IOS Blocks详解-基本介绍

1.什么是Blocks

Blocks 是C预言的扩充功能，简单来说就是带有自动变量（局部变量）的匿名函数,一般来说，blocks包含的内容可以理解为代码块。

2.Blocks 模式

2.1 Block 语法

^  void（int value)  { } 

其中对应关系为 ^ +返回值类型+参数列表+表达式 也可以缩写为

^ （int value)  { } 

2.2 Block的函数传递

int  (^blk)(int) =  ^ (int value){ return value+1; };
int  (^blk1)(int) = blk;
int  (^blk2)(int) = blk1;
blk2 = blk1;
//通过typedef 可以声明blk_t类型变量
typedef  int  (^blk_t)(int) ;
blk_t blk  = ^(int value){ return value+1; };

2.3 Block截取自动变量

int blkValue = 2;
    NSString *blkStr = [NSString stringWithFormat:@"my blkValue is :%d",blkValue];
    void (^blk)(void) = ^{
        NSLog(@"blkStr = %@",blkStr);
    };
    blkValue = 3;
    blkStr = [NSString stringWithFormat:@"my blkValue is :%d",blkValue];
    blk();

打印如下 blkValue 的值被截获

 blkStr = my blkValue is :2

3.Block中改变变量值

如果在Block块之外给变量赋值，如下代码会产生编译错误，需要加__block说明符

int blkValue = 2;
    NSString *blkStr = [NSString stringWithFormat:@"my blkValue is :%d",blkValue];
    void (^blk)(void) = ^{
        blkValue = 3;
        NSLog(@"blkStr = %@",blkStr);
    };
    blkValue = 3;
    blkStr = [NSString stringWithFormat:@"my blkValue is :%d",blkValue];
    blk();

__block  int blkValue = 2;
    NSString *blkStr = [NSString stringWithFormat:@"my blkValue is :%d",blkValue];
    void (^blk)(void) = ^{
        blkValue = 3;
        NSLog(@"blkStr = %@",blkStr);
    };
    blkValue = 3;
    blkStr = [NSString stringWithFormat:@"my blkValue is :%d",blkValue];
    blk();