604	CHAPTER 17: Taps, Touches, and Gestures

Multitouch Terminology

Before we dive into the architecture, let’s go over some basic vocabulary. First, a gesture is any sequence of events that happens from the time you touch the screen with one or more fingers until you lift your fingers off the screen. No matter how long it takes, as long as one or more fingers remain against the screen, you are still within a gesture (unless a system event, such as an incoming phone call, interrupts it). Note that Cocoa Touch doesn’t expose any class or structure that represents a gesture. In some sense, a gesture is a verb, and a running app can watch the user input stream to see if one is happening.

A gesture is passed through the system inside a series of events. Events are generated when you interact with the device’s multitouch screen. They contain information about the touch or touches that occurred.

The term touch refers to a finger being placed on the screen, dragging across the screen, or being lifted from the screen. The number of touches involved in a gesture is equal to the number of fingers on the screen at the same time. You can actually put all five fingers on the screen, and as long as they aren’t too close to each other, iOS can recognize and track them all. Now, there aren’t many useful five-finger gestures, but it’s nice to know the iOS can handle one if necessary. In fact, experimentation has shown that the iPad can handle up to 11 simultaneous touches! This may seem excessive, but could be useful if you’re working on a multiplayer game, where several players are interacting with the screen at the same time.

A tap happens when you touch the screen with a finger and then immediately lift your finger off the screen without moving it around. The iOS device keeps track of the number of taps and can tell you if the user double-tapped, triple-tapped, or even 20tapped. It handles all the timing and other work necessary to differentiate between two single-taps and a double-tap, for example.

A gesture recognizer is an object that knows how to watch the stream of events generated by a user, and recognize when the user is touching and dragging in a way that matches a predefined gesture. The UIGestureRecognizer class and its various subclasses can help take a lot of work off your hands when you want to watch for common gestures, since it nicely encapsulates the work of looking for a gesture, and can be easily applied to any view in your application.

The Responder Chain

Since gestures are passed through the system inside events, and events are passed through the responder chain, you need to have an understanding of how the responder chain works in order to handle gestures properly. If you’ve worked with Cocoa for Mac OS X, you’re probably familiar with the concept of a responder chain, as the same basic mechanism is used in both Cocoa and Cocoa Touch. If this is new material, don’t worry; we’ll explain how it works.

www.it-ebooks.info

CHAPTER 17: Taps, Touches, and Gestures

605

Responding to Events

Several times in this book, we’ve mentioned the first responder, which is usually the object with which the user is currently interacting. The first responder is the start of the responder chain. There are other responders as well. Any class that has UIResponder as one of its superclasses is a responder. UIView is a subclass of UIResponder, and UIControl is a subclass of UIView, so all views and all controls are responders. UIViewController is also a subclass of UIResponder, meaning that it is a responder, as are all of its subclasses, such as UINavigationController and UITabBarController. Responders, then, are so named because they respond to system-generated events, such as screen touches.

If the first responder doesn’t handle a particular event, such as a gesture, it passes that event up the responder chain. If the next object in the chain responds to that particular event, it will usually consume the event, which stops the event’s progression through the responder chain. In some cases, if a responder only partially handles an event, that responder will take an action and forward the event to the next responder in the chain. That’s not usually what happens, though. Normally, when an object responds to an event, that’s the end of the line for the event. If the event goes through the entire responder chain and no object handles the event, the event is then discarded.

Let’s take a more specific look at the responder chain. The first responder is almost always a view or control and gets the first shot at responding to an event. If the first responder doesn’t handle the event, it passes the event to its view controller. If the view controller doesn’t consume the event, the event is then passed to the first responder’s parent view. If the parent view doesn’t respond, the event will go to the parent view’s controller, if it has one.

The event will proceed up the view hierarchy, with each view and then that view’s controller getting a chance to handle the event. If the event makes it all the way up through the view hierarchy without being handled by a view or a controller, the event is passed to the application’s window. If the window doesn’t handle the event, it passes that event to the application’s UIApplication object instance.

If UIApplication doesn’t respond to the event, there’s one more spot where you can build a global catchall as the end of the responder chain: the app delegate. If the app delegate is a subclass of UIResponder (which it normally is if you create your project from one of Apple’s application templates), the app will try to pass it any unhandled events. Finally, if the app delegate isn’t a subclass of of UIResponder or doesn’t handle the event, then the event goes gently into the good night.

This process is important for a number of reasons. First, it controls the way gestures can be handled. Let’s say a user is looking at a table and swipes a finger across a row of that table. What object handles that gesture?

If the swipe is within a view or control that’s a subview of the table view cell, that view or control will get a chance to respond. If it doesn’t respond, the table view cell gets a chance. In an application like Mail, where a swipe can be used to delete a message, the

www.it-ebooks.info