Gtk+ 1.0 was released in in 1998. Here's a list of vastly superior UI toolkits that were developed from scratch before Gtk+ 3.0 release:
* Microsoft's WPF (first release in 2006) * Android's Java-based UI framework * Palm's webOS UI framework * Cocoa in Mac OS X (first release in 2001 - and before you nitpick, I know it was based on earlier tech from NeXT) * iPhone UI (granted, based on Mac OS X UI but a lot of it was new)
The technology behind Gtk+ (C, even when written in a disciplined, object-oriented fashion) makes it painful to program against. It was fine when the alternative was Win32 but the above mentioned toolkits use higher-level languages (java, c#, javascript, objective-c) making them not only easier to program against but also easier to improve the toolkits themselves.
I don't follow Gnome development but my outsider perspective is that Gtk+ is a huge technical debt and there are no signs that the community is planning to ditch it for something that can go head-to-head with those other technologies (mono is the only initiative trying to move the state of the art of implementation technologies on unix but it doesn't have UI component (other than silverlight implementation, which is WPF-lite)).
Open source core libraries are written against the C ABI (even when employing object models like GObject) so that everyone can bind to them easily. For example:
http://www.gtk.org/language-bindings.html
Why has it not been successful? Well IMO because of several reasons: deep dependency on X, making its use on Windows a kludge at best; hostility in the community to making it viable for commercial use (basically, polishing it enough for integration, and willingness to compromise design purity for integration with native widgets), and the horrendous plague that seems to be the bane of many open source products that focus on Linux: how hard it is to compile, the many dependencies, not providing easy to use complete packages for Windows, ... On Linux it's no issue, you've got all dependencies already and otherwise your distro takes care of it. On Window you need(ed?) to download 5+ various packages with different build systems, somehow integrate them (with different build systems) into your build, and if you wanted to use C++ you needed an extra 3 or 4 (for gtkmm); and then the result was still a second-rate citizen on Windows.
Plus there was the senseless bickering between Gnome and KDE (yes yes, competition is healthy, blah - if I had a penny for every hour wasted on that, I'd retire on the spot), the 'GTK = C' FUD, and the lack of people who wanted to go the extra mile of making a produkt rather than a nice proof of concept. The people familiar with it didn't care to make it easier for others, GUI tools were considered to be for 'pussies' and 'VB programmers' and so the self-righteous dogmatist make himself irrelevant by being surpassed by products who didn't think themselves above the average programmer.
I'm still a bit bitter, at the time I was so much cleaner to write GTK application than moc-infested QT code (yes I know things have changed).
Gtk+ development has been badly under-sponsored by the companies that sponsor GNOME development. In recent years, Cairo and then Clutter were the hot new toolkits that many people used to supplement Gtk. (Clutter provides a 2.5D canvas that can be used in Gtk+ applications and Cairo is used by Gtk+ to draw widgets.)
Qt, on the other hand, is backed by Nokia and has been undergoing constant development. The difference is noticeable.
There was a time when people spoke excitedly about creating new GNOME applications using Clutter, because Gtk+ itself didn't support what they wanted to do. Clutter was (and still is) blessed by GNOME. Among other programs, many GNOME games have been rewritten to use Clutter with a small Gtk+ wrapper.
I am no longer involved in GNOME, so I can't say for sure how people feel right now. I would guess that the attitude is what it has always has been: Gtk+ might not be amazing, but it is good enough for what we need. Use Clutter if you need a canvas. It would be insane to switch to another toolkit, as that would kill the GNOME project.
Edit: I want to add a few points I forgot to mention:
1. Gtk+ 3.0's support for CSS theming will level the playing field with other toolkits, and especially with Qt. This is the most exciting addition to Gtk+ 3.0. To be honest, it is long due.
2. Qt is my personal choice for a cross platform toolkit in high-level languages. Qt Designer, QLayouts, and Qt's language bindings are very good, and have all been present for years. Gtk+ has good language bindings, but it is lacking in all of the other areas. Furthermore, Qt's last few releases bring many new features that Gtk+ is missing and wont implement soon.
3. Despite my preference for Qt, I still use Gtk+ for small small C programs because Qt is written in C++. Gtk+ isn't as fun as Qt, but being forced to use C++ is plain out painful.
Cairo drawing throughout. GDK no longer wraps the antiquated X11 drawing API; we've made a clean break and exclusively rely on cairo for all our drawing needs now.
As for Clutter, the OpenGL based canvas, Gnome-Shell (aka the Gnome 3 desktop UI) uses cluttter throughout. The new window manager is called Mutter which is a fork of Metacity to use Clutter for hardware acceleration and all the panels and other desktop UI are also rendered this way.
just curious, have you considered qt jambi (now open source) or qt4dotnet?
I've decided my rule is, I want to either do everything in code (like GTK) or nothing in code (like Cocoa), but none of this "here's a utility that generates code for you" like Windows Forms.
So uic and moc are still there, but they may as well be generating object code - you never have to read the C++ that they spit out.
CSS themeing means themes can be created by designers more readily. This should mean more and better themes in the future.
Anyone knowns precompiled binaries for windows (even mingw should be fine, they link to KERNEL32.lib, and are binary compatible with MS).
