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An introduction to events
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First responders and keyboards
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Before we leave the topic of responders, we d like to mention that the first responder is an important concept. Because this first responder is the object that can accept input, it sometimes takes a special action to show its readiness for input. This is particularly true for text objects like UITextField and UITextView, which (if editable) pop up a keyboard when they become the first responder. This has two immediate consequences. If you want to pop up a keyboard for the text object, you can do so by turning it into the first responder: [myText becomeFirstResponder]; Similarly, if you want to get rid of a keyboard, you must tell your text object to stop being the first responder: [myText resignFirstResponder]; We ll discuss these ideas more when you encounter your first editable text object toward the end of this chapter.
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If an event gets all the way up through the responder chain to the window and it can t deal with an event, then it moves up to the UIApplication, which most frequently punts the event to its own delegate: the application delegate, an object that you ve been using in every program to date. Ultimately, you, the programmer, must decide what in the responder chain will respond to events in your program. You should keep two factors in mind when you make this decision: how classes of events can be abstracted together at higher levels in your chain, and how you can build your event management using the concepts of MVC. At the end of this section, we ll address how you can subvert this responder chain by further regulating events, but for now let s build on its standard setup.
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Touches and events
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Now that you know a bit about how events find their way to the appropriate object, we can dig into how they re encoded by the SDK. First, we want to offer a caveat: usually you won t need to worry about this level of detail because the standard UIKit objects generally convert low-level events into higher-level actions for you, as we discuss in the second half of this chapter. With that said, let s look at the nuts and bolts of event encoding. The SDK abstracts events by combining a number of touches (which are represented by UITouch objects) into an event (which is represented by a UIEvent object). An event typically begins when the first finger touches the screen and ends when the last finger leaves the screen. In addition, it should generally include only those touches that happen in the same view.
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In this chapter, you ll work mainly with UITouches (which make it easy to parse single-touch events) and not with UIEvents (which are more important for parsing multitouch events). Let s lead off with a more in-depth look at each.
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UITOUCH REFERENCE
A UITouch object is created when a finger is placed on the screen, moves on the screen, or is removed from the screen. A handful of properties and instance methods can give you additional information on the touch, as detailed in table 6.1.
Table 6.1 Additional properties and methods can tell you precisely what happened during a touch event. Type Property Summary Returns a touch phase constant, which indicates whether touch began, moved, ended, or was canceled The number of times the screen was tapped When the touch occurred or changed The view where the touch began The window where the touch began The current location of the touch in the specified view The previous location of the touch in the specified view
Method or property phase
tapCount timestamp view window locationInView: previousLocationInView:
Property Property Property Property Method Method
Together, the methods and properties shown in table 6.1 offer considerable information about a touch, including when and how it occurred. Only the phase property requires additional explanation. It returns a constant that can be set to one of five values: UITouchPhaseBegan, UITouchPhaseMoved, UITouchPhaseStationary, UITouchedPhaseEnded, or UITouchPhaseCancelled. You ll often want to have different event responses based on exactly which phase a touch occurred in, as you ll see in the event example.
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