Chapter 9

Navigation Controllers

and Table Views

In the previous chapter, you mastered the basics of working with table views. In this chapter, you’ll get a whole lot more practice, because we’re going to explore navigation controllers.

Table views and navigation controllers work hand in hand. Strictly speaking, a navigation controller doesn’t need a table view in order to do its thing. As a practical matter, however, when you implement a navigation controller, you almost always implement at least one table, and usually several, because the strength of the navigation controller lies in the ease with which it handles complex hierarchical data. On the iPhone’s small screen, hierarchical data is best presented using a succession of table views.

In this chapter, we’re going to build an application progressively, just as we did with the Pickers application back in Chapter 7. We’ll get the navigation controller and the first view controller working, and then we’ll start adding more controllers and layers to the hierarchy. Each view controller we create will reinforce some aspect of table use or configuration:

How to drill down from table views into child tables

How to drill down from table views into content views, where detailed data can be viewed and even edited

How to use a table list to allow the user to select from multiple values

How to use edit mode to allow rows to be deleted from a table view

That’s a lot, isn’t it? Well, let’s get started with an introduction to navigation controllers.

Navigation Controller Basics

The main tool you’ll use to build hierarchical applications is UINavigationController.

UINavigationController is similar to UITabBarController in that it manages, and swaps

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in and out, multiple content views. The main difference between the two is that UINavigationController is implemented as a stack, which makes it well suited to working with hierarchies.

Do you already know everything there is to know about stacks? Scan through the following subsection, and we’ll meet you at the beginning of the next subsection, “A Stack of Controllers.” If you’re new to stacks, continue reading. Fortunately, stacks are a pretty easy concept to grasp.

Stacky Goodness

A stack is a commonly used data structure that works on the principle of last in, first out. Believe it or not, a Pez dispenser is a great example of a stack. Ever try to load one? According to the little instruction sheet that comes with each and every Pez dispenser, there are a few easy steps. First, unwrap the pack of Pez candy. Second, open the dispenser by tipping its head straight back. Third, grab the stack (notice the clever way we inserted the word “stack” in there!) of candy, holding it firmly between your pointer finger and thumb, and insert the column into the open dispenser. Fourth, pick up all the little pieces of candy that flew all over the place because these instructions just never work.

OK, so far this example has not been particularly useful. But what happens next is. As you pick up the pieces and jam them, one at a time, into the dispenser, you are working with a stack. Remember that we said a stack was last in, first out? That also means first in, last out. The first piece of Pez you push into the dispenser will be the last piece that pops out. The last piece of Pez you push in will be the first piece you pop out. A computer stack follows the same rules:

When you add an object to a stack, it’s called a push. You push an object onto the stack.

The first object you push onto the stack is called the base of the stack.

The last object you pushed onto the stack is called the top of the stack (at least until it is replaced by the next object you push onto the stack).

When you remove an object from the stack, it’s called a pop. When you pop an object off the stack, it’s always the last one you pushed onto the stack. Conversely, the first object you push onto the stack will always be the last one you pop off the stack.

A Stack of Controllers

A navigation controller maintains a stack of view controllers. Any kind of view controller is fair game for the stack. When you design your navigation controller, you’ll need to specify the very first view the user sees. As we’ve discussed in previous chapters, that view is called the root view controller, or just root controller, and is the base of the navigation controller’s stack of view controllers. As the user selects the next view to

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display, a new view controller is pushed onto the stack, and the view it controls appears. We refer to these new view controllers as subcontrollers. As you’ll see, this chapter’s application, Nav, is made up of a navigation controller and six subcontrollers.

Take a look at Figure 9–1. Notice the navigation button in the upper-left corner of the current view. The navigation button is similar to a web browser’s back button. When the user taps that button, the current view controller is popped off the stack, and the previous view becomes the current view.

Figure 9–1. The Settings application uses a navigation controller. In the upper left is the navigation button used to pop the current view controller off the stock, returning you to the previous level of the hierarchy. The title of the current content view controller is also displayed.

We love this design pattern. It allows us to build complex hierarchical applications iteratively. We don’t need to know the entire hierarchy to get things up and running. Each controller only needs to know about its child controllers so it can push the appropriate new controller object onto the stack when the user makes a selection. You can build up a large application from many small pieces this way, which is exactly what we’re going to do in this chapter.

The navigation controller is really the heart and soul of many iPhone apps, but when it comes to iPad apps, the navigation controller plays a more marginal role. A typical example of this is the Mail app, which features a hierarchical navigation controller to let the user navigate among all their mail servers, folders, and messages. In the iPad version of Mail, the navigation controller never fills the screen, but appears either as a sidebar or a temporary popover window. We’ll dig into that usage a little later, when we cover iPad-specific GUI functionality in Chapter 11.

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