#AlterViewBlock
##先从文档入手 ###Declaring and Using a Block ####You use the ^ operator to declare a block variable and to indicate the beginning of a block literal. The body of the block itself is contained within {}, as shown in this example (as usual with C, ; indicates the end of the statement): int num = 7; int (^MyBlock)(int) = ^(int multiplier){ // 声明和实现 return num * multiplier; };
####经典的一张图
## ####block可以使用在它定义区块的变量，但是无法直接在作用域内更改，此时要添加*__block*.(A powerful feature of blocks is that they can modify variables in the same lexical scope. You signal that a block can modify a variable using the __block storage type modifier.)。
###Using a Block Directly // qsort(void , size_t, size_t, int ()(const void *, const void *))

char *myCharacters[3] = { "TomJohn", "George", "Charles Condomine" };
qsort_b(myCharacters, 3, sizeof(char *), ^(const void *l, const void *r) {
char *left = *(char **)l;
char *right = *(char **)r;
return strncmp(left, right, 1);
});

####这点可以多加利用，相当于要获得某些变量时，比如我要获得某个bool型变量的值，这时我可能还要去做判断或者请求来确定，那直接用block来写，在block里去做这些事情，是很高效的一种写法。 ###Block Functionality

A block is an anonymous inline collection of code that:

• Has a typed argument list just like a function
• Has an inferred or declared return type
• Can capture state from the lexical scope¹ within which it is defined
• Can optionally modify the state of the lexical scope
• Can share the potential for modification with other blocks defined within the same lexical scope
  *Can continue to share and modify state defined within the lexical scope (the stack frame) after the lexical scope (the stack frame) has been destroyed

###Declaring and Creating Blocks void (^blockReturningVoidWithVoidArgument)(void); int (^blockReturningIntWithIntAndCharArguments)(int, char); void (^arrayOfTenBlocksReturningVoidWithIntArgument[10])(int);

float (^oneFrom)(float);
oneFrom = ^(float aFloat) {
float result = aFloat - 1.0;
return result;
};  

###Global Blocks #import <stdio.h> int GlobalInt = 0; int (^getGlobalInt)(void) = ^{ return GlobalInt; };

##然后简单罗列几种平时的用法：
###属性用法： @property (nonatomic, copy) void (^Block)(); // 声明一个返回值为空，无参block属性 @property (nonatomic, copy) NSString* (^Block)(NSString ); // 声明一个返回值为NSString类型，参数为NSString类型的block属性
###typedef: typedef void(^Block)(); // 通上述属性用法第一个 typedef NSString (^Block)(NSString *); // 第二个 ####这样可能调用的时候会更方便一些,比如 @property (nonatomic, copy) block myBlock; ###在函数体里作为变量进行回调操作:

- (void)workWithBlockOne:(Block)ablock{ // typedef 后直接作为变量
  // ...
}  
- (void)workWithBlockTwo:(void(^)())ablock{ // 直接书写变量
 // ...
}  

Footnotes

1. 变量定义区域. ↩