Navigation Controller

Click anywhere on a view to go to the next one.

The status and navigation bars, not to mention the back button and the leading between the lines of text, looked better in iOS 6. What were they thinking of?

The tab bar controller we saw here let us visit the view controllers in any order. A navigation controller lets us visit the view controllers in only one specific order: Eighth Avenue, Sixth Avenue, Union Square, etc., and back again. You can’t jump directly from Eighth Avenue to Union Square without visiting Sixth Avenue along the way. See Bars and Navigation Bars.

The classic example of a navigation controller is in the Settings app that comes with the iPhone. The navigation controller provides the right-to-left animation as we drill deeper into the menus, and left-to-right animation as we come back out. It does not provide the menus themselves—they will come later, when we have class UITableView. If the navigation bar has no room for the title and the back button, hold the iPhone in landscape orientation.

Even if we have only one view controller and one view, we might still want to put a navigation controller atop the view controller just to see the view controller’s navigation bar. Every view controller has a navigation bar, but the navigation bar is visible only if there is a navigation controller above the view controller. The navigation bar can have attractive titles and buttons.

The NavigateAppDelegate creates the UINavigationController and puts it in the window. The UINavigationController contains one or more ViewControllers, one for each station on the L Train, Manhattan’s only east/west subway line. Each ViewController contains one View.

The application:didFinishLaunchingWithOptions: method of class NavigateAppDelgate creates only the first ViewController, not all of them, because it might be expensive to create objects of this class. We create the ViewControllers only as needed.

Source code in Navigate.zip

  1. main.m
  2. Class NavigateAppDelegate
  3. Class ViewController
  4. Class View

A kludge I resisted

It would be possible for a lowly view to talk directly to the application delegate. For example, the touchesEnded:withEvent: method of class View could be changed to the following. 

- (void) touchesEnded: (NSSet *) touches withEvent: (UIEvent *) event {
	UIApplication *application = [UIApplication sharedApplication];
	NavigateAppDelegate *applicationDelegate = application.delegate;
	[applicationDelegate nextStation];
}
To mention class NavigateAppDelegate in the file View.m, we would have to say 
#import "NavigateAppDelegate.h"
at the top of View.m.

But please resist this temptation. Instead of talking to objects all over the application, a view should restrict itself to talking to its view controller. The view controller can then talk to the model on behalf of the view. In our case, the model consists of the array of stations in the application delegate and the number of currently pushed stations.

Things to try

  1. To see the animation more clearly, give each view a different background color. In the initWithFrame: method of class View, change the background color to the following. The expression (CGFloat)rand() / RAND_MAX is a fraction in the range 0.0 to 1.0 inclusive. We have to cast (convert) at least one of the operands of the division to CGFloat because all-integer division would result in an integer quotient, not a fraction. 
    		self.backgroundColor =
			[UIColor colorWithRed: (CGFloat)rand() / RAND_MAX
			green: (CGFloat)rand() / RAND_MAX
			blue: 1.0
			alpha: 1.0
		];
    To get different random colors each time you run the app, insert the following code at the start of the application:didFinishLaunchingWithOptions: method of class NavigateAppDelegate. 
    	//Seed the random number generator with a different seed
	//each time the app is run.
	srand(time(NULL));

  2. In the nextStation method of class NavigateAppDelegate, what happens if you change the animated: to NO?

  3. The status bar is the top 20 pixels of the iPhone screen. It displays the current time, battery level, etc. Under the status bar, the navigation controller displays a navigation bar, 44 pairs of pixels high in portrait orientation, 32 pairs in landscape. It displays the title of the controller that is currently displayed by the navigation controller. To add a prompt to the top of the navigation bar (occcupying an additional 30 pixels), insert the following statement into the initWithTitle: method of class ViewController after setting the title. 
    		self.navigationItem.prompt =
			[NSString stringWithFormat: @"Welcome to %@.", self.title];
    I acknowledge that this prompt is inane.

  4. The navigation bar already has a button in the upper left corner (except when we’re at 8th Avenue). Let’s add a button in the upper right corner. Insert the following statement into the initWithTitle: method of class ViewController after setting the title and prompt. 
    		self.navigationItem.rightBarButtonItem = [[UIBarButtonItem alloc] initWithTitle: @"Go East"
			style: UIBarButtonItemStylePlain
			target: self
			action: @selector(nextStation)
		];
    You can now remove touchesEnded:withEvent: from class View. How could you prevent the last station (First Avenue) from having the “Go East” button?


  5. Set the toolbarhidden property of the UINavigationController to NO. 
    	((UINavigationController *)self.window.rootViewController).toolbarHidden = NO;
    The tool bar that appears at the bottom of the screen will be empty. We will use a tool bar here.

  6. Instead of puttings the arrays of data into the application delegate, should we have put them into a separate object called the model? Then we would have all three components: model/view/controller.

  7. We currently destroy a ViewController each time we retreat one station to the west, and re-create it each time we advance one station to the east. We can verify this by adding the following method to class ViewController. 
    - (void) dealloc {
	NSLog(@"ViewController %@ is being destroyed", self.title);
}
    It would save time if we could create the ViewController the first time we visit it, and keep it stored in an array in case we visit it again in the future. Give the application delegate the following instance variable. 
    	//all the stations we have already visited at any time
	NSMutableArray *visited;
    Initialize the instance variable in application:didFinishLaunchingWithOptions:. 
    	visited = [NSMutableArray arrayWithObject: firstController];
    Change the nextStation method of the application delegate to the following. 
    - (void) nextStation {
	
	UINavigationController *navigationController =
		(UINavigationController *)self.window.rootViewController;

	NSUInteger i = navigationController.viewControllers.count;
	if (i == titles.count) {
		//We are currently visiting the last station, and can go no farther.
		return;
	}

	if (visited.count <= i) {
		//This station is being visited for the first time.
		NSString *title = [titles objectAtIndex: i];
		ViewController *viewController = [[ViewController alloc] initWithTitle: title];
		[visited addObject: viewController];
	}

	[navigationController pushViewController: [visited objectAtIndex: i] animated: YES];
}