Nib Files

Nib files play an important role in the creation of applications in OS X and iOS. With nib files, you create and manipulate your user interfaces graphically, using Xcode, instead of programmatically. Because you can see the results of your changes instantly, you can experiment with different layouts and configurations very quickly. You can also change many aspects of your user interface later without rewriting any code.

For applications built using the AppKit or UIKit frameworks, nib files take on an extra significance. Both of these frameworks support the use of nib files both for laying out windows, views, and controls and for integrating those items with the application’s event handling code. Xcode works in conjunction with these frameworks to help you connect the controls of your user interface to the objects in your project that respond to those controls. This integration significantly reduces the amount of setup that is required after a nib file is loaded and also makes it easy to change the relationships between your code and user interface later.

Anatomy of a Nib File

A nib file describes the visual elements of your application’s user interface, including windows, views, controls, and many others. It can also describe non-visual elements, such as the objects in your application that manage your windows and views. Most importantly, a nib file describes these objects exactly as they were configured in Xcode. At runtime, these descriptions are used to recreate the objects and their configuration inside your application. When you load a nib file at runtime, you get an exact replica of the objects that were in your Xcode document. The nib-loading code instantiates the objects, configures them, and reestablishes any inter-object connections that you created in your nib file.

The following sections describe how nib files used with the AppKit and UIKit frameworks are organized, the types of objects found in them, and how you use those objects effectively.

About Your Interface Objects

Interface objects are what you add to an nib file to implement your user interface. When a nib is loaded at runtime, the interface objects are the objects actually instantiated by the nib-loading code. Most new nib files have at least one interface object by default, typically a window or menu resource, and you add more interface objects to a nib file as part of your interface design. This is the most common type of object in a nib file and is typically why you create nib files in the first place.

Besides representing visual objects, such as windows, views, controls, and menus, interface objects can also represent non-visual objects. In nearly all cases, the non-visual objects you add to a nib file are extra controller objects that your application uses to manage the visual objects. Although you could create these objects in your application, it is often more convenient to add them to a nib file and configure them there. Xcode provides a generic object that you use specifically when adding controllers and other non-visual objects to a nib file. It also provides the controller objects that are typically used to manage Cocoa bindings.

About the File’s Owner

One of the most important objects in a nib file is the File’s Owner object. Unlike interface objects, the File’s Owner object is a placeholder object that is not created when the nib file is loaded. Instead, you create this object in your code and pass it to the nib-loading code. The reason this object is so important is that it is the main link between your application code and the contents of the nib file. More specifically, it is the controller object that is responsible for the contents of the nib file.

In Xcode, you can create connections between the File’s Owner and the other interface objects in your nib file. When you load the nib file, the nib-loading code recreates these connections using the replacement object you specify. This allows your object to reference objects in the nib file and receive messages from the interface objects automatically.

About the First Responder

In a nib file, the First Responder is a placeholder object that represents the first object in your application’s dynamically determined responder chain. Because the responder chain of an application cannot be determined at design time, the First Responder placeholder acts as a stand-in target for any action messages that need to be directed at the application’s responder chain. Menu items commonly target the First Responder placeholder. For example, the Minimize menu item in the Window menu hides the frontmost window in an application, not just a specific window, and the Copy menu item should copy the current selection, not just the selection of a single control or view. Other objects in your application can target the First Responder as well.

When you load a nib file into memory, there is nothing you have to do to manage or replace the First Responder placeholder object. The AppKit and UIKit frameworks automatically set and maintain the first responder based on the application’s current configuration.

For more information about the responder chain and how it is used to dispatch events in AppKit–based applications, see “Event Architecture” inCocoa Event Handling Guide. For information about the responder chains and handling actions in iPhone applications, see Event Handling Guide for iOS.

About the Top-Level Objects

When your program loads a nib file, Cocoa recreates the entire graph of objects you created in Xcode. This object graph includes all of the windows, views, controls, cells, menus, and custom objects found in the nib file. The top-level objects are the subset of these objects that do not have a parent object. The top-level objects typically include only the windows, menubars, and custom controller objects that you add to the nib file. (Objects such as File’s Owner, First Responder, and Application are placeholder objects and not considered top-level objects.)

Typically, you use outlets in the File’s Owner object to store references to the top-level objects of a nib file. If you do not use outlets, however, you can retrieve the top-level objects from the nib-loading routines directly. You should always keep a pointer to these objects somewhere because your application is responsible for releasing them when it is done using them. For more information about the nib object behavior at load time, see “Managing the Lifetimes of Objects from Nib Files.”

About Image and Sound Resources

In Xcode, you can refer to external image and sound resources from within the contents of your nib files. Some controls and views are able to display images or play sounds as part of their default configuration. The Xcode library provides access to the image and sound resources of your Xcode projects so that you can link your nib files to these resources. The nib file does not store these resources directly. Instead, it stores the name of the resource file so that the nib-loading code can find it later.

When you load a nib file that contains references to image or sound resources, the nib-loading code reads the actual image or sound file into memory and and caches it. In OS X, image and sound resources are stored in named caches so that you can access them later if needed. In iOS, only image resources are stored in named caches. To access images, you use the imageNamed: method of NSImage or UIImage, depending on your platform. To access cached sounds in OS X, use the soundNamed: method of NSSound.

Nib File Design Guidelines

When creating your nib files, it is important to think carefully about how you intend to use the objects in that file. A very simple application might be able to store all of its user interface components in a single nib file, but for most applications, it is better to distribute components across multiple nib files. Creating smaller nib files lets you load only those portions of your interface that you need immediately. They also make it easier to debug any problems you might encounter, since there are fewer places to look for problems.

When creating your nib files, try to keep the following guidelines in mind:

The Nib Object Life Cycle

When a nib file is loaded into memory, the nib-loading code takes several steps to ensure the objects in the nib file are created and initialized properly. Understanding these steps can help you write better controller code to manage your user interfaces.

The Object Loading Process

When you use the methods of NSNib or NSBundle to load and instantiate the objects in a nib file, the underlying nib-loading code does the following:

  1. It loads the contents of the nib file and any referenced resource files into memory:

  2. It unarchives the nib object graph data and instantiates the objects. How it initializes each new object depends on the type of the object and how it was encoded in the archive. The nib-loading code uses the following rules (in order) to determine which initialization method to use.

    1. By default, objects receive an initWithCoder: message.

      In OS X, the list of standard objects includes the views, cells, menus, and view controllers that are provided by the system and available in the default Xcode library. It also includes any third-party objects that were added to the library using a custom plug-in. Even if you change the class of such an object, Xcode encodes the standard object into the nib file and then tells the archiver to swap in your custom class when the object is unarchived.

      In iOS, any object that conforms to the NSCoding protocol is initialized using the initWithCoder: method. This includes all subclasses of UIView and UIViewController whether they are part of the default Xcode library or custom classes you define.

    2. Custom views in OS X receive an initWithFrame: message.

      Custom views are subclasses of NSView for which Xcode does not have an available implementation. Typically, these are views that you define in your application and use to provide custom visual content. Custom views do not include standard system views (like NSSlider) that are part of the default library or part of an integrated third-party plug-in.

      When it encounters a custom view, Xcode encodes a special NSCustomView object into your nib file. The custom view object includes the information it needs to build the real view subclass you specified. At load time, the NSCustomView object sends an alloc and initWithFrame: message to the real view class and then swaps the resulting view object in for itself. The net effect is that the real view object handles subsequent interactions during the nib-loading process.

      Custom views in iOS do not use the initWithFrame: method for initialization.

    3. Custom objects other than those described in the preceding steps receive an init message.

  3. It reestablishes all connections (actions, outlets, and bindings) between objects in the nib file. This includes connections to File’s Owner and other placeholder objects. The approach for establishing connections differs depending on the platform:

    • Outlet connections

      • In OS X, the nib-loading code tries to reconnect outlets using the object’s own methods first. For each outlet, Cocoa looks for a method of the form setOutletName: and calls it if such a method is present. If it cannot find such a method, Cocoa searches the object for an instance variable with the corresponding outlet name and tries to set the value directly. If the instance variable cannot be found, no connection is created.

        Setting an outlet also generates a key-value observing (KVO) notification for any registered observers. These notifications may occur before all inter-object connections are reestablished and definitely occur before any awakeFromNib methods of the objects have been called.

      • In iOS, the nib-loading code uses the setValue:forKey: method to reconnect each outlet. That method similarly looks for an appropriate accessor method and falls back on other means when that fails. For more information about how this method sets values, see its description in NSKeyValueCoding Protocol Reference.

        Setting an outlet in iOS also generates a KVO notification for any registered observers. These notifications may occur before all inter-object connections are reestablished and definitely occur before any awakeFromNib methods of the objects have been called.

    • Action connections

      • In OS X, the nib-loading code uses the source object’s setTarget: and setAction: methods to establish the connection to the target object. If the target object does not respond to the action method, no connection is created. If the target object is nil, the action is handled by the responder chain.

      • In iOS, the nib-loading code uses the addTarget:action:forControlEvents: method of the UIControl object to configure the action. If the target is nil, the action is handled by the responder chain.

    • Bindings

      • In OS X, Cocoa uses the bind:toObject:withKeyPath:options: method of the source object to create the connection between it and its target object.

      • Bindings are not supported in iOS.

  4. It sends an awakeFromNib message to the appropriate objects in the nib file that define the matching selector:

    • In OS X, this message is sent to any interface objects that define the method. It is also sent to the File’s Owner and any placeholder objects that define it as well.

    • In iOS, this message is sent only to the interface objects that were instantiated by the nib-loading code. It is not sent to File’s Owner, First Responder, or any other placeholder objects.

  5. It displays any windows whose “Visible at launch time” attribute was enabled in the nib file.

The order in which the nib-loading code calls the awakeFromNib methods of objects is not guaranteed. In OS X, Cocoa tries to call the awakeFromNib method of File’s Owner last but does not guarantee that behavior. If you need to configure the objects in your nib file further at load time, the most appropriate time to do so is after your nib-loading call returns. At that point, all of the objects are created, initialized, and ready for use.

Managing the Lifetimes of Objects from Nib Files

Each time you ask the NSBundle or NSNib class to load a nib file, the underlying code creates a new copy of the objects in that file and returns them to you. (The nib-loading code does not recycle nib file objects from a previous load attempt.) You need to ensure that you maintain the new object graph as long as necessary, and disown it when you are finished with it. You typically need strong references to top-level objects to ensure that they are not deallocated; you don’t need strong references to objects lower down in the graph because they’re owned by their parents, and you should minimize the risk of creating strong reference cycles.

From a practical perspective, in iOS and OS X outlets should be defined as declared properties. Outlets should generally be weak, except for those from File’s Owner to top-level objects in a nib file (or, in iOS, a storyboard scene) which should be strong. Outlets that you create should therefore typically be weak, because:

@property (weak) IBOutlet MyView *viewContainerSubview;
@property (strong) IBOutlet MyOtherClass *topLevelObject;

Outlets are generally considered private to the defining class; unless there is a reason to expose the property publicly, hide the property declarations a class extension. For example:

// MyClass.h
 
@interface MyClass : MySuperclass
@end
 
// MyClass.m
 
@interface MyClass ()
@property (weak) IBOutlet MyView *viewContainerSubview;
@property (strong) IBOutlet MyOtherClass *topLevelObject;
@end

These patterns extend to references from a container view to its subviews where you have to consider the internal consistency of your object graph. For example, in the case of a table view cell, outlets to specific subviews should again typically be weak. If a table view contains an image view and a text view, then these remain valid so long as they are subviews of the table view cell itself.

Outlets should be changed to strong when the outlet should be considered to own the referenced object:

Classes that you expect to be subclassed (in particular abstract classes) expose outlets publicly so that they can be used appropriately by subclasses (e.g. UIViewController’s view outlet). Outlets might also be exposed where there is an expectation that consumers of the class will need to interact with the property; for example a table view cell might expose subviews. In this latter case, it may be appropriate to expose a read-only public outlet that is redefined privately as read-write, for example:

// MyClass.h
 
@interface MyClass : UITableViewCell
@property (weak, readonly) MyType *outletName;
@end
 
// MyClass.m
 
@interface MyClass ()
@property (weak, readwrite) IBOutlet MyType *outletName;
@end

Top-level Objects in OS X May Need Special Handling

For historical reasons, in OS X the top-level objects in a nib file are created with an additional reference count. The Application Kit offers a couple of features that help to ensure that nib objects are properly released:

  • NSWindow objects (including panels) have an isReleasedWhenClosed attribute, which if set to YES instructs the window to release itself (and consequently all dependent objects in its view hierarchy) when it is closed. In the nib file, you set this option through the “Release when closed” check box in the Attributes pane of the Xcode inspector.

  • If the File’s Owner of a nib file is an NSWindowController object (the default in document nibs in document-based applications—recall that NSDocument manages an instance of NSWindowController) or an NSViewController object, it automatically disposes of the windows it manages.

If the File’s Owner is not an instance of NSWindowController or NSViewController, then you need to decrement the reference count of the top level objects yourself. You must cast references to top-level objects to a Core Foundation type and use CFRelease. (If you don’t want to have outlets to all top-level objects, you can use the instantiateNibWithOwner:topLevelObjects: method of the NSNib class to get an array of a nib file’s top-level objects.)

Action Methods

Broadly speaking, action methods (see Target-Action in OS X or Target-Action in iOS) are methods that are typically invoked by another object in a nib file. Action methods use type qualifier IBAction, which is used in place of the void return type, to flag the declared method as an action so that Xcode is aware of it.

@interface MyClass
- (IBAction)myActionMethod:(id)sender;
@end

You may choose to regard action methods as being private to your class and thus not declare them in the public @interface. (Because Xcode parses implementation files, there is no need to declare them in the header.)

// MyClass.h
 
@interface MyClass
@end
 
// MyClass.m
 
@implementation MyClass
- (IBAction)myActionMethod:(id)sender {
    // Implementation.
}
@end

You should typically not invoke an action method programmatically. If your class needs to perform the work associated with the action method, then you should factor the implementation into a different method that is then invoked by the action method.

// MyClass.h
 
@interface MyClass
@end
 
// MyClass.m
 
@interface MyClass (PrivateMethods)
- (void)doSomething;
- (void)doWorkThatRequiresMeToDoSomething;
@end
 
@implementation MyClass
- (IBAction)myActionMethod:(id)sender {
    [self doSomething];
}
 
- (void)doSomething {
    // Implementation.
}
 
- (void)doWorkThatRequiresMeToDoSomething {
    // Pre-processing.
    [self doSomething];
    // Post-processing.
}
 
@end

Built-In Support For Nib Files

The AppKit and UIKit frameworks both provide a certain amount of automated behavior for loading and managing nib files in an application. Both frameworks provide infrastructure for loading an application’s main nib file. In addition, the AppKit framework provides support for loading other nib files through the NSDocument and NSWindowController classes. The following sections describe the built-in support for nib files, how you can take advantage of it, and ways to modify that support in your own applications.

The Application Loads the Main Nib File

Most of the Xcode project templates for applications come preconfigured with a main nib file already in place. All you have to do is modify this default nib file in the nib file and build your application. At launch time, the application’s default configuration data tells the application object where to find this nib file so that it can load it. In applications based on either AppKit and UIKit, this configuration data is located in the application’s Info.plist file. When an application is first loaded, the default application startup code looks in the Info.plist file for the NSMainNibFile key. If it finds it, it looks in the application bundle for a nib file whose name (with or without the filename extension) matches the value of that key and loads it.

Each View Controller Manages its Own Nib File

The UIViewController (iOS) and NSViewController (OS X) classes support the automatic loading of their associated nib file. If you specify a nib file when creating the view controller, that nib file is loaded automatically when you try to access the view controller’s view. Any connections between the view controller and the nib file objects are created automatically, and in iOS, the UIViewController object also receives additional notifications when the views are finally loaded and displayed on screen. To help manage memory better, the UIViewController class also handles the unloading of its nib file (as appropriate) during low-memory conditions.

For more information about how you use the UIViewController class and how you configure it, see View Controller Programming Guide for iOS.

Document and Window Controllers Load Their Associated Nib File

In the AppKit framework, the NSDocument class works with the default window controller to load the nib file containing your document window. The windowNibName method of NSDocument is a convenience method that you can use to specify the nib file containing the corresponding document window. When a new document is created, the document object passes the nib file name you specify to the default window controller object, which loads and manages the contents of the nib file. If you use the standard templates provided by Xcode, the only thing you have to do is add the contents of your document window to the nib file.

The NSWindowController class also provides automatic support for loading nib files. If you create custom window controllers programmatically, you have the option of initializing them with an NSWindow object or with the name of a nib file. If you choose the latter option, the NSWindowController class automatically loads the specified nib file the first time a client tries to access the window. After that, the window controller keeps the window around in memory; it does not reload it from the nib file, even if the window’s “Release when closed” attribute is set.

Loading Nib Files Programmatically

Both OS X and iOS provide convenience methods for loading nib files into your application. Both the AppKit and UIKit framework define additional methods on the NSBundle class that support the loading of nib files. In addition, the AppKit framework also provides the NSNib class, which provides similar nib-loading behavior as NSBundle but offers some additional advantages that might be useful in specific situations.

As you plan out your application, make sure any nib files you plan to load manually are configured in a way that simplifies the loading process. Choosing an appropriate object for File’s Owner and keeping your nib files small can greatly improve their ease of use and memory efficiency. For more tips on configuring your nib files, see “Nib File Design Guidelines.”

Loading Nib Files Using NSBundle

The AppKit and UIKit frameworks define additional methods on the NSBundle class (using Objective-C categories) to support the loading of nib file resources. The semantics for how you use the methods differs between the two platforms as does the syntax for the methods. In AppKit, there are more options for accessing bundles in general and so there are correspondingly more methods for loading nib files from those bundles. In UIKit, applications can load nib files only from their main bundle and so fewer options are needed. The methods available on the two platforms are as follows:

Whenever loading a nib file, you should always specify an object to act as File’s Owner of that nib file. The role of the File’s Owner is an important one. It is the primary interface between your running code and the new objects that are about to be created in memory. All of the nib-loading methods provide a way to specify the File’s Owner, either directly or as a parameter in an options dictionary.

One of the semantic differences between the way the AppKit and UIKit frameworks handle nib loading is the way the top-level nib objects are returned to your application. In the AppKit framework, you must explicitly request them using one of the loadNibFile:externalNameTable:withZone: methods. In UIKit, the loadNibNamed:owner:options: method returns an array of these objects directly. The simplest way to avoid having to worry about the top-level objects in either case is to store them in outlets of your File’s Owner object (see “Managing the Lifetimes of Objects from Nib Files”).

Listing 1-1 shows a simple example of how to load a nib file using the NSBundle class in an AppKit–based application. As soon as the loadNibNamed:owner: method returns, you can begin using any outlets that refer to the nib file objects. In other words, the entire nib-loading process occurs within the confines of that single call. The nib-loading methods in the AppKit framework return a Boolean value to indicate whether the load operation was successful.

Listing 1-1  Loading a nib file from the current bundle

- (BOOL)loadMyNibFile
{
    // The myNib file must be in the bundle that defines self's class.
    if (![NSBundle loadNibNamed:@"myNib" owner:self])
    {
        NSLog(@"Warning! Could not load myNib file.\n");
        return NO;
    }
    return YES;
}

Listing 1-2 shows an example of how to load a nib file in a UIKit–based application. In this case, the method checks the returned array to see if the nib objects were loaded successfully. (Every nib file should have at least one top-level object representing the contents of the nib file.) This example shows the simple case when the nib file contains no placeholder objects other than the File’s Owner object. For an example of how to specify additional placeholder objects, see “Replacing Proxy Objects at Load Time.”

Listing 1-2  Loading a nib in an iPhone application

- (BOOL)loadMyNibFile
{
    NSArray*    topLevelObjs = nil;
 
    topLevelObjs = [[NSBundle mainBundle] loadNibNamed:@"myNib" owner:self options:nil];
    if (topLevelObjs == nil)
    {
        NSLog(@"Error! Could not load myNib file.\n");
        return NO;
    }
    return YES;
}

Getting a Nib File’s Top-Level Objects

The easiest way to get the top-level objects of your nib file is to define outlets in the File’s Owner object along with setter methods (or better yet, properties) for accessing those objects. This approach ensures that the top-level objects are retained by your object and that you always have references to them.

Listing 1-3 shows the interface and implementation of a simplified Cocoa class that uses an outlet to retain the nib file’s only top-level object. In this case, the only top-level object in the nib file is an NSWindow object. Because top-level objects in Cocoa have an initial retain count of 1, an extra release message is included. This is fine because by the time the release call is made, the property has already been retained the window. You would not want to release top-level objects in this manner in an iPhone application.

Listing 1-3  Using outlets to get the top-level objects

// Class interface.
@interface MyController : NSObject
- (void)loadMyWindow;
@end
 
// Private class extension.
@interface MyController ()
@property (strong) IBOutlet NSWindow *window;
@end
 
 
// Class implementation
@implementation MyController
 
- (void)loadMyWindow {
    [NSBundle loadNibNamed:@"myNib" owner:self];
 
    // The window starts off with a retain count of 1
    // and is then retained by the property, so add an extra release.
    NSWindow *window = self.window;
    CFRelease(__bridge window);
}
@end

If you do not want to use outlets to store references to your nib file’s top-level objects, you must retrieve those objects manually in your code. The technique for obtaining the top-level objects differs depending on the target platform. In OS X, you must ask for the objects explicitly, whereas in iOS they are returned to you automatically.

Listing 1-4 shows the process for getting the top-level objects of a nib file in OS X. This method places a mutable array into the nameTable dictionary and associates it with the NSNibTopLevelObjects key. The nib-loading code looks for this array object and, if present, places the top-level objects in it. Because each object starts with a retain count of 1 before it is added to the array, simply releasing the array is not enough to release the objects in the array as well. As a result, this method sends a release message to each of the objects to ensure that the array is the only entity holding a reference to them.

Listing 1-4  Getting the top-level objects from a nib file at runtime

- (NSArray*)loadMyNibFile
{
    NSBundle*            aBundle = [NSBundle mainBundle];
    NSMutableArray*      topLevelObjs = [NSMutableArray array];
    NSDictionary*        nameTable = [NSDictionary dictionaryWithObjectsAndKeys:
                                            self, NSNibOwner,
                                            topLevelObjs, NSNibTopLevelObjects,
                                            nil];
 
    if (![aBundle loadNibFile:@"myNib" externalNameTable:nameTable withZone:nil])
    {
        NSLog(@"Warning! Could not load myNib file.\n");
        return nil;
    }
 
    // Release the objects so that they are just owned by the array.
    [topLevelObjs makeObjectsPerformSelector:@selector(release)];
    return topLevelObjs;
}

Obtaining the top-level objects in an iPhone application is much simpler and is shown in Listing 1-2. In the UIKit framework, the loadNibNamed:owner:options: method of NSBundle automatically returns an array with the top-level objects. In addition, by the time the array is returned, the retain counts on the objects are adjusted so that you do not need to send each object an extra release message. The returned array is the only owner of the objects.

Loading Nib Files Using UINib and NSNib

The UINib (iOS) and NSNib (OS X) classes provide better performance in situations where you want to create multiple copies of a nib file’s contents. The normal nib-loading process involves reading the nib file from disk and then instantiating the objects it contains. However, with the UINib and NSNib classes, the nib file is read from disk once and the contents are stored in memory. Because they are in memory, creating successive sets of objects takes less time because it does not require accessing the disk.

Using the UINib and NSNib classes is always a two-step process. First, you create an instance of the class and initialize it with the nib file’s location information. Second, you instantiate the contents of the nib file to load the objects into memory. Each time you instantiate the nib file, you specify a different File’s Owner object and receive a new set of top-level objects.

Listing 1-5 shows one way to load the contents of a nib file using the NSNib class in OS X. The array returned to you by the instantiateNibWithOwner:topLevelObjects: method comes already autoreleased. If you intend to use that array for any period of time, you should make a copy of it.

Listing 1-5  Loading a nib file using NSNib

- (NSArray*)loadMyNibFile
{
    NSNib*      aNib = [[NSNib alloc] initWithNibNamed:@"MyPanel" bundle:nil];
    NSArray*    topLevelObjs = nil;
 
    if (![aNib instantiateNibWithOwner:self topLevelObjects:&topLevelObjs])
    {
        NSLog(@"Warning! Could not load nib file.\n");
        return nil;
    }
    // Release the raw nib data.
    [aNib release];
 
    // Release the top-level objects so that they are just owned by the array.
    [topLevelObjs makeObjectsPerformSelector:@selector(release)];
 
    // Do not autorelease topLevelObjs.
    return topLevelObjs;
}

Replacing Proxy Objects at Load Time

In iOS, it is possible to create nib files that include placeholder objects besides the File’s Owner. Proxy objects represent objects created outside of the nib file but which have some connection to the nib file’s contents. Proxies are commonly used to support navigation controllers in iPhone applications. When working with navigation controllers, you typically connect the File’s Owner object to some common object such as your application delegate. Proxy objects therefore represent the parts of the navigation controller object hierarchy that are already loaded in memory, because they were created programmatically or loaded from a different nib file.

Each placeholder object you add to a nib file must have a unique name. To assign a name to an object, select the object in Xcode and open the inspector window. The Attributes pane of the inspector contains a Name field, which you use to specify the name for your placeholder object. The name you assign should be descriptive of the object’s behavior or type, but really it can be anything you want.

When you are ready to load a nib file containing placeholder objects, you must specify the replacement objects for any proxies when you call the loadNibNamed:owner:options: method. The options parameter of this method accepts a dictionary of additional information. You use this dictionary to pass in the information about your placeholder objects. The dictionary must contain the UINibExternalObjects key whose value is another dictionary containing the name and object for each placeholder replacement.

Listing 1-6 shows a sample version of an applicationDidFinishLaunching: method that loads the application’s main nib file manually. Because the application’s delegate object is created by the UIApplicationMain function, this method uses a placeholder (with the name “AppDelegate”) in the main nib file to represent that object. The proxies dictionary stores the placeholder object information and the options dictionary wraps that dictionary.

Listing 1-6  Replacing placeholder objects in a nib file

- (void)applicationDidFinishLaunching:(UIApplication *)application
{
    NSArray*    topLevelObjs = nil;
    NSDictionary*    proxies = [NSDictionary dictionaryWithObject:self forKey:@"AppDelegate"];
    NSDictionary*    options = [NSDictionary dictionaryWithObject:proxies forKey:UINibExternalObjects];
 
    topLevelObjs = [[NSBundle mainBundle] loadNibNamed:@"Main" owner:self options:options];
    if ([topLevelObjs count] == 0)
    {
        NSLog(@"Warning! Could not load myNib file.\n");
        return;
    }
 
    // Show window
    [window makeKeyAndVisible];
}
 

For more information about the options dictionary of the loadNibNamed:owner:options: method, see NSBundle UIKit Additions Reference.

Accessing the Contents of a Nib File

Upon the successful loading a nib file, its contents become ready for you to use immediately. If you configured outlets in your File’s Owner to point to nib file objects, you can now use those outlets. If you did not configure your File’s Owner with any outlets, you should make sure you obtain a reference to the top-level objects in some manner so that you can release them later.

Because outlets are populated with real objects when a nib file is loaded, you can subsequently use outlets as you would any other object you created programmatically. For example, if you have an outlet pointing to a window, you could send that window a makeKeyAndOrderFront: message to show it on the user’s screen. When you are done using the objects in your nib file, you must release them like any other objects.

Connecting Menu Items Across Nib Files

The items in an OS X application’s menu bar often need to interact with many different objects, including your application’s documents and windows. The problem is that many of these objects cannot (or should not) be accessed directly from the main nib file. The File’s Owner of the main nib file is always set to an instance of the NSApplication class. And although you might be able to instantiate a number of custom objects in your main nib file, doing so is hardly practical or necessary. In the case of document objects, connecting directly to a specific document object is not even possible because the number of document objects can change dynamically and can even be zero.

Most menu items send action messages to one of the following:

Messaging fixed objects is a relatively straightforward process that is usually best handled through the application delegate. The application delegate object assists the NSApplication object in running the application and is one of the few objects that rightfully belongs in the main nib file. If the menu item refers to an application-level command, you can implement that command directly in the application delegate or just have the delegate forward the message to the appropriate object elsewhere in your application.

If you have a menu item that acts on the contents of the frontmost window, you need to link the menu item to the First Responder placeholder object. If the action method associated with the menu item is specific to one of your objects (and not defined by Cocoa), you must add that action to the First Responder before creating the connection.

After creating the connection, you need to implement the action method in your custom class. That object should also implement the validateMenuItem: method to enable the menu item at appropriate times. For more information about how the responder chain handles commands, see Cocoa Event Handling Guide.