obj-c编程09:块的语法

    在obj-c中，有一种和C截然不同的东西——块。块可以在外边定义，也可以在函数或方法内部定义，可以被赋值给一个变量，然后用该变量调用。默认情况下块对外部变量的访问只能读不能写，除非用__block显示声明可写。对于块这么个东东，让我想到了ruby中的block,不过ruby中block是闭包，而且外部变量变化会影响block的值，但obj-c的块不会，即obj-c块的外部变量是编译时求值而不是运行时求值哦：

#import <Foundation/Foundation.h>int main(int argc,char *argv[]){@autoreleasepool{int i = 10;//foo是一个块变量被赋值为一个块。int (^foo)(int) = ^(int x) {int ret = x * i;//i = 100;编译错误，不能改写外部变量的值NSLog(@"%d * i(%d) = %d",x,i,ret);return ret;};i = 1000;foo(11);//i此时为10而不是1000哦。}return 0;}

可以看到在块内是不能写外部i变量的，除非用__block修饰:

#import <Foundation/Foundation.h>int main(int argc,char *argv[]){@autoreleasepool{__block int i = 10;//foo是一个块变量被赋值为一个块。int (^foo)(int) = ^(int x) {i = 100;int ret = x * i;NSLog(@"%d * i(%d) = %d",x,i,ret);return ret;};i = 1000;NSLog(@"ret is %d",foo(11));//i此时为100哦。}return 0;}

如果是在ruby中则外部变量变化会导致block的结果发生变化，也给个例子吧：

apple@kissAir: objc_src$irb2.1.1 :001 > $v = 100 => 100 2.1.1 :002 > b = ->(x){puts "#$v * #{x} = #{$v * x}"}  => #<Proc:0x0000010182c0d8@(irb):2 (lambda)> 2.1.1 :003 > b[11]100 * 11 = 1100 => nil 2.1.1 :004 > $v=11 => 11 2.1.1 :005 > b[11]11 * 11 = 121 => nil 

[2014.07.06第一次新增内容]:块功能扩展

我们还可以用block来实现集合对象的map功能，比如ruby中数组或散列的map：

#import <Foundation/Foundation.h>#define msg(...) NSLog(__VA_ARGS__)NSArray *map(NSArray *ary,id (^block)(id)){NSMutableArray *ary_ret = [NSMutableArray array];for(id x in ary)[ary_ret addObject:block(x)];return ary_ret;}int main(int argc,char *argv[]){@autoreleasepool{NSArray *ary_init = [NSArray arrayWithObjects:@"big",@"small",\@"love",@"pluck",@"save",nil];NSArray *ary = map(ary_init,^(id item){\return [item capitalizedString];});msg(@"%@",ary);}return 0;}

运行结果如下：

apple@kissAir: objc_src$./blk

2014-07-06 14:47:57.758 blk[1009:507] (

    Big,

    Small,

    Love,

    Pluck,

    Save

)

块能做的事可不止这些哦，我们可以借助GCD中对块的支持并发的执行任务：

#import <Foundation/Foundation.h>#define msg(...) NSLog(__VA_ARGS__)int main(int argc,char *argv[]){@autoreleasepool{NSMutableArray *ary_m = [NSMutableArray array];//获取GCD全局队列dispatch_queue_t g_queue = dispatch_get_global_queue(0,0);//创建新的执行组dispatch_group_t grp = dispatch_group_create();srand(time(NULL));__block int s = 0;for(int i = 0;i < 1000;++i){dispatch_block_t block = ^{s = rand() % 5;sleep(s);msg(@"at %4d:sleeped %d s",i,s);};//在组中异步执行块dispatch_group_async(grp,g_queue,block);}//等待组中所有块任务执行完毕dispatch_group_wait(grp,DISPATCH_TIME_FOREVER);}return 0;}

运行如下：

apple@kissAir: objc_src$./blk

2014-07-06 15:25:07.318 blk[1231:3903] at   19:sleeped 0 s

2014-07-06 15:25:07.318 blk[1231:3303] at   17:sleeped 0 s

2014-07-06 15:25:07.319 blk[1231:4703] at   21:sleeped 0 s

2014-07-06 15:25:07.319 blk[1231:4c03] at   22:sleeped 0 s

2014-07-06 15:25:07.319 blk[1231:4003] at   20:sleeped 0 s

2014-07-06 15:25:07.322 blk[1231:3e03] at   31:sleeped 0 s

2014-07-06 15:25:07.323 blk[1231:5e03] at   36:sleeped 0 s

2014-07-06 15:25:07.324 blk[1231:6403] at   40:sleeped 0 s

2014-07-06 15:25:07.325 blk[1231:6803] at   43:sleeped 0 s

2014-07-06 15:25:07.327 blk[1231:7a03] at   53:sleeped 0 s

2014-07-06 15:25:07.328 blk[1231:7a03] at   56:sleeped 0 s

2014-07-06 15:25:07.328 blk[1231:7a03] at   58:sleeped 2 s

2014-07-06 15:25:07.330 blk[1231:8a03] at   64:sleeped 0 s

2014-07-06 15:25:07.333 blk[1231:9e03] at   75:sleeped 0 s

2014-07-06 15:25:07.333 blk[1231:9e03] at   77:sleeped 0 s

2014-07-06 15:25:07.333 blk[1231:9e03] at   78:sleeped 0 s

2014-07-06 15:25:08.315 blk[1231:1903] at    4:sleeped 2 s

2014-07-06 15:25:08.315 blk[1231:1103] at    1:sleeped 2 s

2014-07-06 15:25:08.315 blk[1231:1503] at    3:sleeped 2 s

2014-07-06 15:25:08.315 blk[1231:1f03] at    7:sleeped 2 s

2014-07-06 15:25:08.315 blk[1231:2303] at    9:sleeped 2 s

2014-07-06 15:25:08.317 blk[1231:2f03] at   15:sleeped 2 s

2014-07-06 15:25:08.319 blk[1231:2303] at   84:sleeped 0 s

2014-07-06 15:25:08.323 blk[1231:4c03] at   28:sleeped 2 s

2014-07-06 15:25:08.323 blk[1231:5203] at   30:sleeped 2 s

2014-07-06 15:25:08.323 blk[1231:5803] at   32:sleeped 2 s

2014-07-06 15:25:08.323 blk[1231:5c03] at   35:sleeped 2 s

2014-07-06 15:25:08.324 blk[1231:5e03] at   38:sleeped 2 s

2014-07-06 15:25:08.324 blk[1231:6603] at   41:sleeped 2 s

2014-07-06 15:25:08.325 blk[1231:5203] at   88:sleeped 0 s

2014-07-06 15:25:08.325 blk[1231:6a03] at   44:sleeped 0 s

2014-07-06 15:25:08.325 blk[1231:5c03] at   90:sleeped 0 s

2014-07-06 15:25:08.325 blk[1231:6c03] at   46:sleeped 0 s

2014-07-06 15:25:08.326 blk[1231:7003] at   48:sleeped 1 s

2014-07-06 15:25:08.327 blk[1231:6a03] at   94:sleeped 0 s

2014-07-06 15:25:08.328 blk[1231:5c03] at   97:sleeped 0 s

2014-07-06 15:25:08.328 blk[1231:5c03] at   99:sleeped 0 s

2014-07-06 15:25:08.329 blk[1231:7a03] at   61:sleeped 3 s

2014-07-06 15:25:08.330 blk[1231:8c03] at   65:sleeped 3 s

2014-07-06 15:25:08.331 blk[1231:8c03] at  102:sleeped 0 s

2014-07-06 15:25:08.333 blk[1231:a003] at   76:sleeped 4 s

2014-07-06 15:25:08.333 blk[1231:9a03] at   73:sleeped 4 s

2014-07-06 15:25:08.334 blk[1231:9a03] at  105:sleeped 0 s