XIB, Interface Builder File (.xib)
The XIB Developer file format
Interface Builder, a graphic-based editor for the development and testing of Apple user interfaces, stores its data using XIB files. They basically contain all information about an application interface. The XIB file format was introduced in the Interface Builder version 3. Previously, Interface Builder used the NIB file format. Both formats are available for saving in Interface Builder 3.0 and onward.
Technical details of XIB files
XIB files are basically text files as the data contained in them is stored in a plain-text format. The data inside the XIB files are made up from flattened user interface objects. Thus, the content of the file has to be loaded very early in the execution process of the program to provide all necessary information. They differ from their predecessor, the NIB file format, which are commonly known as “deployment” files. The XIB file, on the other hand, is also called a “development-time” file. The latter provides an even tighter integration of data within an Xcode project. If Xcode requires a NIB file instead, it automatically converts the XIB files into the needed format.