GCD中的dispatch_apply的用法及作用

GCD中的dispatch_apply的用法及作用

(一)dispatch_apply的基本用法

dispatch_apply函数是dispatch_sync函数和Dispatch Group的关联API,该函数按指定的次数将指定的Block追加到指定的Dispatch Queue中,并等到全部的处理执行结束

 /*!
  *  @brief  dispatch_apply的用法
  */
 - (void)dispatchApplyTest1 {
     //生成全局队列
     dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, );

     /*! dispatch_apply函数说明
      *
      *  @brief  dispatch_apply函数是dispatch_sync函数和Dispatch Group的关联API
      *         该函数按指定的次数将指定的Block追加到指定的Dispatch Queue中,并等到全部的处理执行结束
      *
      *  @param 10    指定重复次数  指定10次
      *  @param queue 追加对象的Dispatch Queue
      *  @param index 带有参数的Block, index的作用是为了按执行的顺序区分各个Block
      *
      */
     dispatch_apply(, queue, ^(size_t index) {
         NSLog(@"%zu", index);
     });
     NSLog(@"done");

     /*!
      *  @brief  输出结果
      *
      2016-02-25 19:24:39.102 dispatch_apply测试[2985:165004] 0
      2016-02-25 19:24:39.102 dispatch_apply测试[2985:165086] 1
      2016-02-25 19:24:39.104 dispatch_apply测试[2985:165004] 4
      2016-02-25 19:24:39.104 dispatch_apply测试[2985:165004] 5
      2016-02-25 19:24:39.104 dispatch_apply测试[2985:165004] 6
      2016-02-25 19:24:39.103 dispatch_apply测试[2985:165088] 3
      2016-02-25 19:24:39.104 dispatch_apply测试[2985:165004] 7
      2016-02-25 19:24:39.105 dispatch_apply测试[2985:165004] 8
      2016-02-25 19:24:39.105 dispatch_apply测试[2985:165004] 9
      2016-02-25 19:24:39.102 dispatch_apply测试[2985:165087] 2
      2016-02-25 19:24:39.105 dispatch_apply测试[2985:165004] done
      *  !!!因为在Global Dispatch Queue中执行,所以各个处理的执行时间不定
      但done一定会输出在最后的位置,因为dispatch_apply函数会等待所以的处理结束
      */
 }

(二)dispatch_apply的使用技巧:模拟for循环

 /*!
  *  @brief  实例:当要对NSArray类对象的所有元素执行处理时,不必一个一个的编写for循环部分
  */
 - (void)dispatchApplyTest2 {
     //1.创建NSArray类对象
     NSArray *array = @[@"a", @"b", @"c", @"d", @"e", @"f", @"g", @"h", @"i", @"j"];

     //2.创建一个全局队列
     dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, );

     //3.通过dispatch_apply函数对NSArray中的全部元素进行处理,并等待处理完成,
     dispatch_apply([array count], queue, ^(size_t index) {
         NSLog(@"%zu: %@", index, [array objectAtIndex:index]);
     });
     NSLog(@"done");
     /*!
      *  @brief  输出结果
      *
      2016-02-25 19:37:17.308 dispatch_apply测试[3010:167871] 0: a
      2016-02-25 19:37:17.308 dispatch_apply测试[3010:167956] 1: b
      2016-02-25 19:37:17.308 dispatch_apply测试[3010:167957] 3: d
      2016-02-25 19:37:17.308 dispatch_apply测试[3010:167871] 4: e
      2016-02-25 19:37:17.309 dispatch_apply测试[3010:167957] 6: g
      2016-02-25 19:37:17.309 dispatch_apply测试[3010:167871] 7: h
      2016-02-25 19:37:17.309 dispatch_apply测试[3010:167957] 8: i
      2016-02-25 19:37:17.309 dispatch_apply测试[3010:167871] 9: j
      2016-02-25 19:37:17.308 dispatch_apply测试[3010:167956] 5: f
      2016-02-25 19:37:17.308 dispatch_apply测试[3010:167955] 2: c
      *  !!!因为在Global Dispatch Queue中执行,所以各个处理的执行时间不定
         但done一定会输出在最后的位置,因为dispatch_apply函数会等待所以的处理结束
      */
 }

(三)在dispatch_async函数中异步执行dispatch_apply函数,模拟dispatch_sync的同步效果

 /*!
  *  @brief  推荐在dispatch_async函数中异步执行dispatch_apply函数
     效果     dispatch_apply函数与dispatch_sync函数形同,会等待处理执行结束
  */
 - (void)dispatchApplyTest3 {
     NSArray *array = @[@"a", @"b", @"c", @"d", @"e", @"f", @"g", @"h", @"i", @"j"];
     dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, );

     dispatch_async(queue, ^{

         dispatch_apply([array count], queue, ^(size_t index) {
             NSLog(@"%zu: %@", index, [array objectAtIndex:index]);
         });

         dispatch_async(dispatch_get_main_queue(), ^{
             NSLog(@"回到主线程执行用户界面更新等操作");
         });

     });
     /*!
      *  @brief  执行结果
      *
      2016-02-25 19:49:53.189 dispatch_apply测试[3060:171856] 3: d
      2016-02-25 19:49:53.189 dispatch_apply测试[3060:171852] 1: b
      2016-02-25 19:49:53.189 dispatch_apply测试[3060:171853] 2: c
      2016-02-25 19:49:53.189 dispatch_apply测试[3060:171850] 0: a
      2016-02-25 19:49:53.189 dispatch_apply测试[3060:171856] 4: e
      2016-02-25 19:49:53.189 dispatch_apply测试[3060:171852] 5: f
      2016-02-25 19:49:53.190 dispatch_apply测试[3060:171853] 6: g
      2016-02-25 19:49:53.190 dispatch_apply测试[3060:171850] 7: h
      2016-02-25 19:49:53.190 dispatch_apply测试[3060:171852] 9: j
      2016-02-25 19:49:53.190 dispatch_apply测试[3060:171856] 8: i
      2016-02-25 19:49:53.218 dispatch_apply测试[3060:171760] 回到主线程执行用户界面更新等操作
      *
      */
 }