Say you wanted an object that could take two numbers, add them together, and then return the result. You could probably write that pretty easily. But what if you now wanted that object to perform subtraction? Then multiplication? Division? You’d end up writing a number of methods for this object.

Instead of writing these methods, we can give this object an instance variable that points at a block. When we want to swap out the operation this object uses, we can construct a block literal that performs the appropriate operation. When the object is asked to compute the result, it executes the operation in the block.

Create a new NSObject subclass for Blocky and name it BNRExecutor. In BNRExecutor.h, add an instance variable and two methods to the BNRExecutor class.