The files with the .xib file extension contain development time format files. These files contain the interface files created with the Interface Builder software.
From the point of view of the UI designer, XIB files contain the views or windows that you design in Interface Builder — the controls, their layout and properties, and their connections to your code.
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It is important to understand that on a technical level, XIB files are stored object graphs. That means that an XIB file, essentially, is a hierarchical set of objects descriptions. When an XIB file gets loaded at runtime, all the objects defined in the XIB file get instantiated, configured, and connected as described in the XIB.
Each XIB file also knows about a special object called the "File's Owner". This object will not be created when the XIB is loaded. Rather, the object that initiated the loading of the XIB file will take the place of the File's Owner within the XIB's object graph — including any connections and references to it. We will see how that is useful and important soon.