Archive for April, 2014

Being objective about Objective C

(Sorry, couldn’t resist the pun.) A few months ago, I started a new job that required me to learn Objective C and Cocoa/Cocoa Touch development on the Apple platform. In this post, I’ll relate a few concepts that I learned in other languages that crossed over to the Apple platform.

MVC architecture

In web app development, the view is (arguably) the DOM. The DOM is loaded at a certain point in time: when the browser is finished parsing the initial payload, a load event is generated, and as a developer you can specify a handler for this event. This allows you to do things when that view appears, like populate it with data.

In iOS, the load event is similar to viewWillAppear. A NSViewController subclass can implement this method to perform custom behavior when the view appears, such as updating the UI based on the contents of some property.

Connecting the view to the controller

In Cocoa/Cocoa Touch, outlets and actions are used to connect objects in the view with code in the controller. Outlets are for linking objects in the view to a property in the controller. This allows the controller to manipulate that object, for example, setting the title of a button object. Actions are triggered when an event happens to an object. For example, an action responds to a click on a button. Action methods are passed the sender of that action.

I would compare outlets to DOM objects, and I would compare actions to DOM event handlers. Xcode makes creating outlets and actions a drag and drop operation, but in Javascript, you use document.getElementById or jQuery. DOM events are attached to an object, and specify a callback function, which can also take an argument of the sender (the this keyword).

Event loops

An event loop handles all incoming events and allows the application to respond appropriately. More generally, an event loop takes a queue of messages and processes them one at a time. An example of a message would be “call this callback function”. Processing that message would be actually executing the callback. For example, in Javascript, you specify a callback for the click event. When the click happens, the message to call that callback is placed on the message queue. The loop processes messages one at a time, so if you have another Javascript function that is currently processing a table (maybe shading the rows), the click callback message will not be processed immediately.

The Cocoa main event loop, GLib main event loop, and Javascript event loop are conceptually the same.

Node.js uses a single-threaded Javascript event loop to great advantage. Using Node, it is fairly easy to implement a scalable, event-driven web application.

Cocoa extends the main event loop, allowing you to create other queues. Queues are event loops that may or may not run on another thread. However, messages dispatched on the same serial queue will always be executed one at a time. This allows you to protect variables that aren’t thread safe by only accessing them from a particular queue.

The Glib event loop is used by a GUI library such as Gtk+ to make a Linux application. I’ve used these libraries while fixing a few Gnucash bugs and while writing a Twitter client.


My experience with Gtk+ and Javascript allowed me to quickly pick up core Cocoa concepts and design patterns. Now if I could only remember to do all UI stuff on the main queue!

Yet more website tweaks

Improve server response time

In my original post about hosting my own blog, I mentioned that Google PageSpeed Insights was complaining about server response time. After some research, I realized that my home page was quite large. It was over 1 MB, mainly because of a particular post which contained some large images. (I was complaining about the way graphics are configured on Windows, and included some large screenshots.) The fix was two parts. First, I cropped two of the screenshots to bring the size down a bit more. Second, I added the “More” tag, which makes users click a “Continue reading” link from the home page if they want to see the whole post.

If you want to measure the page load size on your own blog, clear your browser cache, then open up developer tools. Reload the page. On the “Network” tab (“Net” in Firebug), there is a summary of the number of HTTP requests, the amount of data downloaded, and the time it took.

Add LaTeX and YouTube to WordPress

Recently I found another post that needed some special care. In 2012 I posted about my computer animation project and included a YouTube video and some math equations. These used features unique to by simply pasting the link, it will embed a YouTube video, and by using a special tag, you can include math equations using the popular typesetting language LaTeX. One way to bring these into my blog would be to use’s plugin Jetpack. Jetpack brings a lot of features to, but I didn’t want all of them. Instead, I opted for two small plugins, WP-Latex and YouTube Embed Plus.

For people setting up their own blog, I’d recommend either Jetpack or a combination of smaller plugins to enable these features.