A data source is like a delegate, except that its methods supply the data for another object to display. The only Cocoa classes with data sources are UITableView and UIPickerView. A table view displays data in rows; a picker view displays selectable choices using a rotating drum metaphor. In each case, the data source must formally conform to a protocol with required methods (UITableViewDataSource and UIPickerViewDataSource, respectively).
It comes as a surprise to some beginners that a data source is necessary at all. Why isn’t a table’s data just part of the table? Or why isn’t there at least some fixed data structure that contains the data? The reason is that such policies would violate generality. Use of a data source separates the object that displays the data from the object that manages the data, and leaves the latter free to store and obtain that data however it likes (see on model–view–controller in Chapter 12). The only requirement is that the data source must be able to supply information quickly, because it will be asked for it in real time when the data needs displaying.
Another surprise is that the data source is different from the delegate. But this again is only for generality; it’s an option, not a requirement. There is no reason why the data source and the delegate should not be the same object, and most of the time they probably will be.
In this simple example, we implement a UIPickerView that allows the user to select by name a day of the week (the ...