GCD中的dispatch_apply的用法及作用 iOS

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, 0);
    
    
    /*! 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(10, 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, 0);
    
    //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, 0);
    
    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
     *
     */
}