KVC and KVO

What is KVC and KVO? Give an example of using KVC to set a value.

KVC stands for Key-Value Coding. It's a mechanism by which an object's properties can be accessed using string's at runtime rather than having to statically know the property names at development time. KVO stands for Key-Value Observing and allows a controller or class to observe changes to a property value.

Let's say there is a property name on a class:

@property (nonatomic, copy) NSString *name;

We can access it using KVC:

NSString *n = [object valueForKey:@"name"]

And we can modify it's value by sending it the message:

[object setValue:@"Mary" forKey:@"name"]

Question 12

What are blocks and how are they used?

Blocks are a way of defining a single task or unit of behavior without having to write an entire Objective-C class. Under the covers Blocks are still Objective C objects. They are a language level feature that allow programming techniques like lambdas and closures to be supported in Objective-C. Creating a block is done using the ^ { } syntax:

 myBlock = ^{
    NSLog(@"This is a block");
 }

It can be invoked like so:

myBlock();

It is essentially a function pointer which also has a signature that can be used to enforce type safety at compile and runtime. For example you can pass a block with a specific signature to a method like so:

- (void)callMyBlock:(void (^)(void))callbackBlock;

If you wanted the block to be given some data you can change the signature to include them:

- (void)callMyBlock:(void (^)(double, double))block {
    ...
    block(3.0, 2.0);
}