Programming iOS 6, 3rd Edition

Blocks

A block is an extension to the C language, introduced in Mac OS X 10.6 and available in iOS 4.0 or later. It’s a way of bundling up some code and handing off that entire bundle as an argument to a C function or Objective-C method. This is similar to what we did in Example 3-1, handing off a pointer to a function as an argument, but instead we’re handing off the code itself. The latter has some major advantages over the former, which I’ll discuss in a moment.

As an example, I’ll rewrite Example 3-1 to use a block instead of a function pointer. Instead of calling sortedArrayUsingFunction:context:, I’ll call sortedArrayUsingComparator:, which takes a block as its parameter. The block is typed like this:

NSComparisonResult (^)(id obj1, id obj2)

That’s similar to the syntax for specifying the type of a pointer to a function, but a caret character is used instead of an asterisk character. So this means a block that takes two id parameters and returns an NSComparisonResult (which is merely an NSInteger, with just the same meaning as in Example 3-1). We can define the block and hand it off as the argument to sortedArrayUsingComparator: all in a single move, as in Example 3-2.

Example 3-2. Using a block instead of a callback function

NSArray* arr2 = [arr sortedArrayUsingComparator: ^(id obj1, id obj2) { NSString* s1 = (NSString*) obj1; NSString* s2 = (NSString*) obj2; NSString* string1end = [s1 substringFromIndex:[s1 length] - 1]; NSString* string2end = [s2 substringFromIndex:[s2 length] ...