Key Value Coding is integrated into NSObject using NSKeyValueCoding protocol.

What this means?

It means that any id object is capable of calling valueForKey method and its various variants like valueForKeyPath etc. '

It also means that any id object can invoke setValue method and its various variants too.

Example:

id obj = [[MyClass alloc] init];
id value = [obj valueForKey:@"myNumber"];

int myNumberAsInt = [value intValue];
myNumberAsInt = 53;
[obj setValue:@(myNumberAsInt) forKey:@"myNumber"];

Exceptions:

Above example assumes that MyClass has an NSNumber Property called myNumber. If myNumber does not appear in MyClass interface definition, an NSUndefinedKeyException can be raised at possibly both lines 2 and 5 - popularly known as:

this class is not key value coding-compliant for the key myNumber.

Why this is SO powerful:

You can write code that can access properties of a class dynamically, without needing interface for that class. This means that a table view can display values from any properties of an NSObject derived object, provided its property names are supplied dynamically at runtime.

In the example above, the code can as well work without MyClass being available and id type obj being available to calling code.

if ([[dataObject objectForKey:@"yourVariable"] isEqualToString:"Hello World"]) {
    return YES;
} else {
    return NO;
}

You can query values stored using KVC quickly and easily, without needing to retrieve or cast these as local variables.

Collection Operators can be used in a KVC key path to perform an operation on a “collection-type” property (i.e. NSArray, NSSet and similar). For example, a common operation to perform is to count the objects in a collection. To achieve this, you use the @count collection operator:

self.array = @[@5, @4, @3, @2, @1];
NSNumber *count = [self.array valueForKeyPath:@"@count"];
NSNumber *countAlt = [self valueForKeyPath:@"array.@count"];
// count == countAlt == 5

While this is completely redundant here (we could have just accessed the count property), it can be useful on occasion, though it is rarely necessary. There are, however, some collection operators that are much more useful, namely @max, @min, @sum, @avg and the @unionOf family. It is important to note that these operators also require a separate key path following the operator to function correctly. Here's a list of them and the type of data they work with:

@max and @min will return the highest or lowest value, respectively, of a property of objects in the collection. For example, look at the following code:

// “Point” class used in our collection
@interface Point : NSObject

@property NSInteger x, y;

+ (instancetype)pointWithX:(NSInteger)x y:(NSInteger)y;

@end

...

self.points = @[[Point pointWithX:0 y:0],
                [Point pointWithX:1 y:-1],
                [Point pointWithX:5 y:-6],
                [Point pointWithX:3 y:0],
                [Point pointWithX:8 y:-4],
];

NSNumber *maxX = [self valueForKeyPath:@"[email protected]"];
NSNumber *minX = [self valueForKeyPath:@"[email protected]"];
NSNumber *maxY = [self valueForKeyPath:@"[email protected]"];
NSNumber *minY = [self valueForKeyPath:@"[email protected]"];

NSArray<NSNumber*> *boundsOfAllPoints = @[maxX, minX, maxY, minY];

...

In just a 4 lines of code and pure Foundation, with the power of Key-Value Coding collection operators we were able to extract a rectangle that encapsulates all of the points in our array.

It is important to note that these comparisons are made by invoking the compare: method on the objects, so if you ever want to make your own class compatible with these operators, you must implement this method.

@sum will, as you can probably guess, add up all the values of a property.

@interface Expense : NSObject

@property NSNumber *price;

+ (instancetype)expenseWithPrice:(NSNumber *)price;

@end

...

self.expenses = @[[Expense expenseWithPrice:@1.50],
                  [Expense expenseWithPrice:@9.99],
                  [Expense expenseWithPrice:@2.78],
                  [Expense expenseWithPrice:@9.99],
                  [Expense expenseWithPrice:@24.95]
];

NSNumber *totalExpenses = [self valueForKeyPath:@"[email protected]"];

Here, we used @sum to find the total price of all the expenses in the array. If we instead wanted to find the average price we're paying for each expense, we can use @avg:

NSNumber *averagePrice = [self valueForKeyPath:@"[email protected]"];

Finally, there's the @unionOf family. There are five different operators in this family, but they all work mostly the same, with only small differences between each. First, there's @unionOfObjects which will return an array of the properties of objects in an array:

// See "expenses" array above

NSArray<NSNumber*> *allPrices = [self valueForKeyPath:
    @"[email protected]"];

// Equal to @[ @1.50, @9.99, @2.78, @9.99, @24.95 ]

@distinctUnionOfObjects functions the same as @unionOfObjects, but it removes duplicates:

NSArray<NSNumber*> *differentPrices = [self valueForKeyPath:
    @"[email protected]"];

// Equal to @[ @1.50, @9.99, @2.78, @24.95 ]

And finally, the last 3 operators in the @unionOf family will go one step deeper and return an array of values found for a property contained inside dually-nested arrays:

NSArray<NSArray<Expense*,Expense*>*> *arrayOfArrays =
    @[
        @[ [Expense expenseWithPrice:@19.99],
           [Expense expenseWithPrice:@14.95],
           [Expense expenseWithPrice:@4.50],
           [Expense expenseWithPrice:@19.99]
         ],

        @[ [Expense expenseWithPrice:@3.75],
           [Expense expenseWithPrice:@14.95]
         ]
     ];

// @unionOfArrays
NSArray<NSNumber*> allPrices = [arrayOfArrays valueForKeyPath:
    @"@unionOfArrays.price"];
// Equal to @[ @19.99, @14.95, @4.50, @19.99, @3.75, @14.95 ];

// @distinctUnionOfArrays
NSArray<NSNumber*> allPrices = [arrayOfArrays valueForKeyPath:
    @"@distinctUnionOfArrays.price"];
// Equal to @[ @19.99, @14.95, @4.50, @3.75 ];

The one missing from this example is @distinctUnionOfSets, however this functions exactly the same as @distinctUnionOfArrays, but works with and returns NSSets instead (there is no non-distinct version because in a set, every object must be distinct anyway).

And that's it! Collection operators can be really powerful if used correctly, and can help to avoid having to loop through stuff unnecessarily.

One last note: you can also use the standard collection operators on arrays of NSNumbers (without additional property access). To do this, you access the self pseudo-property that just returns the object:

NSArray<NSNumber*> *numbers = @[@0, @1, @5, @27, @1337, @2048];

NSNumber *largest = [numbers valueForKeyPath:@"@max.self"];
NSNumber *smallest = [numbers valueForKeyPath:@"@min.self"];
NSNumber *total = [numbers valueForKeyPath:@"@sum.self"];
NSNumber *average = [numbers valueForKeyPath:@"@avg.self"];