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CHAPTER 7: Dragging, Rotating, and Scaling
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We will now focus on one of the concepts I mentioned earlier only in passing: eventhandling. The four lines of code that follow deal with related events and the methods by which we want the computer to deal with these events.
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(void) (void) (void) (void) touchesBegan:(NSSet*)touches withEvent:(UIEvent*)event touchesMoved:(NSSet*)touches withEvent:(UIEvent*)event touchesEnded:(NSSet*)touches withEvent:(UIEvent*)event touchesCancelled:(NSSet*)touches withEvent:(UIEvent*)event
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In order to dig this code, we first need to remind ourselves what methods and arguments are. These four touch methods are called event-handling methods. Explaining an event-handling method is a bit tricky because it s a very abstract tool like using a time warp in order to advise Thomas Jefferson about drafting The Declaration of Independence on a computer. So before we talk about these four event-handling methods, let s take a close look at how a generic event-handling method works. Suppose you hear your phone ring. You know that somebody from the outside world is calling you. Do you: A) pick up the phone and say, Hello B) pick up the phone and say Sorry I need to call you back. or C) let the phone ring through to voicemail These are event-handling methods. You have different methods of handling a phone call, and choosing one depends on several factors: who is calling, what you are doing, how tired you are, how hungry you are, and so forth. Bearing this analogy in mind, the lines we re examining are event-handling methods for handling a touch from the user who is operating your iPhone or iPad app. The programmers at Apple have created event-handling methods that make it easy for you to decide whether you are going to pick up the phone, answer it, and so on. In our case though, we want to handle four different kinds of events. These touch events are events that are propagated through the responder chain. What s a responder chain Here s another analogy. The phone rings, but you don t want to answer it. You ask somebody (brother, sister, mother) to pick up the phone for you. What they do with the phone is totally up to them because you gave up your chance to answer the phone. That is how the responder chain works: events come in and objects can either handle the events (answer the phone yourself) or continue to pass the event down the responder chain (tell someone else to pick up the phone). Along these same lines you may see the term first responder in Interface Builder or some of the Apple documentation. This is the first object in the responder chain, and the first responder always gets the first opportunity with generated events. In the phone
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CHAPTER 7: Dragging, Rotating, and Scaling
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example, you would be the first responder, since you were given the opportunity to handle the phone call first. Most controls without targets (such as a button that does not have its target set) send their actions to the first responder by default. When a touch is placed on the device, the window uses hit testing to determine which view the touch was in and then passes the touch event information down the responder chain. If a view s userInteractionEnabled property is set to NO, the event will continue down the responder chain until a view is able to handle that event. A touch and its associated view are linked for the lifetime of the touch. This means that, even if the touch moves off the view that received the touch initially, that view is still in charge of that touch and no other views will receive touch information about that touch no matter where it moves. The generated events hold information on the touches that triggered them, as well as a timestamp, the event s type, and the event s subtype, all of which can be accessed through corresponding properties. New to iPhone OS 3.0 are motion events that are triggered similarly to touch events. They are targeted events that default to the first responder. The event object for a motion may have its subtype set to UIEventTypeMotionShake, which provides an easy way to detect shake events. The types and subtypes of a UIEvent object provide lots of useful information that can help to determine how an incoming event should be handled. Cocoa Touch, the view system of the iPhone OS, works according to a hierarchy. That is, views handle drawing themselves and their subviews. When a call like [[self view] addSubview:aView]; is made, a view is made a subview of self s view. Pretty simple, right Well, each view has a transform that describes the view s location, rotation, scaling, and other factors relative to that view s superview. This is exactly what we need in order to make our custom view scale, rotate, and move around in its superview, whenever a touch is sensed and identified. We can change the transform in many ways, but, for the most part, we only need to deal with distances and angles between touches to do everything we need. The structure CGAffineTransform is used to store and manipulate the transforms of views. Now that you re in the world of advanced programming, you should be comfortable using the Cstyle calls for CGAffineTransform. Take some time to peruse the documentation on CGAffineTransform and take a look at the view programming guides to get an Appleapproved in-depth description of how all this stuff works.
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