iOS Programming: The Big Nerd Ranch Guide

Variable Capturing

There is one special characteristic of blocks that makes them very useful: they capture variables.

A block, like a function, can use the arguments passed to it and can declare local variables. Modify the block in BNRAppDelegate.m’s application:​didFinishLaunchingWithOptions: so that it declares and uses a local variable.

[​e​x​e​c​u​t​o​r​ ​s​e​t​E​q​u​a​t​i​o​n​:​^​i​n​t​(​i​n​t​ ​x​,​ ​i​n​t​ ​y​)​ ​{​
 ​ ​ ​ ​i​n​t​ ​s​u​m​ ​=​ ​x​ ​+​ ​y​;​
 ​ ​ ​ ​r​e​t​u​r​n​ ​x​ ​+​ ​y​;​
 ​ ​ ​ ​r​e​t​u​r​n​ ​s​u​m​;​
}​]​;​

The variable sum is local to the block, so sum can be used inside this block. Both x and y are arguments of the block, so they, too, can be used inside this block.

A block can also use any variables that are visible ...