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Monitoring events and actions
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Figure 6.9 A text field and a slider conspire to set the color of the iPhone s background.
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how you can combine action management by letting multiple controls point to a single method, a technique that will be useful in more complex programs. As usual, more information about both of these controls is available in the Apple class references, including lots of methods and properties that we didn t talk about.
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Throughout the latter half of this chapter, you ve seen controls that are tied to the fully fledged target-action mechanism. In the next chapter, that will change when you see the same idea in a somewhat simplified form. Sometimes, buttons or other controls are built into other classes of objects (such as the button that can be built into the navigation bar). These controls have special methods that allow them to automatically create a target-action pair. As a result, you don t have to go through the nuisance of calling the addTarget:action:forControlEvents: method separately. We ll point out this technique when we encounter it as part of the navigation controller.
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There are numerous control objects that we ve opted not to cover here, mainly because they use the same general principles as those we ve talked about. Nonetheless, they ll remain an important factor throughout the rest of this book.
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Introducing notifications
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In particular, controls represent one of the main ways that users can offer input to your programs, and we ll discuss them when we talk about data in chapter 9. We ll also offer more complex programs that use a variety of controls from chapter 9 on. Through those examples, the majority of the UI controls will receive some coverage in this book.
Introducing notifications
As we mentioned in chapter 2, there s one other way that a program can learn about events: through notifications. When directly manipulating events or actions, as you have throughout this chapter, individual objects receive events because the events occurred in their view, because the events occurred in a subview, or because the events occurred in a view that has delegated to them. Notifications step outside this paradigm. Now, an object registers to receive notice when certain events occur. These are often events that lie beyond the standard view hierarchy, such as information when a network connection closes or when the device s orientation changes. Notably, these notifications are also broadcast messages: many different objects can be notified when the event occurs. All notifications occur through the NSNotificationCenter. You must create a copy of this shared object to use it:
[NSNotificationCenter defaultCenter]
Afterward, you may use the addObserver:selector:name:object: method to request a certain notification. The Observer: is the object that receives the notification method (usually, self), the selector: is the method that is called in the observer, name: is the name of the notification (which is in the class reference), and the object: can be used if you want to restrict which objects you receive notification from (but it s usually set to nil). For example, to receive the UIDeviceOrientationDidChangeNotification notification that we ll talk about in chapter 10, you might use the following code:
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(deviceDidRotate:) name:@"UIDeviceOrientationDidChangeNotification" object:nil];
Overall, notification programming tends to have four steps:
You learn that there s a notification by reading the appropriate class reference (UIDevice in this case). You may need to explicitly turn on the notification (as is indeed the case for UIDeviceOrientationDidChangeNotification). You write a method that will respond to the notification (in this case, deviceDidRotate:). You connect the notification to the method with the NSNotificationCenter.
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Monitoring events and actions
There is considerably more power in the notification system. Not only can you set up multiple observers, but you can also post your own notifications. If you want more information on these advanced features, you should read the class references on NSNotificationCenter, NSNotification, and NSNotificationQueue.
Summary
The iPhone OS includes an extensive set of frameworks that takes care of a lot of details for you, making your programming as painless as possible. You ve seen this to date in everything you ve done, as sophisticated objects appear on screen with almost no work. The same applies to the iPhone s and iPad s event system. There is a complex underlying methodology. It centers on a responder chain and granular reporting of touches and allows you to follow precise user gestures. You may occasionally have to manipulate events via these more complex means. But the iPhone and iPad also support a higher-level action system that lets programs respond to specific actions applied to controls rather than more freeform gestures. We ve explained how to use both, but it s the target-action mechanism that you re more likely to rely on when programming. With actions and events out of the way, we re ready to look at the final fundamental building block of the SDK. We ve already discussed views, controls, and basic view controllers, but another category of object is critical for most SDK programming: the advanced view controller that allows for navigation over multiple screens of content. That s the basis of the next chapter.
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