Manually Loading nib Files

June 15, 2011

Interface Builder is the standard Mac OS and iOS user interface design tool. Layouts are saved as “nib” (NeXT Interface Builder) files. Recent versions of Interface Builder write these files as XML archives using the .xib file extension.

At runtime, nib files are loaded and assigned an “owner” object that forms the basis of the layout’s object hierarchy. This “owner” is very often a view controller object. In this article, we’ll focus specifically on UIKit and its UIViewController class.

The nib loading process is generally abstracted away as part of the standard UIViewController initialization process. Here’s a simple example:

- (id)init
{
if ((self = [super initWithNibName:@"SomeNib" bundle:nil])) {
// additional view controller initialization
}

return self;
}

-initWithNibName:bundle: handles all of the nib loading and instantiation work. While this is quite convenient, it has one major limitation: the nib file must have been compiled and bundled (in the NSBundle sense) along with the application itself. Fortunately, it is possible to manually load a nib from arbitrary data and associate its contents with a view controller.

The UINib Class

The UINib class provides all of the functionality needed to manually load nib files. One reason to do this is to pre-cache the contents of a nib file in memory, ready for quick deserialization and instantiation later in the application’s lifetime. Another reason (and our focus here) is to load the raw nib data from an alternate (non-NSBundle) source.

The UINib class provides two methods for loading nibs:

Once loaded, the nib’s objects can be instantiated and assigned an owner using the -instantiateWithOwner:options: instance method.

Manual View Controller Loading

How can the UINib class be used in conjunction with a view controller?

First, the view controller needs to call UIViewController’s designated initializer without a nib name. Note that passing nil as the nib name will trigger an automatic lookup for a nib named the same as the view controller class, so take care not to fall into that case unintentionally.

[super initWithNibName:nil bundle:nil]

Because this view controller hasn’t loaded a view during initialization, we must implement the -loadView method. This is where all of the manual loading is performed.

- (void)loadView
{
[super loadView];

UINib *nib = [UINib nibWithNibName:@"SomeNib" bundle:nil];
[nib instantiateWithOwner:self options:nil];
}

This code effectively reproduces the standard nib loading process that was previously being handled by -initWithNibName:bundle: itself.

Loading Raw nib Data

At this point, our code is now manually loading the nib’s objects, but it’s still reading the nib itself from our application bundle. We can go a step further and load arbitrary nib data using the +nibWithData:bundle: method.

- (void)loadView
{
[super loadView];

NSData *data = [NSData dataWithContentsOfFile:@"SomeNib.xib.bin"];

UINib *nib = [UINib nibWithData:data bundle:nil];
[nib instantiateWithOwner:self options:nil];
}

The nib data can come from pretty much anywhere, such as from a bundled file or a network-fetched resource. The only requirement is that the nib data be compiled. To compile an existing .xib file, use ibtool:

ibtool SomeNib.xib --compile SomeNib.xib.bin

Additional Considerations

Now that the basic approach has been established, there are a few practical considerations to keep in mind. First, we’ve largely ignored the bundle parameter in the above code examples. This argument instructs the nib loader where to look for dependent resources, such as images. Passing nil implies that the main application bundle ([NSBundle mainBundle]) should be used. There unfortunately doesn’t appear to be a way to redirect these lookups to non-bundle locations, however.

Next, the nib data needs to be available by the time the view controller is initialized. All I/O needs to block, and any stalls could result in a poor user experience. Given that, any non-bundled nib data needs to be fetched before the view is loaded.

Lastly, nibs can contain references to “external” objects. One such example is the “owner” object, which is explicitly resolved by the call to -instantiateWithOwner:options:. If the nib references any additional external objects, the loading code must provide those objects via a dictionary mapping passed through the options: interface’s UINibExternalObjects key.