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Creating Your Own Popover

There’s still one piece of iPad GUI technology that we haven’t dealt with in quite enough detail yet: the creation and display of your own popover. So far, we’ve had a UIPopoverController handed to us from a UISplitView delegate method, which let us keep track of it in an instance variable so we could force it to go away, but popovers really come in handy when you want to present your own view controllers.

To see how this works, we’re going to add a popover to be activated by a permanent toolbar item (unlike the one that the UISplitView delegate method gives us, which is meant to come and go). This popover will display a table view containing a list of languages. If the user picks a language from the list, the web view will load whatever Wikipedia entry was already showing in the new language. This will be simple enough to do, since switching from one language to another in Wikipedia is just a matter of changing a small piece of the URL that contains an embedded country code.

NOTE: Both uses of popovers in this example are in the service of showing a UITableView, but don’t let that mislead you—UIPopoverController can be used to handle the display of any view controller content you like! We’re sticking with table views for this example because it’s a common use case, it’s easy to show in a relatively small amount of code, and it’s something with which you should already be quite familiar.

Start off by right-clicking the Presidents folder in Xcode and selecting New File… from the contextual menu. When the assistant appears, select Cocoa Touch, then select UIViewController subclass, and then click Next. On the next screen, name the new class

BIDLanguageListController and select UITableViewController from the Subclass of field. Turn on the checkbox next to Targeted for iPad, and turn off the checkbox next to With XIB for user interface. Click Next, double-check the location where you’re saving the file, and click Create.

The BIDLanguageListController is going to be a pretty standard table view controller class. It will display a list of items and let the detail view controller know when a choice is made by using a pointer back to the detail view controller. Edit BIDLanguageListController.h, adding the bold lines shown here:

#import <UIKit/UIKit.h>

@class BIDDetailViewController;

@interface BIDLanguageListController : UITableViewController

@property (weak, nonatomic) BIDDetailViewController *detailViewController; @property (strong, nonatomic) NSArray *languageNames;

@property (strong, nonatomic) NSArray *languageCodes;

@end

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These additions define a pointer back to the detail view controller (which we’ll set from code in the detail view controller itself when we’re about to display the language list), as well as a pair of arrays for containing the values that will be displayed (English, French, and so on) and the underlying values that will be used to build an URL from the chosen language (en, fr, and so on).

If you copied and pasted this code from the book’s source archive (or e-book) into your own project or typed it yourself a little sloppily, you may not have noticed an important difference in how the detailViewController property was declared earlier. Unlike most properties that reference an object pointer, we declared this one using weak instead of strong. This is something that we must do in order to avoid a retain cycle.

What’s a retain cycle? It’s a situation where a set of two or more objects have retained one another in a circular fashion. Each object has a retain counter of one or higher and will therefore never release the pointers it contains, so they will never be deallocated either. Most potential retain cycles can be avoided by carefully considering the creation of your objects, often by trying to figure out who “owns” whom. In this sense, an instance of BIDDetailViewController owns an instance of BIDLanguageListController, because it’s the BIDDetailViewController that actually creates the BIDLanguageListController in order to get a piece of work done. Whenever you have a pair of objects that each needs to refer to one another, you’ll usually want the owner object to retain the other object, while the other object should specifically not retain its owner. Since we’re using the ARC feature that Apple introduced in Xcode 4.2, the compiler does most of the work for us. Instead of paying attention to the details about releasing and retaining objects, all we need to do is declare a property with the weak keyword instead of strong. ARC will do the rest!

Now, switch over to BIDLanguageListController.m to implement the following changes. At the top of the file, start by importing the header for BIDDetailViewController, and then synthesize getters and setters for the properties you declared:

#import "BIDLanguageListController.h"

#import "BIDDetailViewController.h"

@implementation BIDLanguageListController

@synthesize languageNames; @synthesize languageCodes; @synthesize detailViewController;

.

.

.

Then scroll down a bit to the viewDidLoad method, and add a bit of setup code:

- (void)viewDidLoad { [super viewDidLoad];

self.languageNames = [NSArray arrayWithObjects:@"English", @"French", @"German", @"Spanish", nil];

self.languageCodes = [NSArray arrayWithObjects:@"en", @"fr", @"de", @"es", nil]; self.clearsSelectionOnViewWillAppear = NO;

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self.contentSizeForViewInPopover = CGSizeMake(320.0, [self.languageCodes count] * 44.0);

}

This sets up the language arrays and also defines the size that this view will use if shown in a popover (which, as we know, it will be). Without defining the size, we would end up with a popover stretching vertically to fill nearly the whole screen, even with only four entries in it.

Farther down, we have a few methods generated by Xcode’s template that don’t contain particularly useful code—just a warning and some placeholder text. Let’s replace those with something real:

- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView { #warning Potentially incomplete method implementation.

// Return the number of sections. return 0;

return 1;

}

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section

{

#warning Incomplete method implementation.

// Return the number of rows in the section. return 0;

return [self.languageCodes count];

}

Then add a line near the end of tableView:cellForRowAtIndexPath: to put a language name into a cell:

// Configure the cell.

cell.textLabel.text = [languageNames objectAtIndex:[indexPath row]]; return cell;

Next, fix tableView:didSelectRowAtIndexPath: by eliminating the comment block it contains and adding this new code instead:

- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath {

detailViewController.languageString = [self.languageCodes objectAtIndex: [indexPath row]];

}

Note that BIDDetailViewController doesn’t actually have a languageString property. We’ll take care of that in just a bit. But first, finish up BIDLanguageListController by making the following bookkeeping changes:

- (void)viewDidUnload { [super viewDidUnload];

self.detailViewController = nil; self.languageNames = nil; self.languageCodes = nil;

}

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Now, it’s time to make the changes required for BIDDetailViewController to handle the popover, as well as generate the correct URL whenever the user either changes the display language or picks a different president. Start by making the following changes in

BIDDetailViewController.h:

#import <UIKit/UIKit.h>

@interface BIDDetailViewController : UIViewController <UISplitViewControllerDelegate>

@property (strong, nonatomic) id detailItem;

@property (strong, nonatomic) IBOutlet UILabel *detailDescriptionLabel; @property (weak, nonatomic) IBOutlet UIWebView *webView;

@property (strong, nonatomic) UIBarButtonItem *languageButton;

@property (strong, nonatomic) UIPopoverController *languagePopoverController; @property (copy, nonatomic) NSString *languageString;

- (IBAction)touchLanguageButton;

@end

All we need to do now is fix BIDDetailViewController.m so that it can handle the language popover and the URL construction. Start by adding this import somewhere at the top:

#import "BIDLanguageListController.h"

Then synthesize the new properties just below the @implementation line:

@synthesize languageButton;

@synthesize languagePopoverController;

@synthesize languageString;

The next thing we’re going to add is a function that takes as arguments a URL pointing to a Wikipedia page and a two-letter language code, and returns a URL that combines the two. We’ll use this at appropriate spots in our controller code later. You can place this function just about anywhere, including within the class’s implementation. The compiler is smart enough to always treat a function as just a function. Why don’t you place it just after the last synthesize statement toward the top of the file?

static NSString * modifyUrlForLanguage(NSString *url, NSString *lang) { if (!lang) {

return url;

}

//We're relying on a particular Wikipedia URL format here. This

//is a bit fragile!

NSRange languageCodeRange = NSMakeRange(7, 2);

if ([[url substringWithRange:languageCodeRange] isEqualToString:lang]) { return url;

} else {

NSString *newUrl = [url stringByReplacingCharactersInRange:languageCodeRange withString:lang];

return newUrl;

}

}

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Why make this a function instead of a method? There are a couple of reasons. First, instance methods in a class are typically meant to do something involving one or more instance variables. This function does not make use of any instance variables. It simply performs an operation on two strings and returns another. We could have made it a class method, but even that feels a bit wrong, since what the method does isn’t really related specifically to our controller class. Sometimes, a function is just what you need.

Our next move is to update the setDetailItem: method. This method will use the function we just defined to combine the URL that’s passed in with the chosen languageString to generate the correct URL. It also makes sure that our second popover, if present, disappears just like the first popover (the one that was defined for us) does.

- (void)setDetailItem:(id)newDetailItem { if (detailItem != newDetailItem) {

_detailItem = newDetailItem;

_detailItem = modifyUrlForLanguage(newDetailItem, languageString);

// Update the view. [self configureView];

}

if (self.masterPopoverController != nil) { [self.masterPopoverController dismissPopoverAnimated:YES];

}

}

Now, let’s update the viewDidLoad method. Here, we’re going to create a UIBarButtonItem and put it into the UINavigationItem at the top of the screen.

- (void)viewDidLoad

{

[super viewDidLoad];

// Do any additional setup after loading the view, typically from a nib. self.languageButton = [[UIBarButtonItem alloc] init]; languageButton.title = @"Choose Language";

languageButton.target = self;

languageButton.action = @selector(touchLanguageButton); self.navigationItem.rightBarButtonItem = self.languageButton;

[self configureView];

}

Next, we implement setLanguageString:. This also calls our modifyUrlForLanguage() function so that the URL can be regenerated (and the new page loaded) immediately. Add this method to the bottom of the file, just above the @end:

- (void)setLanguageString:(NSString *)newString {

if (![newString isEqualToString:languageString]) { languageString = [newString copy];

self.detailItem = modifyUrlForLanguage(_detailItem, languageString);

}

if (languagePopoverController != nil) { [languagePopoverController dismissPopoverAnimated:YES];

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