Touch Events and UIResponder - iOS Programming: The Big Nerd Ranch Guide - page 540

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Touch Events and UIResponder
In Chapter 6 , you created a UIScrollView that handled touch events to scroll a view
and even handled a multi-touch event to zoom. The UIScrollView class makes
scrolling and zooming easy to implement. But what if you want to do something else,
something special or unique, with touch events?
In this chapter, you are going to create a view that lets the user draw lines by dragging
across the view ( Figure 19.1 ). Using multi-touch, the user will be able to draw more than
one line at a time. Double-tapping will clear the screen and allow the user to begin again.
Figure 19.1 A drawing program