gistfile1.js

var Image = require('parse-image');
var Comic = Parse.Object.extend('Comic');

Parse.Cloud.job('updateCache', function(request, response) {
	getLatestStoredNum(function(num) {
		console.log('Latest stored comic is ' + num);
		checkForAndCacheNewComicsSince(num, function(success) {
			if (success)
				response.success('OK');
			else
				response.error('failed to fetch & cache');
		});
	})
});

function getLatestStoredNum(callback) {
	var query = new Parse.Query(Comic);
	query.select('num');
	query.descending('num');
	query.limit(1);
	query.find().then(function(result) {
		callback(result.length ? result[0].get('num') : null);
	});
}

function checkForAndCacheNewComicsSince(sinceNum, callback) {
	// if we didn't get a number, we don't have any we should cache all since 0
	sinceNum || (sinceNum = 0);

	// fetch the latest comic
	fetchComic(null, function(comic) {
		// if we didn't get a comic, bail out
		if (!comic) return callback(false);

		// if this comic number is the same as our "since number",
		// there are no updates and we have succeeded at... something
		if (comic.get('num') == sinceNum) {
			console.log('No updates');
			return callback(true);
		}

		// debug!
		console.log('Latest comic is ' + comic.get('num'));

		// ok, fine, we need to fetch and cache all comics since the last one we have
		// oh and by the way, here's the latest one
		fetchAndCacheComicsSinceWithLatestComic(sinceNum, callback, comic);
	});
}

function fetchAndCacheComicsSinceWithLatestComic(sinceNum, callback, latestComic) {
	// set our current number as 1 above the "since" number
	var currentNum = sinceNum + 1;

	// attach image dimensions to the latest
	attachDimensions(latestComic, function(success) {
		// fail?
		if (!success) return callback(false);

		// save our latest
		persistComic(latestComic, function(success) {
			// fail?
			if (!success) return callback(false);

			// begin downloading the rest
			__downloadNext();
		});
	});

	// callback
	function __downloadNext() {
		// if the current number is equal to the latest comic's number, we're done!
		if (currentNum == latestComic.get('num')) return callback(true);

		// fetch the comic
		fetchComic(currentNum, function(comic) {
			// if we didn't get a comic, failure :(
			if (!comic) return callback(false);

			// attach the image dimensions
			attachDimensions(comic, function(success) {
				// fail?
				if (!success) return callback(false);

				// save it
				persistComic(comic, function(success) {
					// fail?
					if (!success) return callback(false);

					// increment our current number
					currentNum++;

					// if the number is 404, skip it, because Randall's a funny bitch
					if (currentNum == 404) currentNum++;

					// proceed to the next
					__downloadNext();
				});
			});
		});
	}
}

function fetchComic(id, callback) {
	var url = 'http://xkcd.com/';
	id && (url += id + '/');
	url += 'info.0.json';

	Parse.Cloud.httpRequest({
		url: url
	}).then(function(httpResponse) {
		if (!httpResponse.data || !httpResponse.data.num) {
			console.error('Unexpected response from URL ' + url);
			callback(false);
		}
		else {
			console.log('Fetched comic ' + httpResponse.data.num);
			var comic = new Comic;
			for (key in httpResponse.data)
				comic.set(key, httpResponse.data[key]);
			callback(comic);
		}
	}, function(httpResponse) {
		console.error('Request failed with response code ' + httpResponse.status + ' to URL ' + url);
		callback(false);
	});
}

function attachDimensions(comic, callback) {
	Parse.Cloud.httpRequest({
		url: comic.get('img'),
		success: function(response) {
			console.log('Retrieved image for comic ' + comic.get('num'));

			var image = new Image();
			image.setData(response.buffer, {
				success: function() {
					comic.set('img_w', image.width());
					comic.set('img_h', image.height());
					callback(true);
				},
				error: function(error) {
					console.error('Failed to load image for comic ' + comic.get('num') + ': ' + error.code);
					callback(false);
				}
			});
		},
		error: function(error) {
			console.error('Failed to receive image for comic ' + comic.get('num') + ' from URL ' + comic.get('img') + ': ' + error.code);
			callback(false);
		}
	});
}

function persistComic(comic, callback) {
	comic.save(null, {
		success: function() {
			console.log('Stored comic ' + comic.get('num'));
			callback(true);
		},
		error: function(error) {
			console.log('Failed to store comic ' + comic.get('num') + ': ' + error.code);
			callback(false);
		}
	});
}