547
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$irb
2.1.1 :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
最后更新：2017-04-03 05:38:53