Wednesday, November 02, 2005

Working on purely proprietary code makes it awkward to write about it in public. On top of that, it leaves my time a little too committed to do open source work.

So what is there to blog about? Well, there are still a few things, but having the baby due about now (yesterday, in fact) has been enough excuse to avoid writing. All the same, maybe I need to put some effort in. I keep finding myself wondering how I did something a few weeks ago, and the blog was supposed to help with that.

For a change of pace, I've spent my last two evenings learning Cocoa and Objective C. I'd avoided it before now because I didn't know Objective C, and wasn't sure I wanted to learn it when almost no other system makes use of it. Similarly, Cocoa is only available on the Mac, so there's no portability, unlike with QT or Java's Swing.

However, I like the Mac, and it's been bothering me that I haven't done any GUI coding for a while. I did some work with QT earlier this year, but it never looked quite right to me. (That would be more due to my knowledge of QT than anything else, as I've seen some attractive interfaces built with QT). I enjoy drawing mathematical shapes (yes, I could just learn more GNUPlot), and Quartz 2D sounds like fun. Since my after-hours work is all about fun, these seemed as good a set of reasons as any to try it.

I reasoned that there were two approaches I could try for this. The first was the traditional one of reading the docs and then applying my knowledge. That's very successful, and gives a very thorough knowledge of the subject material, but it also takes a long time. The other option is to learn just enough to get started, and then experiment with the documentation nearby. This technique leads to a much less thorough understanding of the system, and can result in accidentally taking a poor approach to a problem (since the best approach may not have been apparent). However, it gets you up and running faster. It's also more fun, because of the immediate feedback that you get. Given my criteria of fun and lack of time, the second approach seemed like a better idea.

What could I write that would be fun, and easy to write in my limited time in the evening? I decided to draw a simple curve which meets the equation:
x = sin(theta)
y = cos(3 * theta)

This looks like 3 cycles of a sine wave, wrapped around a clear cylinder. If I got it working, then I thought I should animate it (rotate the cylinder), and maybe introduce some controls to control the speed and number of cycles.

Objective C
I started with an ObjectiveC/Cocoa tutorial. This covered the development tools (Xcode and InterfaceBuilder) more than coding, but it covered a couple of the basics, and explained the basic syntax for method calls in Objective C (which I'd had trouble working out when I read example code). It turns out that calling methods like this is called "sending a message", which was the first hint I had of some of the "disconnected" nature of some of the features in the runtime system.

Once I thought I understood how to use Objective C, I left Xcode and tried my hand at some "Hello world" style programs on the command line. This worked pretty well, and helped me work out some more of the syntax of classes. I still have a LOT to learn (for instance, while I know @interface and @implementation, I haven't worked out what @protocol does). Still, I've always felt that if you can't do something like this manually from the command line, then you'll never be able to properly use the GUI tool that automates it for you.

The only thing I struggled with was the need to inherit from NSObject. This was needed for the alloc method, which does the heap allocation. I think I could implement this myself using malloc, but it was easier to use the Cocoa system instead.

It was only later that I discovered that ObjectiveC/Cocoa does memory reclamation when objects are no longer in use. An object called an allocator is used to assign memory for objects, and register them for later cleanup. The allocator is normally provided by the system, but it can also be set manually. This is essential to know when launching new threads, as there is no default allocator (it was while reading about Cocoa threads that I learned about automated object reclamation.

Other than memory management, there are two other features to ObjectiveC that stood out for me. The first is a kind of reflection which lets you analyze the available methods at runtime. This is interesting as it means that many objects don't need to inherit an interface, but just need to implement any required methods, ignoring anything not needed. This is the technique used by delegates.

The other feature is based on the numerous message passing mechanisms supported by the runtime. This reminds me of Windows messages, but seems to be built into the runtime (it was hard to tell what was part of the ObjectiveC runtime and what was provided by the Cocoa libraries). It can sometimes be difficult initializing an object with references to every other object that it needs to talk to, so a global message passing system can be really useful on occasion.

Over the years I've done GUI programming with the Windows API, MFC, Delphi, Xlib, Gnome/GTK, QT, Java AWT and Visual Basic. I'm certainly not an expert in any of these (at least, not anymore) but it's given me a taste of a number of approaches.

Just to provide some perspective on my impressions of Cocoa, here is a quick and dirty rundown on my impressions of each:

  • Xlib: Lots of work to do anything at all. I wouldn't use it without a modern library to wrap it, to provide an attractive and consistent interface, and to provide useful modern widgets.
  • Windows API: Less work than XLib, but there is still a lot of boilerplate code. Unfortunately a lot of Windows functionality now comes through object systems (COM+, ActiveX and more), and these can take a lot of code in C. All the same, once you have your head around the parts of the system you need, it's pretty easy to code. It can just take a lot of code to do some common tasks.
  • MFC: At face value this wraps the Windows API, and implements a lot of common tasks, making it easier and quicker to write code. However, the action and messaging system is a dogs breakfast. If I want to respond to something, then do I implement a virtual function in a subclass, or do I add an entry to the message map? If it goes into the message map, then is there already a macro for it, or do I register for a particular windows message? It is also too dependent on Windows messages. Some basic functions are provided to wrap message (eg. setText), but most of the time the response to any message is to send a new message. Where's the OO in this? Sidestepping the class framework is possible when needed (reimplementing the message loop, for instance), but painful. Not to mention the difficulties working with MFC and ATL together. My biggest gripe was the need to learn a new API every week.
  • Gnome/GTK: An extremely flexible system, running on Windows, OS X, and any X11 system. I've only used the standard C binding, but it comes with bindings for practically every useful language, including binary OO languages like C++. Glade also does a lot of the work in designing and implementing a GUI. My experience here was extremely limited, but I felt a little overwhelmed with options. The object design in C is good, but it does force certain development patterns on the programmer (this is probably a good thing, but feels constraining). The main reason I didn't go too far here was the sheer number of libraries needed to install on a non-Linux system to make your application work.
  • QT: The big competitor to GTK/Gnome, running on the same systems. The GUI editor feels a little clunky, and I found it hard to make layout managers work exactly the way I wanted on all platforms. It feels more like MFC than any of the other frameworks here, but the messaging system is a lot cleaner and more consistent. I think Trolltech could have improved over MFC if they'd chosen to integrate with the C++ STL, but they haven't. This is demonstrate in their QString class, whose operator+(...) methods don't always do as you expect.
  • Java AWT: Anyone using this interface will know why I stopped using it. Even Sun realized that the "lowest common denominator between windowing system" was never going to work, and moved on to Swing. However, it was easy enough to use, with all the work done with subclassing and methods to receive events. Layouts were limited, but their use was clearly documented. The main problem was the coding work required to set up the interface. This could be quite verbose, and needed to be run before the GUI was visible. I should just learn to use Swing, but I still hear about "clunkiness" problems. It's also buggy on OS X.4.
  • Visual Basic: Back to being Windows only, but very easy to set up and code. The moment you try to do anything useful you discover that you're badly hamstrung and can't do much. It was necessary to call directly into the Windows API much more than it should have been. I used to write a lot of DLLs and call them from VB. Even worse was the propensity to launch background threads that you were never told about, leading to code that could fail on some invocations, but not on others.
  • Delphi: I always found the syntax of this language to be too saccharine, but I can't deny the usefulness of this system. As easy to set up as VB, but with the power of MFC. I was particularly impressed when Borland made it fully compatible with C++ Builder. Most functionality is available in classes and methods rather than windows messages. Releasing Kylix for Linux was also a great move. It's a shame that there is no way to run it on any other system, such as OS X or Solaris. However, I can't afford the costs of Delphi, so I decided to leave it to the corporate types.

Overall, developing in Cocoa is really very nice. With object management and reflection, I almost felt like I was using Java, only easier. Everything I needed was in virtual methods, meaning I could do everything by subclassing. However, I decided that I wanted to shut down the application when the main window was closed, and I was concerned that I would therefore need to create a new subclass of NSWindow and use that as the class for the window. This is where I discovered delegates and some of the other notification mechanisms in use by ObjectiveC and Cocoa.

Delegates are objects given to a notifying object (such as an instance of NSWindow) which will be told when anything significant happens to the notifying object. The delegate object does not implement any interface and need not implement any methods except the ones it's interested in. This meant that I could simply have the main window inform an object of my choice when it was about to close, rather than having to introduce a new instance of a subclass of NSWindow, implementing a single method.

Of course, one of the nicest aspects of the system was access to subsystems like Quartz 2D. The resulting drawing had some nice extra features by default, such as antialiasing and buffering. The buffering became really apparent when I started animating the curve. I expected to do offscreen buffering and then start displaying completed images at the correct rate (there's even the NSViewAnimation class which will take these images and do the work for you), but Quartz did all the buffering for me. It even seemed to introduce some blur from one image to the next. The old drawing was wiped before the new one was created, so this appeared to be an intentional effect to make the animation seem smoother.

Some of the detractions include threads and the Interface Builder tool. Threads are managed with the NSThread class, but this really acts as a wrapper over some non-OO code that sets up a POSIX thread. The entry point to the thread can just be a method anywhere, rather than in a class specifically designed for threads. Also, the new thread has no default auto-release pool for objects, so to stop memory leaking it is important to set one up. This is trivial, but seems like the sort of thing that should have been done already. Also, operations like timers require a run loop to be processing, and again this has to be called manually. It works, but it's hardly OO.

The biggest problem with the Interface Builder was lack of documentation. The basics are covered in several tutorials, or when the help button is pressed, but I could not find advanced options mentioned anywhere. For instance, what does it mean to bind the value of a control to a user class instance? It seems to introduce a new dictionary with a key/value for the object's value, but where is this dictionary to be found?

To start with, the layout of the controls was difficult to work out, but I finally got it. I later found an obscure reference to it in some documents, but it still wasn't properly explained.

I also had several attempts to merge class files updated by Interface Builder with the originals in Xcode, but none were successful. In the end, I started generating new files from scratch, and manually copying the new elements over to the files in Xcode. I'm sure this can be made to work, but I didn't get it going. Again, documentation may be useful here.

Finally, on three separate occasions the Xcode IDE crashed when I clicked on a link in the documentation window. Normally the documentation is a separate program from the IDE, but Xcode can make requests of the documentation window all the time, so it all seems to be one program.

All these complaints aside, it is still a nice system. The Interface Builder lets you instantiate classes and then store them in the binary NIB file. These objects are then de-serialized at runtime, and are therefore instantiated on startup. This solves certain problems that can come up in object lifecycles, so I like it here. Connections between objects (between outlets and actions) are not clear unless the source object is selected in the GUI, but the concept is still nice. This is particularly useful as the objects at either end are extracted from the NIB, otherwise there would be no easy way to get references to either object to programmatically initialize this connection.

What do I think?
Weighing it up, this was one of the easier systems I've encoded on, and I was happy with the resulting program. For someone who likes coding (as opposed to a pure GUI configuration, as provided in VB or Delphi) then this is a nice and powerful system. The tool for building interfaces is very good, but doesn't integrate into the code well, which is why I deduct marks when comparing it to Delphi. I'm looking forward to writing more code in it. Maybe I'll check out why the curve isn't appearing when I do a print preview.

I commented the code, so I'm happy to give it to anyone who wants to see some example Cocoa code.

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