Composite Objects in Objective-C

In Objective-C, composite objects are objects that are composed of other objects. This means that an object can have one or more objects as its instance variables, and it can also be a part of another object's instance variables.

Composite objects are useful in many scenarios where complex data structures are required. For instance, a complex user interface that has multiple components can be represented using composite objects. Similarly, a game that has multiple levels, enemies, and weapons can also use composite objects.

Types of Composite Objects 

1.  NSArray

NSArray is a class in Objective-C that represents an ordered collection of objects. It is used to store and retrieve a list of objects of the same type. The objects are stored in a linear sequence, and each object is assigned an index number starting from zero.

Syntax:

NSArray *array = [[NSArray alloc] initWithObjects: object1, object2, object3, ..., nil];

'NSArray' is the class name.

'array' is the name of the instance of the class.

'alloc' is a class method that allocates memory for the array.

' initWithObjects' is an instance method that initializes the array with the specified objects.

'object1', 'object2', 'object3', ... are the objects to be stored in the array.

'nil' is a sentinel value that marks the end of the list of objects.

 Example:

#import <Foundation/Foundation.h>

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        
        // Create an array of NSString objects
        NSArray *vegetables = @[@"Brinjal", @"Onion", @"Cabbage", @"Potato"];
        
        // Print out each vegetable in the array
        for (NSString *vegetable in vegetables) {
            NSLog(@"%@", vegetable);
        }
        
    }
    return 0;
}

Approach: In this example, we create an NSArray object called array that contains four string objects: "Brinjal", "Onion", "Cabbage" and "Potato". And print the results using NSLog().

Output:

2. NSDictionary

NSDictionary is a class in Objective-C that is used to store collections of key-value pairs, also known as an associative array or a map. The keys are used to uniquely identify each value, and they can be any object that conforms to the 'NSCopying' protocol. It provides a number of methods to manipulate and retrieve data from the collection.

Syntax:

NSDictionary *dictionary = [[NSDictionary alloc] initWithObjectsAndKeys:

                            value1, key1,

                            value2, key2,

                            value3, key3,

                            nil];

In the above syntax, 'initWithObjectsAndKeys' is the initializer method that creates an instance of NSDictionary. It takes a variable number of arguments, where each key-value pair is represented by two consecutive arguments.

Example:

#import <Foundation/Foundation.h>

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        
        // Create an instance of NSDictionary
        NSDictionary *dictionary = [[NSDictionary alloc] initWithObjectsAndKeys:
                                    @"XYZ", @"firstName",
                                    @"abc", @"lastName",
                                    @"30", @"age",
                                    nil];
        
        // Access values in the dictionary
        NSString *firstName = [dictionary objectForKey:@"firstName"];
        NSString *lastName = [dictionary objectForKey:@"lastName"];
        NSNumber *age = [dictionary objectForKey:@"age"];
        
        // Print the values
        NSLog(@"First name: %@", firstName);
        NSLog(@"Last name: %@", lastName);
        NSLog(@"Age: %@", age);
        
        // Add a new key-value pair to the dictionary
        [dictionary setValue:@"New York" forKey:@"city"];
        
        // Print the updated dictionary
        NSLog(@"Dictionary: %@", dictionary);
    }
    return 0;
}

Approach: In this example, we create an instance of NSDictionary with three key-value pairs: "firstName"-"XYZ", "lastName"-"abc", and "age"-"30". We then access these values using the objectForKey: method and print them to the console using NSLog.

We also add a new key-value pair to the dictionary using the setValue:forKey: method and print the updated dictionary using NSLog.

Output:

3. NSSet

NSSet is a collection class that stores an unordered set of unique objects. It is similar to an array, but unlike arrays, sets do not store objects in a specific order, and they can only contain unique objects. To create an NSSet, you can use the setWithObjects: method, which takes a variable number of arguments. One real-life example of NSSet in Objective-C is in a music streaming application.

Syntax:

NSSet *mySet = [NSSet setWithObjects:obj1, obj2, obj3, ..., nil];

This creates an NSSet object mySet containing the objects 'obj1', 'obj2', 'obj3', and so on. The last argument to the method is nil, indicating the end of the argument list.

#import <Foundation/Foundation.h>

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        
        // Create a set of NSString objects
        NSSet *stringSet = [NSSet setWithObjects:@"Apple", @"Banana", @"Cherry", nil];
        
        // Loop through the set and print each string
        for (NSString *string in stringSet) {
            NSLog(@"%@", string);
        }
        
        // Check if the set contains a specific object
        BOOL containsApple = [stringSet containsObject:@"Apple"];
        if (containsApple) {
            NSLog(@"The set contains Apple");
        } else {
            NSLog(@"The set does not contain Apple");
        }
        
    }
    return 0;
}

Approach: In this example, we first create a set of NSString objects using the setWithObjects: method. We then loop through the set using a for-in loop and print each string to the console using NSLog. Finally, we check if the set contains the string "Apple" using the containsObject: method and print a message to the console depending on the result.

Output: