gistfile1.js

(function( global, undefined ) {

var slice = [].slice,
	subscriptions = {};

if (global.bus) {
  return; // avoid  existing instance override
}

var bus = global.bus = {
	trigger: function( topic ) {
		if ( typeof topic !== "string" ) {
			throw new Error( "You must provide a valid topic to publish." );
		}

		var args = slice.call( arguments, 1 ),
			topicSubscriptions,
			subscription,
			length,
			i = 0,
			ret;

		if ( !subscriptions[ topic ] ) {
			return true;
		}

		topicSubscriptions = subscriptions[ topic ].slice();
		for ( length = topicSubscriptions.length; i < length; i++ ) {
			subscription = topicSubscriptions[ i ];
			ret = subscription.callback.apply( subscription.context, args );
			if ( ret === false ) {
				break;
			}
		}
		return ret !== false;
	},

	on: function( topic, context, callback, priority ) {
		if ( typeof topic !== "string" ) {
			throw new Error( "You must provide a valid topic to create a subscription." );
		}

		if ( arguments.length === 3 && typeof callback === "number" ) {
			priority = callback;
			callback = context;
			context = null;
		}
		if ( arguments.length === 2 ) {
			callback = context;
			context = null;
		}
		priority = priority || 10;

		var topicIndex = 0,
			topics = topic.split( /\s/ ),
			topicLength = topics.length,
			added;
		for ( ; topicIndex < topicLength; topicIndex++ ) {
			topic = topics[ topicIndex ];
			added = false;
			if ( !subscriptions[ topic ] ) {
				subscriptions[ topic ] = [];
			}

			var i = subscriptions[ topic ].length - 1,
				subscriptionInfo = {
					callback: callback,
					context: context,
					priority: priority
				};

			for ( ; i >= 0; i-- ) {
				if ( subscriptions[ topic ][ i ].priority <= priority ) {
					subscriptions[ topic ].splice( i + 1, 0, subscriptionInfo );
					added = true;
					break;
				}
			}

			if ( !added ) {
				subscriptions[ topic ].unshift( subscriptionInfo );
			}
		}

		return callback;
	},

	off: function( topic, context, callback ) {
		if ( typeof topic !== "string" ) {
			throw new Error( "You must provide a valid topic to remove a subscription." );
		}

		if ( arguments.length === 2 ) {
			callback = context;
			context = null;
		}

		if ( !subscriptions[ topic ] ) {
			return;
		}

		var length = subscriptions[ topic ].length,
			i = 0;

		for ( ; i < length; i++ ) {
			if ( subscriptions[ topic ][ i ].callback === callback ) {
				if ( !context || subscriptions[ topic ][ i ].context === context ) {
					subscriptions[ topic ].splice( i, 1 );
					
					// Adjust counter and length for removed item
					i--;
					length--;
				}
			}
		}
	}
};

}( this ) );