Gorm Manual

Guide to the Gorm application


Documentation authors: Gregory John Casamento <greg_casamento@yahoo.com> Richard Frith-Macdonald <richard@brainstorm.co.uk> Copyright 1999,2000,2004,2005 Free Software Foundation, Inc. License: FDL 1.2

Gorm Developers

Gregory John Casamento <greg_casamento@yahoo.com> Is the current maintaner of Gorm. Has implemented lots of new features and rewritten large portions of the existing code.
Richard Frith-Macdonald <richard@brainstorm.co.uk> wrote the original version of Gorm as part of the GNUstep project.
Pierre-Yves Rivaille <gnustep@rivaille.net> Is also a major contributor to the Gorm application.

Overview

File:GormFull.png

Gorm is an application for creating the user interface (and possibly entire applications) for a GNUstep application. Initially a close clone of the old NeXTstep 3.3 Interface Builder application, I expect that Gorm will mutate beyond the capabilities of that app.

GNUstep is an object-oriented programming framework and a collection of tools developed for or using the GNUstep libraries.

You can find out more about GNUstep at [1]

The basic idea behind Gorm is simple - it provides a graphical user interface with which to connect together objects from the GNUstep libraries (as well as custom-written objects) and set their attributes in an easy to use manner.

The collection of objects is then saved as a document which can either be re-loaded into Gorm for further editing, or can be loaded into a running GNUstep application in order to provide that application with a user interface or some subsystem.

This manual assumes a working knowledge of Objective-C and C. These are necessary prerequisites to understanding some of the technical details and examples given here.

This manual is ment to cover basic operation of the Gorm application. More about the GNUstep development and programming, please refer to the links at the end of this document.

Major features

Drag-and-drop creation of GUI elements from palettes.
Direct on-screen manipulation of GUI elements
Manipulation and examination of objects via inspectors.
Drag-and-drop creation of connections between objects.
Interactive test mode for interfaces/object-networks under development.
Saving data in a format loadable by GNUstep applications.
Run-time loading of additional palettes that may be written using an API

First contact

To launch the Gorm application, select it in the workspace's file viewer or from the dock. If you are using terminal, then type following on the command line:

openapp Gorm

- New Application: This creates a new application document within Gorm, you may then use the Palettes panel to drag new objects into the document.
- New Module: Contains a submenu, which also contains:
  - New Empty: produces an empty document with only NSFirst and NSOwner.
  - New Inspector: produces a document with NSOwner, NSFirst and a window which is the correct size for an Inspector.
  - New Palette: produces a document which is like the one by New Inspector, but it's window is the right size for a Palette.
- Save: This saves the current document
- Save As: This saves the current document to a new file and changes the document name to match the new name on disk.
- Save All: This saves all documents currently being edited by Gorm.
- Revert To Saved: This removes all changes made to the document sunce the last save, or since the document was opened.
- Test Interface: This provides interactive testing of the active document. To end testing, you need to select the quit menu item.
- Miniaturize: This miniaturises the active document (or whatever panel is currently key).
- Close: This closes the currenly active document.
- Debug: Prints some useful internal information.
- Load Sound: Loads a sound into the .gorm file.
- Load Image: Loads an image into the .gorm file.

Edit: In addition to the usual Cut, Copy, Paste, Delete Select All, this menu also contains:

- Group: Which produces a submenu
  - In Splitview: Groups views into an NSSplitView. Gorm does this based on the relative positions of the views being grouped. It determines the orientation and the order of th views and then groups them either vertically or horizontally in the order they appear on the screen.
  - In Box: Simply groups all of the views into one NSBox.
  - In ScrollView: Simply groups all of the views into one NSScrollView.
  - Ungroup: Ungroups the contained views.

- Set Name: This allows the user to set a name for a given object in the Objects view in the main document window.

- Disable Guideline: This item toggles between Enable Guideline and Disable Guideline. This allows the user to turn on or off the guides which appear when placing views in a window or view in Gorm.

Classes: Contains menus for working with classes.
- Create Subclass: Creates a subclass of the currently selected class in the current document classes view.
- Load Class: Loads a class from a .h file into the current document.
- Create Class Files: Generates a .h and .m file from the currently selected class in the current document classes view.
- Instantiate: Creates an instance of the selected class in the current document classes view.
- Add Outlet/Action: Adds an outlet or an action depending on what is selected in the document classes view. If the outlet icon is selected, it will add an outlet, if it the action icon is selected it will add an action.
- Remove: Removes the currently selected outlet, action or class.

Tools: Contains the inspector and the palette menus
- Inspector: Shows the inspector
- Palette: Shows the palette
- Load Palette: Opens a file panel and allows the user to load a palette into Gorm.

Windows: Shows currently open windows.

Services: Shows currently available services.

Hide: Hides the application.

Quit: Quits the application.

Preferences

The preferences panel contains a number of useful customizable options which can be used to modify the behavior of Gorm.

Some of these defaults can be safely modified from the command line by the user.

PreloadHeaders:

The user can define a set of headers to load when Gorm starts creation of a new .gorm file. This is useful when the user is building a framework or a set of interfaces for a large application.

ShowInspectors:

Controls whether the inspector shows when Gorm is started.

ShowPalettes:

Controls whether the palettes window shows when Gorm is started.

BackupFile:

Determines if the old .gorm is moved to .gorm~ when the modified version is saved.

Basic Concepts

This chapter will explain some of the basic things you need to understand before starting work on a new application.

First you need to understand a few basic concepts. Gorm's main window includes a few standard entries which must be explained before we can proceed. The basic objects are abstract and they are being concretised on the runtime. They are:

an owner of the object model or the user interface - NSOwner
first responder - an object or a view that will receive actions as first - NSFirst
a font manager representation - NSFont

The document window of Gorm showing NSOwner and NSFirst.

NSOwner is the class which "owns" the interface. This is, by default, NSApplication, but it can be any class you like. You can change it by selecting NSOwner in the document window and going to the "Custom Class" inspector in the inspectors window. From there, you should see all of the classes which the NSOwner can assume. We'll discuss more about this later when we go over how to create a new application

NSFirst is your interface to the responder chain. NSFirst is representative of the current "first responder" in the application. When you want a message, such as a changeFont: message, to go to the current first responder from, say, a menu, you connect the menu item to the NSFirst object in the document window. By doing this, it means that whichever object has first responder status at that time in the application will become the reciever of the "changeFont:" message.

NSFont represents the NSFontManager object for the application. This object is a shared singleton. This means that, for any given app, there should be only one instance of the object. This object is generally added to the document window when another objec, such as a Font menu item, is added to the interface, which, in turn, requires that this object be added to the document.

Responder and responder chain

A responder is any subclass of NSResponder. This includes NSWindow, NSView and all of the NSControl subclasses.

The responder chain is a sequence of objects which are called to determine where a message sent to the first responder will go. A message invoked on the first responder will be invoked on the first object in the responder chain which responds to that message.

The object which this message will be called on is determined in the method [NSApplication targetForAction:]. The call sequence is as follows, it will only proceed to the next step in each case if the current step fails to respond to the message which was invoked:

The firstResponder of the keyWindow, if one exists.
Iterates through all responders by pulling each in the linked list of responders for the key window.
It then tries the keyWindow.
Then the keyWindow's delegate
if the application is document based it tries the document controller object for the key window.
then it tries the mainWindow's list of responders (as above)
the mainWindow's delegate
if the app is document based, it tries the document controller for the main window
and finally, it tries the NSApplication delegate.

If all of the options in this list are exhausted, it then gives up and returns nil for the object which is to respond.

Creating an Application Interface

Gorm provides you with prebuilt interface template for an application. Note that to create whole application you would need to use the ProjectCenter or other development tools.

If you have ProjectCenter create an "Application" project. Create it with the name "FirstApp". From there you can open the MainMenu.gorm by clicking on interfaces and selecting MainMenu.gorm. If Gorm.app is properly installed, you Gorm should start up.

If you don't have ProjectCenter, you can create the Gorm file by hand. First you need to start Gorm. You can either do this by doing:

You then need to select the Document menu, and then New Application. This should produce a new document window, with a menu and an empty window. This should be the same as with the ProjectCenter gorm file since this is the basic starting point for an application.

For the sections below... only do one or the other, not both.

Editing the interface

Adding Controls from the Palette

Go to the Gorm menu and select Tools, then Palettes. This will bring the palette window to the front. The second palette from the left is the "ControlsPalette". Select that one and find the button object (it should have the word "Button" in it). Drag that to the window and drop it anywhere you like.

Repeat this operation with the text field. It's the control with "Text" in it. We are now ready to start making connections between different objects in the document.

Making Connections

The type of application we are creating is known as a "NSApplication delegate" this means that the MyController object will be set as the delegate of NSApplication.

To make this connection click on NSOwner and hold down the Control button, keep it pressed as you drag from the NSOwner object to the MyController object. The inspectors window should change to the Connections inspector and should show two outlets "delegate" and "menu". Select the "delegate", at this point you should see a green S and a purple T on the NSOwner and MyController objects respectively, and press the "Connect" button in the inspector. In the "Connections" section of the inspector you should see an entry which looks similar to "delegate (MyController)" this indicates that the connection has been made.

Now we need to make connections from the controller to the textfield and from the controller to the button. Select the MyController object and Control-Drag (as before) from the object to the text field, this will make an outlet connection. You should see the connections inspector again, this time select the "value" outlet and hit Connect.

Next, control-drag from the button to the controller, this will make an action connection. The connections inspector should again appear. This time you need to select the "target" outlet, to get the list of actions. The list should have only one entry, which is "buttonPressed:" since this is the one we added earlier. Press Connect. You should see an entry like "buttonPressed: (MyController" in the Connections section of the inspector.

It is also possible to make this connection to NSFirst, but to keep things simple, make it directly to the object. If you make the connection to buttonPressed: on NSFirst the functionality of the application will be unchanged, but the invocation will take the path described above in the section which describes "The Responder Chain".

Saving the gorm file

At this point you must save the .gorm file. Go to the Gorm menu and click Documents and then select "Save". If the document was opened from a pre-existing .gorm, it will save to that same file name. If it is an UNTITLED .gorm file a file dialog will appear and you will need to select the directory where you want to store the .gorm file and type the name of the .gorm file.

Custom classes

There are two ways to do create a custom class in Gorm. I will take you through each step by step. First click on the classes icon in the toolbar on the top of the Gorm document window. You should see the view below change to an outline view containing a list of class names. Once this happens we're ready to create a class.

Select the class you wish to subclass in the outline view. For our example we will use the simplest: NSObject. Select it by clicking on the class name once. Then go to the Classes menu in the main menu and select Create Subclass (you can also type Alt-Shift-c, which will do this as well. The new class will be created in the list with the name "NewClass".

Using The Outline View

Double click on the subclass name to make it editable. Type the name of the class and hit enter. For our example, please use the class name MyController. When you hit enter an alert panel will appear and warn you about breaking connections, simply select OK and continue.

This method of inputting the classes was inspired by IB in OPENSTEP 4.2/Mach which had functionality very similar to this. For users of that the transition to Gorm will be seamless.

Object awakening

The method awakeFromNib is called on any custom object which is unarchived from a nib/gorm file. This method is called on all objects after the entire archive has been loaded into memory and all connections have been made. Given all of this, you should not make any assumptions at all about which objects have been called and which have not. You should not release any objects in this method.

Outlets and Actions

Too add an outlet, select the round icon with the two horizontal lines in it (it sort of looks like a wall outlet. This should become depressed. Here you need to go to the Gorm Menu, under Classes select "Add Outlet/Action". Each time you press this menu item another outlet will be added with a name similar to newOutlet, as you add more the number at the end will increase. For now add only one outlet.

To rename the outlet simply double click it and change it's name like you did the class above to "value" for the sake of our example.

The steps to add on action are precisely the same as adding an outlet, except you must click on the button which looks like a target (a circle with a + inside). Add an action and name it "buttonPressed:" for the sake of our example.

Using The Class Edit Inspector

This way is much more inline with the "OPENSTEP/GNUstep" philosophy. For each object there is an inspector, even for Class objects.

Once you have created the class as described in the previous section "Creating a Class In Gorm", you must skip to this section to use the inspector. In the Gorm main menu select Tools and then select "Inspectors". This will make certain that the inspectors window is being displayed. Once the inspectors window is up move the pulldown on the top to "Attributes" and select the class you created which should, at this point, have the name "NewClass". You'll notice that the "Class" field at the top which shows the name's background color has turned white, instead of grey. This indicates that this class name is editable. Erase "NewClass" from the text field and type "MyController".

Adding outlets is very intuitive in the inspector. Simply select the "Outlets" tab in the tab view and click "Add" to add more outlets, and "Remove" to remove them. For the sake of our example, add one outlet and name it "value".

Very much like above only with the "Actions" tab, add an action called button pressed.

Instantiating The Class

In the Classes outline view select the new class you've created, now called MyController and then go to the Gorm menu and select Classes, and then Instantiate. The document window should shift from the classes view to the objects view. Amoung the set of objects should be a new object called MyController.

Generating .h and .m files from the class.

This is different than saving, some people have gotten this confused with the idea of Gorm generating the code for the gui. Gorm does nothing of the sort (grin).

Go to the Classes section in the Document window and select the MyController class yet again. Now go to the Gorm menu and select Classes and the select "Create Class Files". This will bring up a file panel and it allow you to select the directory in which to put the files. It will first create the MyController.m file and then the MyController.h file. Simply select the directory in which your app will reside and hit okay for both. You can change the names, but the default ones, which are based on the class name, should be sufficient. When you look at the .m for this class, you should see the buttonPressed: method with the comment /* insert your code here */ in it. Delete this comment and add:

   [value setStringValue: @"Hello"];

The class should look like this after you're done:

   #include <AppKit/AppKit.h>
   #include "MyController.h"
   @implementation MyController
   - (void) buttonPressed: (id)sender
   {
     [value setStringValue: @"Hello"];
   }
   @end

You recall, we connected the textfield to the "value" variable. The call above causes the method setStringValue to be invoked on the textfield you added to the window.

Also, note that the name of the method is "buttonPressed:". This is the action which is bound to the button. When it is pressed the text in the textfield should change to "Hello".

You now need to build the application either by copying in a GNUmakefile and making the appropriate changes or by using ProjectCenter's build capability, depending on if you use it or not.

This app is available as "SimpleApp" in the Examples directory under the Documentation directory distributed with Gorm. Hopefully this has helped to demonstrate, albeit on a small scale, the capabilities of Gorm. In later chapters we will cover more advanced application architectures and topics.

A Simple Application

This chapter will describe an application, very much like the previous one, but using a slightly different structure. This application builds on the previous application and uses WinController as the NSOwner of the app instead of making it the delegate of NSApplication.

Adding Menu Items

Select the first palette in the palette window, this should be the MenusPalette. The palette will have a bunch of pre-made menu items on it that you can add. We want to keep this simple, so grab the one called "Item" and drag it over to the menu in main menu nib (the menu on the screen, not the one in the objects view). As you have this object over the menu, the copy/paste mouse cursor should appear (it looks something like one box over another box at a 45 degree angle). Where you drop the menu determines it's position in the menu. You can always drag it to a new position after you've placed it by simply selecting and dragging up or down. Once you've placed the menu item, double click on the title and change it to "Open"

You can also change the name in the NSMenuItem attributes inspector. Now you must add openWindow: to MyController and make the connection from the "Open" menu item to NSFirst. In the connections inspector, find the "openWindow:" action. You could simply make the connection directly, but this is an exaple to show you that this connection will work as well. Whichever object has First Responder status will be tested to see if it responds to this method.

The implementation for openWindow: in MyController should simply be:

   - (void) openWindow: (id) sender
   {
     winController = [[WinController alloc] init];
   }

Also add the winController attribute and an include to allow WinController to be referenced in the MyController.m file.

Making a Controller-based .gorm file

Create a new .gorm file as described in the previous section using the "New Module" menu item. Under "New Module" select "New Empty". This should produce a .gorm file with only NSOwner and NSFirst. From the WindowsPalette (which should be the second palette in the palette window) drag a window to the location where you want it to appear on the screen. In the window add a button called "Close".

Go through the same steps you went through previously to create MyController, except for adding the outlets/actions, but this time with the name WinController. Add an outlet called window and an action called "closeWindow:".

Now, instead of instantiating the class go back to the objects view and select the NSOwner object. After that select the "Custom Class" inspector. Look for the entry for WinController and select it. You now must connect the "window" outlet to the Window you added previously.

Switch back to the objects view, then Control-Drag not to the window on the screen, but to the window's representation in the objects view. In the connection inspector select the window outlet and click Ok.

Save the .gorm as using the name Controller.gorm in the project directory.

Generate the Controller.h and Controller.h files as described in the previous section.

Add the init method to WinController

Add an implementation of the action "closeWindow:" to WinController and also an init which loads the gorm/nib file and declares itself as the owner. Here's how:

   /* All Rights reserved */
   #include <AppKit/AppKit.h>
   #include "WinController.h"
   @implementation WinController
   - (id) init
   {
     if((self = [super init]) != nil)
       {
         if([NSBundle loadNibNamed: @"Controller" owner: self] == NO)
           {
             NSLog(@"Problem loading interface");
             return nil;
           }
         [window makeKeyAndOrderFront: self];
       }
     return self;
   }
   // close
   - (void) closeWindow: (id) sender
   {
     [window close];
   }
   // dealloc
   - (void) dealloc
   {
     [super dealloc];
     RELEASE(window);
   }
   @end

The Controller gorm will be loaded and the connections will be made to the current instance, i.e. window will point to the window object instantianted in the .gorm file and all actions declared in the .gorm file which are attached to the object NSOwner will be resolved on self.

Running the App

Type the command:

   gopen Controller.app

on the command line in the project directory. Once the application has started it should look very much like the first application. Select the "Open" button from the Menu and you should see the second window pop up, now choose close, this will call the method "closeWindow:" which should cause the window to disappear.

Advanced Topics

Restrictions On Your Custom Subclasses

The restrictions here are the same as those in Apple's InterfaceBuilder. In general, you cannot have additional information which is expected to be decoded in an initWithCoder: method from a custom class which uses one of the methods in the previous section. This is because, by definition, Gorm doesn't know anything about these classes and allowing you to use them in Gorm in this way is a convenience to make it simpler for the developer. Gorm therefore, must use one of the proxies to encode the class since it cannot encode the class directly.

How can you get your classes into Gorm, you say? I'm pleased that you asked me that question. The best way to make your class known to Gorm so that you don't need to worry about the above restriction is to add a palette which contains your class. In this way, because you're literally linking the class into Gorm, you're making the class and it's structure known to Gorm so that it can encode the class directly. With the new palette loaded you can load and save classes containing real instances, not proxies, of your class encoded directly in the .gorm file. How to create a palette is discussed at length in the following section.

Graphical Objects In A Palette

You are, by now, familiar with the built in palettes which are provided with Gorm. Palettes are a powerful feature which allows the developer to add his/her own objects to Gorm. It is possible for a developer to write custom inspectors, editors and palettes for use with Gorm. A good example of a custom palette is palettetest in the dev-apps/test in the GNUstep distribution. Assuming you don't have that, however, I will explain precisely what you need to do in order to create a simple palette. The entire process is very short and suprisingly simple. First open Gorm and select Gorm->Document->New Module->New Palette. This will create a palette sized window. Once that's done go to the classes view in the main document window and find "IBPalette" in the class list. Create a subclass of that, the name can be whatever you want. For the purposes of our example we'll call it MyPalette. Drag a custom view to the window and choose the class you would like to add to the palette from one of your custom classes.

Once you've done this, generate the code for the classes (discussed in previous chapters). In the code, you'll add a method called "-(void) finishInstantiate" leave it empty for now. In the makefile for the palette make sure that the library or framework the view comes from is linked with the palette. Now build the palette.

After the palette is built you're ready to load it into Gorm. Go to the preferences panel and go to "Palettes". This should bring up a table view. Click on add. You should see a open dialog open. Select the palette bundle with this. If the palette is successfully loaded, you should see the name appear in the list. One thing to note here. Once a palette is loaded, it can't be unloaded until you close and restart Gorm. This is because by loading the palette bundle, the code in the bundle is being linked into Gorm. This can't be undone, once it's done.

Now, you should see the palette in the set of palettes in the palette window. Simply scroll over to it and select it's icon. When you do this, you should see the view that you set up using the custom view displayed as an actual instance. Note that we used one of the techniques listed above, it is possible to use any of the three for any object you add to your palette. You can now drag the view from the palette to a new window.

Non-Graphical Objects In A Palette

You may recall the creation of a method called "-(void) finishInstantiate" in the previous section. This section will make full use of that method. Re-open the palette you created before, but this time add an image view to the window. Then add to the image view, the icon you want to represent the non-graphical object. Here you'll need to add an ivar to the MyPalette class in both Gorm and in your source code called, imageView. Once you've done this make the connection between the image view and it's ivar.

Assuming that the class is called "NonUIObject", in finishInstantiate, you'll need to add the following line of code:

   -(void) finishInstantiate
   {
     id obj = [NonUIObject new];
     [self associateObject: obj type: IBObjectPboardType with: imageView];
   }

This code has the effect of associating the non-ui object with the ui object you just added to represent it. When you drag and drop the element which prepresents the object to something, it will copy the object, not the ui element, to the destination.

Congratulations, you now know how Palettes work.

Frequently Asked Questions

See: Gorm FAQ

Implementation details

This chapter describes some details about implementation of various features of Gorm. You should read it if you are a developer that would like to add custom objects, inspectors or would like to do other Gorm development.

The IB documentation on how object selection is managed and how editors and inspectors are used is unclear ... so the author of Gorm gone his own way.

When a document is loaded, the document object creates an editor attached to each top-level object in the user interface (NSMenu and NSWindow objects). These editors must be aware of their edited objects being clicked upon, and clicking on one of these should cause the corresponding editor to become the active editor.

The active editor is responsible for handling selection of the edited object (and any objects below it in the object hierarchy). Upon change of selection, the editor is responsible for sending an IBSelectionChangedNotification with the selection owner (normally the editor itsself) as the notification owner.

The main application watches for these notifications in order to keep track of who has the selection.

Connections

The connection API is the same as that for IB, but with the extension that the document object must implement [-windowAndRect:forObject:] to return the window in which the object is being displayed, and the rectangle enclosing the object (in window base coordinates).

This information is needed by Gorm so that it can mark the connection.

The editors mananging the drag-and-drop operation for a connection must call:

   [NSApp -displayConnectionBetween:and:]

to tell Gorm to update its display. This method sets the values currently returned by:

   [NSApp -connectSource] and 
   [NSApp -connectDestination]

This section will cover some topics which won't be of general interest to most users. The details in this section pertain to the internal workings of Gorm.

Gorm file format

The current Gorm file format is basically just a set of objects, encoded one after another in a continuous stream with some markers indicating when a new class starts or which class is encoded.

The Name Table

Each object in the .gorm file has a name assigned to it by the application. This allows Gorm to refer to the objects by a name once they are loaded rather than an address. Each name is associated with it's object in a dictionary which preserves the overall structure of the GUI which has been created.

The Custom Class Table - This is only used when the user has associated a custom class with an existing instance in the gorm file. If the user has, for instance, added an NSWindow to the gorm, he/she can use the custom class inspector to select a subclass of NSWindow to change to.

Connections Array - This array is used to form the connections after the .gorm file is loaded. The method [... establishConnection] is never called on either NSNibControlConnector or NSNibOutletConnector objects while in Gorm. This prevents the connections from having any effect while they are being edited in Gorm itself. Once they are loaded, the establishConnection method is called and the connections are made.

Custom Class Encoding

Custom objects are an interesting challenge in Gorm. By definition, custom classes are not known to Gorm, unless they are in a palette (covered elsewhere). For classes which are not in a palette instances of these classes in Gorm are encoding in one of three ways:

Proxy - This is a standin object which takes the place of the custom object. This is usually used when the superclass of the object is a non-graphical object, such as a controller. The init message is called on this object when it's unarchived.
Custom View - This is a standin view object similar to the one descrribed above, but it is a subclass of NSView. When this is used the initWithFrame: message is called on the view instance which is created (based on what view subclass the user selects)
Template - Probably the most interesting of the three. This is a standin class which uses an existing instance created in Gorm to build a custom subclass from. For instance when a window subclass is created, call it MyWindow, a template class called GSWindowTemplate is used to hold the NSWindow created in Gorm as well as the name of the subclass to be created when the class is unarchived outside of Gorm as well as some additional information. When the classes are unarchived in the running app, the designated initializer for that class will be invoked, except in the case of NSControl subclasses. See the Apple documentation for more information.

All custom instances have awakeFromNib invoked on them when they are unarchived from the .gorm file. This allows the user to do whatever additional setup that needs to be done, such as setting attribute. Classes which are "known" are, of course, directly encoded into the .gorm file.

NIB File Structure

NIB files contain a number of data structures which are used to unarchive the objects. These include a map which maintains the relationships between the objects themselves, a map which maintains the relationship between each object and it's name, a map which holds the relationship between an object and it's custom class and an array of connections which will determine how all of the objects messages and outlets are connected.

The maps are contained in the objects.nib file, while metadata for use by InterfaceBuilder itself is contained in info.nib and classes.nib. The info.nib file contains all of the information regarding the version of IB used to save the file. The classes.nib file contains any definitions of custom classes which were defined by the user in IB.

References

ProjectCenter - GNUstep integrated development environment

Links

Programming under GNUstep -- An Introduction by Ludovic Marcotte