Object Graphs

Object-oriented applications contain complex webs of interrelated objects. Objects are linked to each other by one object either owning or containing another object or holding a reference to another object to which it sends messages. This web of objects is called an object graph.

Even with very few objects, an application’s object graph becomes very entangled with circular references and multiple links to individual objects. Figure 1 shows an incomplete object graph for a simple Cocoa application in OS X. (Many more connections exist than are shown in this figure.) Consider the window’s view hierarchy portion of the object graph. This hierarchy is described by each view containing a list of all of its immediate subviews. However, views also have links to each other to describe the responder chain and the keyboard focus loop. Views also link to other objects in the application for target-action messages, contextual menus, and much more.

There are situations where you may want to convert an object graph, usually just a section of the full object graph in the application, into a form that can be saved to a file or transmitted to another process or machine and then reconstructed. Nib files and property lists are two examples in OS X where object graphs are saved to a file. Nib files are archives that represent the complex relationships within a user interface, such as a window’s view hierarchy. Property lists are serializations that store the simple hierarchical relationship of basic value objects. More details on archives and serializations, and how you can use them, are described in the following sections.