iOS 5 Programming Cookbook

5.1. Constructing Block Objects

Problem

You want to be able to write your own block objects or use block objects with iOS SDK classes.

Solution

You just need to understand the basic differences between the syntax of block objects and classic C functions. These differences are explained in the Discussion section.

Discussion

Block objects can either be inline or coded as independent blocks of code. Let’s start with the latter type. Suppose you have a method in Objective-C that accepts two integer values of type NSInteger and returns the difference of the two values, by subtracting one from the other, as an NSInteger:

- (NSInteger) subtract:(NSInteger)paramValue
                  from:(NSInteger)paramFrom{

  return paramFrom - paramValue;

}

That was very simple, wasn’t it? Now let’s translate this Objective-C code to a pure C function that provides the same functionality to get one step closer to learning the syntax of block objects:

NSInteger subtract(NSInteger paramValue, NSInteger paramFrom){

  return paramFrom - paramValue;

}

You can see that the C function is quite different in syntax from its Objective-C counterpart. Now let’s have a look at how we could code the same function as a block object:

NSInteger (^subtract)(NSInteger, NSInteger) =
  ^(NSInteger paramValue, NSInteger paramFrom){

  return paramFrom - paramValue;

};

Before I go into details about the syntax of block objects, let me show you a few more examples. Suppose we have a function in C that takes a parameter of type NSUInteger (an unsigned integer) ...