How It Works

The first part of this example shows how you can add a property list file to your application. In the property list file, you add three keys representing the category of applications in the AppStore: Entertainment, Games, and Utilities. Each category contains a list of application titles.

When the view is loaded, you first try to locate a file named Apps.plist in the Documents directory of your application:

//---get the path to the property list file---
    NSString *plistFileName = [[self documentsPath]
                              stringByAppendingPathComponent:@"Apps.plist"];

If the file is found, you then load its content into an NSDictionary object:

//---if the property list file can be found---
    if ([[NSFileManager defaultManager] fileExistsAtPath:plistFileName])
    {
        //---load the content of the property list file into a NSDictionary
        // object---
        NSDictionary *dict = [[NSDictionary alloc]
                                 initWithContentsOfFile:plistFileName];
        //...
    }

Next, you enumerate through all the keys in the dictionary object and print the titles of each application in the Debugger Console window:

//---for each category---
        for (NSString *category in dict)
        {

NSLog(category);
            NSLog(@"========");
            //---return all titles in an array---
            NSArray *titles = [dict valueForKey:category];

            //---print out all the titles in that category---
            for (NSString *title in titles)
            {
                NSLog(title);
            }
        }
        [dict release];

When the application is run for the first time, the Apps.plist file is not available, so you load it from the Resources folder:

else {
        //---load the property list from the Resources folder---
        NSString *pListPath = [[NSBundle mainBundle] pathForResource:@"Apps"
                                  ofType:@"plist"];

        NSDictionary *dict = [[NSDictionary alloc]
                                 initWithContentsOfFile:pListPath];
        //...
    }

Because you are making changes to the dictionary object, you need to make a mutable copy of the dictionary object and assign it to an NSMutableDictionary object:

//---make a mutable copy of the dictionary object---
        NSMutableDictionary *copyOfDict = [dict mutableCopy];

This step is important because the NSDictionary object is immutable, meaning that after the items are populated from the property list, you cannot add content to the dictionary object. Using the mutableCopy method of the NSDictionary class allows you to create a mutable instance of the dictionary object, which is NSMutableDictionary.

You then retrieve an array containing all the keys in the mutable dictionary object:

//---get all the different categories---
        NSArray *categoriesArray = [[copyOfDict allKeys]
                                   sortedArrayUsingSelector:@selector(compare:)];

You use this array to loop through all the keys in the dictionary so that you can add some additional titles to each category:

//---for each category---
        for (NSString *category in categoriesArray)
        {

        }

Note that you cannot enumerate using the NSMutableDictionary object like this:

for (NSString *category in copyOfDict)
        {
            //...
        }

This is because you cannot add items to the NSMutableDictionary object while it is being enumerated. Therefore, you need to loop using an NSArray object.

When you're inside the loop, you extract all the titles of the applications in each category and make a mutable copy of the array containing the titles of the applications:

//---get all the app titles in that category---
            NSArray *titles = [dict valueForKey:category];

            //---make a mutable copy of the array---
            NSMutableArray *mutableTitles = [titles mutableCopy];

You can now add a new title to the mutable array containing the application titles:

//---add a new title to the category---
            [mutableTitles addObject:@"New App title"];

After the additional item is added to the mutable array, set it back to the mutable dictionary object:

//---set the array back to the dictionary object---
            [copyOfDict setObject:mutableTitles forKey:category];
            [mutableTitles release];

Finally, you write the mutable dictionary object to a file using the writeToFile:atomically: method:

//---write the dictionary to file---
        fileName = [[self documentsPath]
                       stringByAppendingPathComponent:@"Apps.plist"];
        [copyOfDict writeToFile:fileName atomically:YES];

        [dict release];
        [copyOfDict release];