else

self.yellowViewController = nil;

}

This newly added code checks to see which view is currently being shown to the user and releases the controller for the other view by assigning nil to its property. This will cause the controller, along with the view it controls, to be deallocated, freeing up its memory.

TIP: Lazy loading is a key component of resource management on iOS, and you should implement it anywhere you can. In a complex, multiview application, being responsible and

flushing unused objects from memory can be the difference between an application that works

well and one that crashes periodically because it runs out of memory.

Implementing the Content Views

The two content views that we are creating in this application are extremely simple. They each have one action method that is triggered by a button, and neither one needs any outlets. The two views are also nearly identical. In fact, they are so similar that they could have been represented by the same class. We chose to make them two separate classes because that’s how most multiview applications are constructed.

Let’s declare an action method in each of the header files. First, in

BIDBlueViewController.h, add the following declaration:

#import <UIKit/UIKit.h>

@interface BIDBlueViewController : UIViewController

- (IBAction)blueButtonPressed;

@end

Save the file. Then add the following line to BIDYellowViewController.h:

#import <UIKit/UIKit.h>

@interface BIDYellowViewController : UIViewController

- (IBAction)yellowButtonPressed;

@end

Save this file as well.

Next, select BlueView.xib to open it in Interface Builder so we can make a few changes. First, we need to specify that the class that will load this nib from the file system is BIDBlueViewController. Single-click the File’s Owner icon and press 3 to bring up the identity inspector. File’s Owner defaults to NSObject; change it to

BIDBlueViewController.

Single-click the View icon in the dock, and then press 4 to bring up the object attributes inspector. In the inspector’s View section, click the color well that’s labeled Background, and use the popup color picker to change the background color of this view to a nice shade of blue. Once you are happy with your blue, close the color picker.

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Next, we’ll change the size of the view in the nib. In the object attributes inspector, the top section is labeled Simulated Metrics (see Figure 6–18). If we set these drop-down menus to reflect which top and bottom elements are used in our application, Interface Builder will automatically calculate the size of the remaining space.

Figure 6–18. The Simulated Metrics section of the view’s attributes inspector

The status bar is already specified, but here’s a tricky spot: since this view is going to be contained inside the view we created in SwitchView.xib, we shouldn’t actually specify a status bar, since doing so will shift our content a bit inside the containing view. So, click the Status Bar popup button, and then click None. Next, select the Bottom Bar popup and choose Toolbar to indicate that the enclosing view has a toolbar.

These settings will cause Interface Builder to calculate the correct size for our view automatically, so that we know how much space we have to work with. You can press5 to bring up the size inspector to confirm this. After making the change, the height of the window should be 436 pixels, and the width should still be 320 pixels.

Drag a Round Rect Button from the library over to the view, using the guidelines to center the button in the view, both vertically and horizontally. Double-click the button, and change its title to Press Me. Next, with the button still selected, switch to the connections inspector (by pressing 6), drag from the Touch Up Inside event to the File’s Owner icon, and connect to the blueButtonPressed action method.

We have one more thing to do in this nib, which is to connect BIDBlueViewController’s view outlet to the view in the nib, just as we did earlier in SwitchView.xib. Control-drag from the File’s Owner icon to the View icon, and select the view outlet.

Save the nib, and then go the project navigator and click YellowView.xib. We’re going to make almost exactly the same changes to this nib file.

First, click the File’s Owner icon in the dock and use the identity inspector to change its class to BIDYellowViewController.

Next, select the view and switch to the object attributes inspector. There, click the Background color well and select a bright yellow, and then close the color picker. Also, in the Simulated Metrics section, select Toolbar from the Bottom Bar popup, and switch the Status Bar popup to None.

Next, drag out a Round Rect Button from the library and use the guidelines to center it in the view. Then change its title to Press Me, Too. With the button still selected, use the connections inspector to drag from the Touch Up Inside event to the File’s Owner icon, and connect to the yellowButtonPressed action method.

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Finally, control-drag from the File’s Owner icon to the View icon, and select the view outlet.

When you’re finished, save the nib, and get ready to enter some more code.

The two action methods we’re going to implement do nothing more than show an alert (as we did in Chapter 4’s Control Fun application), so go ahead and add the following code to BIDBlueViewController.m:

#import "BIDBlueViewController.h"

@implementation BIDBlueViewController

- (IBAction)blueButtonPressed {

UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"Blue View Button Pressed"

message:@"You pressed the button on the blue view" delegate:nil

cancelButtonTitle:@"Yep, I did." otherButtonTitles:nil];

[alert show];

}

...

Save the file. Next, switch over to BIDYellowViewController.m, and add this very similar code to that file:

#import "BIDYellowViewController.h"

@implementation BIDYellowViewController

-(IBAction)yellowButtonPressed { UIAlertView *alert = [[UIAlertView alloc]

initWithTitle:@"Yellow View Button Pressed"

message:@"You pressed the button on the yellow view" delegate:nil

cancelButtonTitle:@"Yep, I did." otherButtonTitles:nil];

[alert show];

}

...

Save your code, and let’s take this bad boy for a spin. If your app crashes on launch or when you switch views, go back and make sure you connected all three view outlets.

When our application launches, it shows the view we built in BlueView.xib. When you tap the Switch Views button, it will change to show the view that we built in YellowView.xib. Tap it again, and it goes back to the view in BlueView.xib. If you tap the button centered on the blue or yellow view, you’ll get an alert view with a message indicating which button was pressed. This alert shows that the correct controller class is being called for the view that is being shown.

The transition between the two views is kind of abrupt, though. Gosh, if only there were some way to make the transition look nicer.

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Of course, there is a way to make the transition look nicer! We can animate the transition in order to give the user visual feedback of the change.

Animating the Transition

UIView has several class methods we can call to indicate that the transition between views should be animated, to indicate the type of transition that should be used, and to specify how long the transition should take.

Go back to BIDSwitchViewController.m, and replace your switchViews: method with this new version:

- (IBAction)switchViews:(id)sender {

[UIView beginAnimations:@"View Flip" context:nil]; [UIView setAnimationDuration:1.25];

[UIView setAnimationCurve:UIViewAnimationCurveEaseInOut];

if (self.yellowViewController.view.superview == nil) { if (self.yellowViewController == nil) {

self.yellowViewController =

[[BIDYellowViewController alloc] initWithNibName:@"YellowView" bundle:nil];

}

[UIView setAnimationTransition: UIViewAnimationTransitionFlipFromRight

forView:self.view cache:YES];

[self.blueViewController.view removeFromSuperview];

[self.view insertSubview:self.yellowViewController.view atIndex:0]; } else {

if (self.blueViewController == nil) { self.blueViewController =

[[BIDBlueViewController alloc] initWithNibName:@"BlueView" bundle:nil];

}

[UIView setAnimationTransition: UIViewAnimationTransitionFlipFromLeft

forView:self.view cache:YES];

[self.yellowViewController.view removeFromSuperview];

[self.view insertSubview:self.blueViewController.view atIndex:0];

}

[UIView commitAnimations];

}

Compile this new version, and run your application. When you tap the Switch Views button, instead of the new view just snapping into place, the old view will flip over to reveal the new view, as shown in Figure 6–19.

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Figure 6–19. One view transitioning to another, using the flip style of animation

In order to tell iOS that we want a change animated, we need to declare an animation block and specify how long the animation should take. Animation blocks are declared by using the UIView class method beginAnimations:context:, like so:

[UIView beginAnimations:@"View Flip" context:NULL]; [UIView setAnimationDuration:1.25];

beginAnimations:context: takes two parameters. The first is an animation block title. This title comes into play only if you take more direct advantage of Core Animation, the framework behind this animation. For our purposes, we could have used nil. The second parameter is a (void *) that allows you to specify an object (or any other C data type) whose pointer you would like associated with this animation block. We used NULL here, since we don’t need to do that.

After that, we set the animation curve, which determines the timing of the animation. The default, which is a linear curve, causes the animation to happen at a constant speed. The option we set here, UIViewAnimationCurveEaseInOut, specifies that the animation should start slow but speed up in the middle, and then slow down again at the end. This gives the animation a more natural, less mechanical appearance.

[UIView setAnimationCurve:UIViewAnimationCurveEaseInOut];

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