Life as Clay

Pesky!

This is another challenge that is easy but can trip you up when you test it… The challenge reads:

Create a nib file with a custom About panel. Add an outlet to AppController to point to the new window. Also add a showAboutPanel: method. Load the nib by using NSBundle, and make AppController File’s Owner.

The easy part of this is that you don’t have to create a subclass like you did with PreferenceController. Read the bits about NSBundle and you’ll see that all you have to do is to modify the File’s Owner (AppController) and create the nib/xib file. That’s simple enough.

I initially didn’t understand the purpose of creating an IBOutlet for the About panel. After testing the application, I realized that the implementation of showAboutPanel: allowed for multiple panels to be opened.

Pesky! Read the rest of the post to see the code and instructions.

To do this properly:

• Create the About.xib file in the same manner as done with the preferences file during the chapter. Make sure that you set the class of File’s Owner to be AppController. You don’t need any of the color wells or check boxes. You do, however, need to change one of the check boxes. Select the Panel object in Interface Builder and then UNcheck the “Release When Closed” check box in the Behavior section of the Attributes inspector. If you do not uncheck this, then the IBOutlet that you declare in AppController.h is released and you can only open the About panel one time without the program crashing.
• Follow the directions and declare IBOutlet NSPanel aboutPanel; in AppController.h. This is part of what I didn’t understand at first, since the bit in the book about NSBundle points out that you can load the nib file without NSWindowController.
• Declare -(IBAction)showAboutPanel:(id)sender; in AppController.h. Your AppController.h file should look like this:

#import <Cocoa/Cocoa.h>
@class PreferenceController;

@interface AppController : NSObject {
	PreferenceController *preferenceController;
	IBOutlet NSPanel *aboutPanel;
}

- (IBAction)showPreferencePanel:(id)sender;
- (IBAction)showAboutPanel:(id)sender;

@end

• Open MainMenu.xib in Interface Builder and connect the About menu item to App Controller. Control-click on AppController and drag from the received actions for showAboutPanel: to the About menu item and release. Follow the same procedure that you did for the preference panel earlier in the chapter. The method show below is, incidentally, the same as control dragging from the About RaiseMan menu item and releasing it over App Controller, then selecting showAboutPanel: from the Received Actions popup.

• Return to About.xib in Interface Builder and control-click on File’s Owner. You need to tell File’s Owner (AppController) what your IBOutlet called aboutPanel points to, so drag from the little circle to the Panel icon in the same window, like this:

• Ok, now you need to write the -(IBAction)showAboutPanel: method in AppController.m. However, keep in mind that you want to check to see whether the aboutPanel already is open. Create a simple if statement. This was the pesky part and is one of the reasons for creating the IBOutlet. The logic is: if the aboutPanel does not exist, create it and bring it to the front. If it does exist, bring it to the front. The code for AppController.m looks like this:

#import "AppController.h"
#import "PreferenceController.h"

@implementation AppController

- (IBAction)showPreferencePanel:(id)sender
{
	// Is preferenceController nil?
	if (!preferenceController) {
		preferenceController = [[PreferenceController alloc] init];
	}
	NSLog(@"showing %@", preferenceController);
	[preferenceController showWindow:self];
}

- (IBAction)showAboutPanel:(id)sender
{
	if (!aboutPanel) {
		BOOL successful = [NSBundle loadNibNamed:@"About" owner:self];

		// This makes the about panel the key window - gives it focus
		[aboutPanel makeKeyAndOrderFront:nil];

		NSLog(@"About panel loaded with success status %d.", successful);
		return;
	}

	// The About panel is open, so just give it focus.
	[aboutPanel makeKeyAndOrderFront:nil];
	NSLog(@"About panel given focus.");
}
@end

Now, assuming that you followed the steps and connected everything in Interface Builder, you should be set to test the application. Make sure that you can’t open multiple about panels.

On a larger scale, think about this from the point of view of using NSBundle to open the About.xib file. The text just above the Challenge in the book says that you could do it like this: BOOL successful = [NSBundle loadNibNamed:@"About" owner:someObject]; (In this instance, since this code exists within AppController and AppController is the File’s Owner, then someObject can be replaced with self.) This code is a ‘dumb’ opening of the nib file. It doesn’t know if the About panel already is open.

To prevent it from doing the same thing repeatedly, you have to insert the if statement to check whether the window already is open. Hence, the declaration of the IBOutlet NSPanel aboutPanel; It allows you to check for the existence of the About window.