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问题:

If we've learned anything from HTML/CSS it's that, declarative languages (like XML) do a good job of describing User Interfaces because:

It's easy to build code preprocessors that can template the code effectively.
The code is in a well defined well structured (ideally) format so it's easy to parse.
The technology to effectively parse or crawl an XML based source file already exists.
The UIs scripted code becomes much simpler and easier to understand.
It simple enough that designers are able to design the interface themselves.
Programmers suck at creating UIs so it should be made easy enough for designers.

I recently took a look at the meat of a WPF application (ie. the XAML) and it looks surprisingly familiar to the declarative language style used in HTML.

The current state of desktop UI development is largely fractionalized, otherwise there wouldn't be so much duplicated effort in the domain of graphical user interface design (IE. GTK, XUL, Qt, Winforms, WPF, etc).

There are 45 GUI platforms for Python alone

What are some Open Source GUI's that represent these characteristics:

standardized
platform independent
declarative markup language
language agnostic

WPF, or more specifically XAML seems like a pretty likely step in the right direction.

Update:

Thanks a lot for the info, keep it comin'. Here's are the options I've gathered from the comments and answers.

GladeXML

Editor: Glade Interface Designer
OS Platforms: All
GUI Platform: GTK+
Languages: C (libglade), C++, C# (Glade#), Python, Ada, Pike, Perl, PHP, Eiffel, Ruby

XRC (XML Resource)

Editors: wxGlade, XRCed, wxDesigner, DialogBlocks (non-free)
OS Platforms: All
GUI Platform: wxWidgets
Languages: C++, Python (wxPython), Perl (wxPerl), .NET (wx.NET)

XML based formats that are either not free, not cross-platform, or language specific

XUL

Editor: Any basic text editor
OS Platforms: Any OS running a browser that supports XUL
GUI Platform: Gecko Engine?
Languages: C++, Python, Ruby as plugin languages not base languages

Note: I'm not sure if XUL deserves mentioning in this list because it's less of a desktop GUI language and more of a make-webapps-run-on-the-desktop language. Plus, it requires a browser to run. IE, it's 'DHTML for the desktop.'

CookSwing

Editor: Eclipse via WindowBuilder, NetBeans 5.0 (non-free) via Swing GUI Builder aka Matisse
OS Platforms: All
GUI Platform: Java
Languages: Java only

XAML (Moonlight)

Editor: MonoDevelop
OS Platforms: Linux and other Unix/X11 based OSes only
GUI Platforms: GTK+
Languages: .NET

Note: XAML is not a pure Open Source format because Microsoft controls its terms of use including the right to change the terms at any time. Moonlight can not legally be made to run on Windows or Mac. In addition, the only platform that is exempt from legal action is Novell. See this for a full description of what I mean. XAML is also not an ECMA standard like C#, Managed C++, and the CLR.

Update: The question has been changed from "Is there an Open source alternative to WPF? because the original question was wrong, and it sucked. The direction of this question has changed direction to match up to align with the new input. My apologies to the people who responded before it changed.

回答1:

Qt is developing QML, which looks a lot like XAML except in JSON. It's available as a preview built against the current version, and is available in snapshots of the next version.

Here's a little snippet from http://doc.qt.nokia.com/4.7-snapshot/declarative-ui-components-progressbar.html

import Qt 4.7
import "content"

Rectangle {
   id: main

    width: 600; height: 405
    color: "#edecec"

    Flickable {
       anchors.fill: parent
       contentHeight: column.height + 20

       Column {
           id: column
           x: 10; y: 10
           spacing: 10

           Repeater {
               model: 25

               ProgressBar {
                 property int r: Math.floor(Math.random() * 5000 + 1000)
                 width: main.width - 20

                 NumberAnimation on value { duration: r; from: 0; to: 100; loops: Animation.Infinite }
                 ColorAnimation on color { duration: r; from: "lightsteelblue"; to: "thistle"; loops: Animation.Infinite }
                 ColorAnimation on secondColor { duration: r; from: "steelblue"; to: "#CD96CD"; loops: Animation.Infinite }
               }
           }
       }
   }
}

回答2:

The Web is taking most of the steam away from desktop apps as it is.

I think that the big reason is that everyone's so focused on the web right now. HTML5 is going to be a quantum leap forward in what the web can do. With fast JavaScript interpreters and capable browsers, the need for a desktop programs will begin to wane over time. That's the horse that Google is betting on, and to a much lesser extent, Apple as well.
Creating something good would have radically different implementations for each OS, so the base toolkit itself wouldn't be very portable.

If you think about it, the Web is the only really common substrate we have upon which to develop this sort of infrastructure in a cross-platform manner. WPF is incredibly different from an architectural perspective vs. WinForms/straight WinAPI code. Adapting something like it to each OS would take a great deal of very different plumbing for each OS if you were to have a prayer of making something that performed well. (Not that web apps are very fast, mind you, but they're getting better).
Look and feel is always going to be somewhat of an issue.

Whose look and feel do you use? Do you try to adapt the UI to the OS chrome so it looks "native", or do you do something like Swing did years ago and develop apps that look distinctively different from everything out there? (Ugh, that was a train wreck...) And if you choose to adapt the UI to each OS's look and feel, you may have all sorts of measurement and design issues.