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Creating advanced view controllers
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Tab-based interfaces Navigation-based interfaces The flipside controller
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When we started our look at view controllers in chapter 13, we promised that we d return to the more advanced view controllers that manage several pages of content at once. That s the purpose of this chapter: to introduce you to the final fundamental building block of the iPhone OS that allows you to build complex multipage applications. In this chapter we ll take an in-depth look at two view controllers: the tab bar controller and the navigation controller. We ll also take a briefer look at the flipside controller that appears in one of Xcode s templates and talk about some modal controllers that we ll see in part 4 of this book. As in our previous chapter on view controllers, we ll offer some more skeletal examples since our main purpose is to provide you with the reusable programming frameworks that will allow you to use these controllers in your own programs. Let s kick off our discussion with the tab bar view controller.
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The tab bar view controller
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15.1 The tab bar view controller
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Of the multipage view controllers, the tab bar is the easiest to use because it supports simple navigation between several views. As with all of the advanced view controllers, it has a complex underlying structure incorporating several objects that work in tandem.
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15.1.1 The anatomy of a tab bar controller
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To function, a tab bar view controller requires a hierarchy of at least six objects:
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One UITabBarController A minimum of two UIViewControllers One UITabBar A minimum of two UITabBarItems
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This hierarchy of objects is depicted in figure 15.1. The tab bar controller and its associated view controllers are the heart of this setup. Essentially the tab bar controller switches off between different pages, each of which uses a view controller to manage its events. In Xcode you d have to create and hook up these view controllers by hand, while in Interface Builder (which is what we ll be using) it s automated. In either case, you ll need to fill in the controllers views once your controllers are ready to go. The tab bar itself is created automatically when you instantiate a tab bar controller. It displays a set of radio buttons that go at the bottom of the page. Each of those buttons is a tab bar item (which Interface Builder also creates automatically). Each tab bar item then links to an individual view controller. Usually you shouldn t have to mess with the tab bar at all; you can do all the modifications you require through either the tab bar controller or the view controllers. The connection between the tab bar controller and its tab bar is a simple delegation, as we ve seen in use in previous chapters. The tab bar has a delegate property that is hooked up to the controller, which must respond to the UITabBarDelegate protocol. The tab bar controller can also designate a delegate. The controller s delegate must follow the UITabBarControllerDelegate protocol. This protocol requires response to two types of high-level events: when the tab bar is rearranged and when a view controller is selected.
UITabBarController
UITabBar UIViewController UIViewController
UITabBarItem
UITabBarItem
Figure 15.1 A collection of six objects (at minimum) is required to create a functioning tab bar controller.
Creating advanced view controllers
15.1.2 Creating a tab bar controller
Each of the advanced view controllers has its own Xcode template that you can use to immediately instantiate the controller. Since this is our first advanced view controller, though, we ll look at how you d create it by hand before we move over to simpler, template-driven object creation.
CREATING YOUR TAB BAR CONTROLLER BY HAND
To create a tab bar controller manually, begin with the Window-Based Application template. Use it to create a project imaginatively called tabex. Once you ve created your project, you should pop straight over to Interface Builder by clicking on the MainWindow.xib file. To create a tab bar controller:
Drag the Tab Bar Controller object from the Library window (where you ll find it under Controllers) to the nib display window. Drop the Controller down next to your window object. When you do that, a tab bar controller Main display window should appear. Dismiss your old Main display; you won t need it anymore. Instead you ll create new objects as subviews of your tab bar controller.
The results are shown in figure 15.2. Believe it or not, that s it. All six objects of note have been created. The tab bar controller is accessible from the nib display window. The other five objects are accessible from the black bar at the bottom of the Main display window. Click a button once to get its UIViewController and a second time to get its UITabBarItem. Click in the middle of the strip (between the buttons) to access the UITabBar. By Figure 15.2 Just dragging a tab bar selecting these items, you can set their attributes, con- controller to the nib display window creates the whole tab bar interface. nections, size, and identity. We took this slight diversion into the harder side of tab bar controller design to show what all the objects look like in Interface Builder. If you ve been following on a computer, we suggest clicking around for a while to see how everything works. Once you ve seen all of the fundamental objects that are created as part of an advanced view controller, we ve played the WindowBased Application template s last trick. In the future we re just going to jump straight to the appropriate template for each sort of view controller starting with the tab bar controller template.
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