Question

A UITableView object has two properties: one delegate, and one dataSource. Are they exactly the same design pattern? I don’t see books or reference calling dataSource a second delegate for the UITableView.

Answer 1

No, they are not the same thing but they are using the delegate pattern.

They are both different and distinct delegates that serve different purposes. One property is for the UITableViewDataSource and the other is for UITableViewDelegate.

Here is how they are defined in UITableView.h:

@property(nonatomic, assign) id<UITableViewDataSource> dataSource
@property(nonatomic, assign) id<UITableViewDelegate> delegate

The delegate pattern is pretty simple and the Wikipedia entry sums it up pretty well:

In software engineering, the delegation pattern is a design pattern in
object-oriented programming where an object, instead of performing one
of its stated tasks, delegates that task to an associated helper
object. There is an Inversion of Responsibility in which a helper
object, known as a delegate, is given the responsibility to execute a
task for the delegator. The delegation pattern is one of the
fundamental abstraction patterns that underlie other software patterns
such as composition (also referred to as aggregation), mixins and
aspects.