Programming under GNUstep—An Introduction
When porting an application from GNUstep to Mac OS X (or the other way around), some important things must be considered. For example, when porting to Mac OS X, you have to redo the user interface using Interface Builder under Mac OS X. The following steps are needed to port the application to Mac OS X:
From the File menu of Project Builder, select New Project... and select Cocoa Application. Click on the Next button.
Specify the project name (Tiff Viewer) and the project location. Click on the Finish button.
Now, select the Classes node and add the files AppController.m and ImageModel.m from the Project menu.
Expand the Other Sources node and delete the main.m file.
Expand the Resources node and double-click on the MainMenu.nib node. This will start Interface Builder.
Much like you did under Gorm, drag and drop the NSImageView in the empty window and the Load Image menu item in the File menu.
Because the controller class (AppController) was created in the previous section, you simply can reuse it under Mac OS X. In Interface Builder, from the Classes menu, choose Read Files.. and select AppController.h.
From the File window, click on the Classes tab, select AppController and from the Classes menu, choose Instantiate AppController.
Now, connect the outlets and the action like we did under Gorm.
Save the modified interface, and quit Interface Builder.
From the Build menu in Project Builder, choose Build and run. This will compile and launch the application.
Once the application is launched, choose Load Image from the file menu and select a picture to show. The final result should look like Figure 10.
As you can see, we have ported the application without rewriting a single line of code. Even if the application is quite simple, complex applications can be developed under GNUstep and easily ported to Mac OS X. Affiche and GNUMail.app are good examples of applications that are portable between GNUstep and Mac OS X.
Going the other way, more care should be taken when porting applications from Mac OS X to GNUstep. First, you have to redo the user interface of the application using Gorm. Secondly, GNUstep currently does not provide an implementation of some Cocoa classes like NSToolbar, NSDrawer or any core foundation services. To avoid problems when porting a Mac OS X application using those unimplemented functionalities to GNUstep, you will need to use conditional compilation. Finally, one or more GNU Makefiles must be created in order to compile the application under GNUstep.
As we have seen in this article, developing a GNUstep application is relatively easy. GNUstep offers a rich, clean and consistent API for developing true cross-platform applications in the Objective-C language.
New application kit back ends are being developed for Microsoft Windows, DirectFB and Ghostscript, thus allowing support for a wider range of computing environments. Also, OpenGL support has recently been added through the implementation of the NSOpenGLView class.
Finally, GNUstep-based distributions are emerging. For example, the LinuxSTEP Project aims to create a fully integrated, desktop Linux operating environment that is not bound by some of the more traditional approaches of common Linux distributions.
All listings referred in this article are available by anonymous download at ftp.linuxjournal.com/pub/lj/listings/issue108/6418.tgz.
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