iPhone and iPad App 24-Hour Trainer

Chapter 9

Pickers

A picker view (Figure 9-1) is a user interface component that enables a user to pick a value from a set of related values using a slot machine–style interface.

Each wheel of the picker view is called a component, and it is fairly common to have picker view with multiple components. Each component can have a different number of items in it (Figure 9-2).

Figure 9-1

Figure 9-2

A picker view is encapsulated by the UIPickerView class, which is part of the UIKit framework. Apple provides a special picker for allowing the user to select date and time. This component is called the date picker and is covered in the next lesson.

A picker requires a data source object and a delegate object. The data source object is one that implements the UIPickerViewDataSource protocol and provides information on the number of components, and rows-per-component of the picker.

The delegate object implements the UIPickerViewDelegate protocol and has methods that are called when the current selection in a component has changed.

The delegate and data source objects could both be the same object, and in many cases the duties of these objects are performed by the view controller. However, it is very possible for them to be independent objects.

Creating a picker view is a simple matter of ...