANEOUS_DL = 4;
	// TODO : le fading ne marche pas bien actuellement
	var FADING_ENABLED = false;
	var FADING_DURATION = 700; // in milliseconds
	
	
	var Downloader = function(view) {
		this.view = view; // reference to the view to be able to request redraw
		this.nbDownloads = 0; // number of current downloads
		this.dlQueue = []; // queue of items being downloaded
        this.urlsInQueue = {};
	};
	
	Downloader.prototype.requestDownload = function(img, url) {
        // first check if url already in queue
        if (url in this.urlsInQueue)  {
            return;
        }
		// put in queue
		this.dlQueue.push({img: img, url: url});
        this.urlsInQueue[url] = 1;
		
		this.tryDownload();
	};
	
	// try to download next items in queue if possible
	Downloader.prototype.tryDownload = function() {
		while (this.dlQueue.length>0 && this.nbDownloads<NB_MAX_SIMULTANEOUS_DL) {
			this.startDownloadNext();
		}
	};
	
	Downloader.prototype.startDownloadNext = function() {
		// get next in queue
		var next = this.dlQueue.shift();
		if ( ! next) {
			return;
		}

		
		this.nbDownloads++;
		var downloaderRef = this;
		next.img.onload = function() {
			downloaderRef.completeDownload(this, true); // in this context, 'this' is the Image
		};
			
		next.img.onerror = function() {

			downloaderRef.completeDownload(this, false); // in this context, 'this' is the Image
		};
		
		next.img.src = next.url;
	};
	
	Downloader.prototype.completeDownload = function(img, success) {
        delete this.urlsInQueue[img.src];
		img.onerror = null;
		img.onload = null;
		this.nbDownloads--;
		if (success) {
			if (FADING_ENABLED) {
				var now = new Date().getTime();
				img.fadingStart = now;
				img.fadingEnd = now + FADING_DURATION;
			}
			this.view.requestRedraw();
		}
		
		this.tryDownload();
	};
	
	
	
	return Downloader;
})();