iOS开发——高级篇——多线程dispatch_apply

我们知道遍历数组是一个相对耗时的操作,而同时手机的核是越来越多,所以我们需要充分利用iOS多核的作用.

特别是在遍历操作中还有其他耗时操作.像我们平时直接遍历数组的操作

for (NSInteger i= 0,i< 100,i++){

 //do something

 } 

这样的代码不会随着手机的不断升级而提高效率.

但是利用GCD可以实现充分利用多核.并且随着手机不断升级,同样的代码,执行效率会自动提高,十分OK.下面提供两种常用方法.

NSArray * arr = @[@"1",@"2",@"3",@"4",@"5"];

   dispatch_queue_t queue =  dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

   dispatch_group_t group =  dispatch_group_create();

   

    for (NSInteger i = 0; i < arr.count; i++) {

        NSLog(@"%ld",(long)i);

        dispatch_group_async(group, queue, ^{

            sleep(5);

            NSLog(@"%ld -- %@",(long)i,[NSThread currentThread]);

        });

    }

    //dispatch_group_wait(group, DISPATCH_TIME_FOREVER);// 等待queue任务执行完,才往下走;

    dispatch_group_notify(group, queue, ^{ // 往下走,回调block

        NSLog(@"group end");

 

    });

NSLog(@"end”);

2016-07-26 15:59:09.566 学习之路[4448:1770686] 0

2016-07-26 15:59:09.566 学习之路[4448:1770686] 1

2016-07-26 15:59:09.566 学习之路[4448:1770686] 2

2016-07-26 15:59:09.567 学习之路[4448:1770686] 3

2016-07-26 15:59:09.567 学习之路[4448:1770686] 4

2016-07-26 15:59:09.567 学习之路[4448:1770686] end

2016-07-26 15:59:14.569 学习之路[4448:1772333] 4 -- <NSThread: 0x7fc412631120>{number = 6, name = (null)}

2016-07-26 15:59:14.569 学习之路[4448:1772330] 1 -- <NSThread: 0x7fc4124bf8e0>{number = 2, name = (null)}

2016-07-26 15:59:14.569 学习之路[4448:1771326] 0 -- <NSThread: 0x7fc4126255e0>{number = 3, name = (null)}

2016-07-26 15:59:14.569 学习之路[4448:1772332] 3 -- <NSThread: 0x7fc41261d900>{number = 5, name = (null)}

2016-07-26 15:59:14.569 学习之路[4448:1772331] 2 -- <NSThread: 0x7fc4124bb230>{number = 4, name = (null)}

2016-07-26 15:59:14.570 学习之路[4448:1772331] group end

dispatch group 只能异步,并且不会在主线程走任务

NSArray * arr = @[@"1",@"2",@"3",@"4",@"5"];

 

dispatch_apply([arr count], dispatch_get_global_queue(0, 0), ^(size_t index) {

        sleep(1);

        NSLog(@"%@ %@",arr[index],[NSThread currentThread]);

        

    });

 

NSLog(@"end”);

它以指定的次数将指定的Block加入到指定的队列中,并等待队列中操作全部完成,NSLog(@"end”)在上面任务执行完才会执行

2016-07-26 16:00:58.106 学习之路[4463:1787375] 2 <NSThread: 0x7f98fa61aff0>{number = 4, name = (null)}

2016-07-26 16:00:58.106 学习之路[4463:1787363] 3 <NSThread: 0x7f98fa61e680>{number = 2, name = (null)}

2016-07-26 16:00:58.106 学习之路[4463:1787587] 4 <NSThread: 0x7f98fa747a00>{number = 3, name = (null)}

2016-07-26 16:00:58.106 学习之路[4463:1787047] 1 <NSThread: 0x7f98fa402c30>{number = 1, name = main}

2016-07-26 16:00:59.110 学习之路[4463:1787375] 5 <NSThread: 0x7f98fa61aff0>{number = 4, name = (null)}

2016-07-26 16:00:59.110 学习之路[4463:1787047] end

 

dispatch apply 是同步的,可以在主线程走任务,如果想异步可以在外面包一层

    NSArray * arr = @[@"1",@"2",@"3",@"4",@"5"];

  dispatch_async(dispatch_get_global_queue(0, 0), ^{

      dispatch_apply([arr count], dispatch_get_global_queue(0, 0), ^(size_t index) {

          sleep(1);

          NSLog(@"%@ %@",arr[index],[NSThread currentThread]);

          

      });

  });

    NSLog(@"end");

 

}

但是这样效果就和dispatch group一样了,不会在主线程走任务