Refactoring Code in to an Object-Oriented Paradigm #4: Extendible, Readable, Descriptive

This is article number 4 of a series of article, which simply document my refactoring of some code I originally wrote (pretty poorly) last year. To get the jist of what’s going on. Check the original post – Refactoring Code in to an Object-Oriented Paradigm.

Making code Extensive, Readable and Descriptive

We’re nearly there with the Javascript, the worst has been cleaned up generally. Now we’re getting on to being developer-friendly, which is pretty important, you want people to extend your program. Even if you want to have control over the way they interface with your program, you still want people to interface with it somehow.

There are a few ways you can make things easier for developers, we’ve reorganised the code in the previous step, but it’s an important step, just by putting the constructor code up the top, you can suddenly look at the code and understand it just by scrolling down the page once. At least, that’s the idea, but sometimes it doesn’t hurt to actually explain what a method is doing, or what a specifically tricky part of the code does. This is why we use comments above each function, and in places actually within the code.

Finally, while it’s not quite keeping exactly the same functionality, we’re going to implement a system which allows other functions to listen for certain events. This means there’s a properly implemented way of extending our code. To ease our compatibility stage at the end, I’m going to use a custom function for custom event handling, that I’ve developed myself.

/** AutoScrobbler is a bookmarklet/plugin which extends the Universal Scrobbler
*   web application, allowing automatic scrobbling of frequently updating track
*   lists such as radio stations.
*
*   This is the constructor, injecting the user controls and starting the first
*   scrobble.
*/
function AutoScrobbler() {

var userControls = "<div id=\"autoScrobbler\" style=\"background: #FFFFFF; border-top: 1px solid #000000; border-left: 1px solid #000000; position: fixed; bottom: 0; height: 50px; width: inherit;\">"+
"<input id=\"autoScrobblerStart\" type=\"button\" value=\"Start auto-scrobbling\" onclick=\"autoScrobbler.start();\" /> | <input id=\"autoScrobblerStop\" type=\"button\" value=\"Stop auto-scrobbling\" onclick=\"autoScrobbler.stop();\" />"+
"<p><span id=\"autoScrobblerScrobbleCount\">0</span> tracks scrobbled</p>"+
"</div>";
document.querySelector("#disclaimersContainer").innerHTML += userControls;
this.startElm = document.getElementById("autoScrobblerStart");
this.stopElm = document.getElementById("autoScrobblerStop");
this.loopUID = -1;
this.lastTrackUID = undefined;
this.scrobbled = 0;
this.countReport = document.getElementById("autoScrobblerTracksScrobbled");
this.evtInit(["addLatest", "loadThenAdd", "start", "stop"]);
this.listen("addLatest", this.reportScrobble);
this.start();

}

/** Hashing function for event listener naming. Similar implementation to
*   Java's hashCode function. Hash collisions are possible.
*
*   @param toHash The entity to hash (the function will attempt to convert
*                 any variable type to a string before hashing)
*   @return A number up to 11 digits long identifying the entity.
*/
AutoScrobbler.prototype.hasher = function(toHash) {

var hash = 0;
toHash = "" + toHash;
for (var i = 0; i < toHash.length; i++)

hash = ((hash << 5) - hash) + hash.charCodeAt(i);

}

/** Custom event initiator for events in AutoScrobbler.
*
*   @param eventName The name of the event. This may be an array of names.
*/
AutoScrobbler.prototype.evtInit = function(eventName) {

//Initialise the evtLstnrs object and the event register if it doesn't exist.
if (typeof this.evtLstnrs == "undefined")

this.evtLstnrs = {"_EVTLST_reg": {}};

if (typeof eventName == "object") {

for (var i = 0; i < eventName.length; i++) {

var event = eventName[i];
this.evtLstnrs[""+event] = [];

}

} else

this.evtLstnrs[""+eventName] = [];

}

/** Custom event listener for events in AutoScrobbler.
*
*   @param toWhat A string specifying which event to listen to.
*   @param fcn A function to call when the event happens.
*   @return A boolean value, true if the listener was successfully set. False
*           otherwise.
*/
AutoScrobbler.prototype.listen = function(toWhat, fcn) {

if (this.evtLstnrs.hasOwnProperty(toWhat)) {

//Naming the function so we can remove it if required. Uses hasher.
var fcnName = this.hasher(fcn);

//Add the function to the list.
var event = this.evtLstnrs[toWhat];
event[event.length] = fcn;
this.evtLstnrs._EVTLST_reg[toWhat+"->"+fcnName] = event.length;
return true;

} else

return false;

}

/** Custom event listener trigger for events in AutoScrobbler
*
*   @param what Which event has happened.
*/
AutoScrobbler.prototype.trigger = function (what) {

if (this.evtLstnrs.hasOwnProperty(what)) {

var event = this.evtLstnrs[what];
for (var i = 0; i < event.length; i++)

event[i]();

}

}

/** Custom event listener removal for events in AutoScrobbler
*
*   @param toWhat A string to specify which event to stop listening to.
*   @param fcn The function which should no longer be called.
*   @return A boolean value, true if removal was successful, false otherwise.
*/
AutoScrobbler.prototype.unlisten = function(toWhat, fcn) {

var fcnName = this.hasher(fcn);
if (this.evtLstnrs._EVTLST_reg[toWhat+"->"+fcnName) {

var event = this.evtLstnrs[toWhat];
var fcnPos = this.evtLstnrs._EVTLST_reg[toWhat+"->"+fcnName];
event[fcnPos] = void(0);
delete this.evtLstnrs._EVTLST_reg[toWhat+"->"+fcnName];

return true;

}

return false;

}

/** Starts the auto-scrobbler, scrobbles immediately and schedules an update
*   every 5 minutes.
*/
AutoScrobbler.prototype.start = function() {

this.loadThenAdd();
autoScrobbler.loopUID = setInterval(this.loadThenAdd, 300000);
autoScrobbler.start.disabled = true;
autoScrobbler.stop.disabled = false;

}

/** Stops the auto-scrobbler, ends the recurring update and zeros the required
*   variables.
*/
AutoScrobbler.prototype.stop = function() {

clearInterval(this.loopUID);
this.lastTrackUID = undefined;
this.loopUID = -1;
this.stop.disabled = true;
this.start.disabled = false;

}

/** Loads the new track list using Universal Scrobbler and schedules a scrobble
*   of the latest tracks 30 seconds afterwards.
*/
AutoScrobbler.prototype.loadThenAdd = function() {

doRadioSearch();
setTimeout(this.addLatest, 30000);

}

/** Selects all the tracks which have not been seen before and scrobbles them
*   using Universal Scrobbler.
*/
AutoScrobbler.prototype.addLatest = function() {

var tracks = document.querySelectorAll(".userSong");
this.lastTrackUID = (typeof this.lastTrackUID == "undefined") ? tracks[1].querySelector("input").value : this.lastTrackUID;

//Check every checkbox until the last seen track is recognised.
for (var i = 0; i < tracks.length; i++) {

var item = tracks[i];
if (item.querySelector("input").value == this.lastTrackUID) {

i = tracks.length;
this.lastTrackUID = tracks[0].querySelector("input").value;

} else {

item.querySelector("input").checked = true;
this.scrobbled++;

}

}
doUserScrobble();
this.trigger("addLatest");

}

/** Updates the user interfaces to reflect new scrobbles.
*/
AutoScrobbler.prototype.reportScrobble = function() {

this.countReport.innerHTML = this.scrobbled;

}

// Create a new instance of the AutoScrobbler.
autoScrobbler = new AutoScrobbler();

Reorganisation

There’s quite a few changes to the structure of the code here. You’ll notice that the code now reads in the same way as the code would be used. When you run the bookmarklet initially, you’d run the constructor code, which then calls start(), which in turn calls loadAndAdd() and so on.

The only exception to this is the stop() command, starting and finishing methods like these are often grouped at the top of the code, to make it clear to the programmer what the basic functions are. As stop() doesn’t use any method which hasn’t been seen further up the code, it also won’t confuse the programmer, as it’s clear how the method works, without knowing anymore about the code.

Comments

You’ll also notice that each method has a comment to go with it, and in certain places, helpful hints have been added in the code. This can be very important for your code, as your way of thinking about the architecture of the code is very rarely the same as someone else’s way of thinking. Comments communicate in natural language, what the function or method’s overall goal is, they should generally not dissect the code and describe each operation (this can be done within the code if really required).

The way that I’ve written them is one of several loose standards for commenting. This style is similar to the way comments are written in Java. It is often useful to write comments in a common standard, as developers will be able to understand them more immediately, but other programs may also be able to understand them. There are many programs which have been written to interpret comments like these into formatted technical documentation (The type that you’d see on the MDN, PHP.net or Oracle Docs), saving you a great deal of time and effort and being a neighbourly developer to anyone that wants to look, use or edit your code.

I’ve used a few of these programs myself, I’ve had most success with Natural Docs (for Javascript, though it has support for other languages), PHP Documentor (A bit of a fiddle, but made great PHP documentation) and Oracle’s own Javadoc (For Java of course).

Extensibility

You will also notice that I’ve added three new methods, two of which help me and other developers extend the code in a documented way. The alternative to this would be for other developers to rely on the variables and functions to remain the same in name and the action they perform, this is a bad idea. The two new functions (listen() and trigger()), allow any other functions to hook on to the function/event that they want to know about and have a function called when the function/event happens.

By using this system, events can be named independently of the internal workings of the code, which means more stability for us and other developers.

The third function is a demonstration of this implemented extensibility, in the constructor method, I start “listening” for the “addLatest” event with reportScrobble(), which for now just signifies that the addLatest() method has run and calls reportScrobble() to update any user interface. The event gets triggered simply by calling this.trigger(“addLatest”).

I haven’t included a “removeListen” function here, as it gets quite complicated in pure Javascript, but this would also be possible, and would allow code to stop listening to our events.

Note (07/02/2013)

I have since added a remove and a evtInit function to the event listener mechanism (which I have added above in the code). It has slightly complicated the implementation and you’ll notice a new hasher function which is used to name functions in a way that they can be identified later. I decided that it’d be better to have a custom implementation which was at least complete (often an issue with custom implementations). The rest of the code here is unchanged (except for the initiation of events), I have in fact refactored my own code, making sure that all existing functionality remains.

I have further researched native implementations for custom events, which I haven’t used because of issues with compatibility (the final article in the series will have more on this). If you are interested in using these native methods, Sitepoint has a pretty good article for the basics.

In terms of refactoring our Javascript, we’re just about done. We now have a far cleaner implementation, which is now a lot more developer friendly. But for all the code, there’s a user interface, and everything about it has been neglected thus far, so next we’ll be looking at Cleaning Up Your Messy User Controls.

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