Downloading files with a Firefox addon - firefox

Question

I'm new to Firefox addon development, and it's going well so far, but I'm stuck on how to, essentially, download a file from the Web, given a URI, and save it to disk. Mozilla's MDN documentation has information on how to upload files, but the downloading files section is empty and yet to be written. Sadly, I haven't found any documentation which describes how to do this.
Does anyone know of relevant documentation on how to do this?
The old Facebook Photo Album Downloader addon uses this function call in its overlay JavaScript:
saveURL(images[i].replace(/\/s/g, "/n"), null, null, false, true, null);
Obviously, the first argument is the URI to request. The saveURL function isn't defined anywhere, so I assume it's an extension API function. I have tried it in my new addon, and it does work. I would, however, like to know what the other arguments mean.

Answer 1

The standard way to do this is with nsIWebBrowserPersist:
var persist =
Cc["#mozilla.org/embedding/browser/nsWebBrowserPersist;1"].
createInstance(Ci.nsIWebBrowserPersist);
persist.saveURI(serverURI, null, null, null, "", targetFile);
See https://developer.mozilla.org/en/Code_snippets/Downloading_Files for more info.

Answer 2

There actually is some MDN documentation on this: https://developer.mozilla.org/en/Code_snippets/Downloading_Files.

Answer 3

Here's an easy copy/paste option for anyone looking for a quick solution without any further messing about. Put it in your main.js and change the filename, directory and url.
function DownloadFile(sLocalFileName, sRemoteFileName)
{
var saveToDirectory = 'C:\\Users\\louis\\downloads\\';
var chrome = require("chrome");
var oIOService = chrome.Cc["#mozilla.org/network/io-service;1"].getService(chrome.Ci.nsIIOService)
var oLocalFile = chrome.Cc["#mozilla.org/file/local;1"].createInstance(chrome.Ci.nsILocalFile);
oLocalFile.initWithPath(saveToDirectory + sLocalFileName);
var oDownloadObserver = {onDownloadComplete: function(nsIDownloader, nsresult, oFile) {console.log('download complete...')}};
var oDownloader = chrome.Cc["#mozilla.org/network/downloader;1"].createInstance();
oDownloader.QueryInterface(chrome.Ci.nsIDownloader);
oDownloader.init(oDownloadObserver, oLocalFile);
var oHttpChannel = oIOService.newChannel(sRemoteFileName, "", null);
oHttpChannel.QueryInterface(chrome.Ci.nsIHttpChannel);
oHttpChannel.asyncOpen(oDownloader, oLocalFile);
}
DownloadFile("saveAsThis.mp3","http://domain.com/file.mp3");

Answer 4

As of 2015, the APIs for managing (starting, stopping, etc.) downloads have changed since this question was answered. The new APIs are (links to documentation on MDN):
Downloads.jsm
Download
DownloadTarget
PlacesUtils.There is good info at paa's answer to the question: API to modify Firefox downloads list