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函數是dispatch_sync函數和Dispatch Group的關聯API,該函數按指定的次數將指定的Block追加到指定的Dispatch Queue中,並等到全部的處理執行結束

/*!
 *  @brief  dispatch_apply的用法
 */
- (void)dispatchApplyTest1 {
    //生成全局隊列
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    
    
    /*! dispatch_apply函數說明
     *
     *  @brief  dispatch_apply函數是dispatch_sync函數和Dispatch Group的關聯API
     *         該函數按指定的次數將指定的Block追加到指定的Dispatch Queue中,並等到全部的處理執行結束
     *
     *  @param 10    指定重復次數  指定10次
     *  @param queue 追加對象的Dispatch Queue
     *  @param index 帶有參數的Block, index的作用是為了按執行的順序區分各個Block
     *
     */
    dispatch_apply(10, queue, ^(size_t index) {
        NSLog(@"%zu", index);
    });
    NSLog(@"done");
    
    /*!
     *  @brief  輸出結果
     *
     2016-02-25 19:24:39.102 dispatch_apply測試[2985:165004] 0
     2016-02-25 19:24:39.102 dispatch_apply測試[2985:165086] 1
     2016-02-25 19:24:39.104 dispatch_apply測試[2985:165004] 4
     2016-02-25 19:24:39.104 dispatch_apply測試[2985:165004] 5
     2016-02-25 19:24:39.104 dispatch_apply測試[2985:165004] 6
     2016-02-25 19:24:39.103 dispatch_apply測試[2985:165088] 3
     2016-02-25 19:24:39.104 dispatch_apply測試[2985:165004] 7
     2016-02-25 19:24:39.105 dispatch_apply測試[2985:165004] 8
     2016-02-25 19:24:39.105 dispatch_apply測試[2985:165004] 9
     2016-02-25 19:24:39.102 dispatch_apply測試[2985:165087] 2
     2016-02-25 19:24:39.105 dispatch_apply測試[2985:165004] done
     *  !!!因為在Global Dispatch Queue中執行,所以各個處理的執行時間不定
     但done一定會輸出在最后的位置,因為dispatch_apply函數會等待所以的處理結束
     */
}

 

(二)dispatch_apply的使用技巧:模擬for循環

/*!
 *  @brief  實例:當要對NSArray類對象的所有元素執行處理時,不必一個一個的編寫for循環部分
 */
- (void)dispatchApplyTest2 {
    //1.創建NSArray類對象
    NSArray *array = @[@"a", @"b", @"c", @"d", @"e", @"f", @"g", @"h", @"i", @"j"];
    
    //2.創建一個全局隊列
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    
    //3.通過dispatch_apply函數對NSArray中的全部元素進行處理,並等待處理完成,
    dispatch_apply([array count], queue, ^(size_t index) {
        NSLog(@"%zu: %@", index, [array objectAtIndex:index]);
    });
    NSLog(@"done");
    /*!
     *  @brief  輸出結果
     *
     2016-02-25 19:37:17.308 dispatch_apply測試[3010:167871] 0: a
     2016-02-25 19:37:17.308 dispatch_apply測試[3010:167956] 1: b
     2016-02-25 19:37:17.308 dispatch_apply測試[3010:167957] 3: d
     2016-02-25 19:37:17.308 dispatch_apply測試[3010:167871] 4: e
     2016-02-25 19:37:17.309 dispatch_apply測試[3010:167957] 6: g
     2016-02-25 19:37:17.309 dispatch_apply測試[3010:167871] 7: h
     2016-02-25 19:37:17.309 dispatch_apply測試[3010:167957] 8: i
     2016-02-25 19:37:17.309 dispatch_apply測試[3010:167871] 9: j
     2016-02-25 19:37:17.308 dispatch_apply測試[3010:167956] 5: f
     2016-02-25 19:37:17.308 dispatch_apply測試[3010:167955] 2: c
     *  !!!因為在Global Dispatch Queue中執行,所以各個處理的執行時間不定
        但done一定會輸出在最后的位置,因為dispatch_apply函數會等待所以的處理結束
     */
}

(三)在dispatch_async函數中異步執行dispatch_apply函數,模擬dispatch_sync的同步效果

/*!
 *  @brief  推薦在dispatch_async函數中異步執行dispatch_apply函數
    效果     dispatch_apply函數與dispatch_sync函數形同,會等待處理執行結束
 */
- (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(@"回到主線程執行用戶界面更新等操作");
        });
        
    });
    /*!
     *  @brief  執行結果
     *
     2016-02-25 19:49:53.189 dispatch_apply測試[3060:171856] 3: d
     2016-02-25 19:49:53.189 dispatch_apply測試[3060:171852] 1: b
     2016-02-25 19:49:53.189 dispatch_apply測試[3060:171853] 2: c
     2016-02-25 19:49:53.189 dispatch_apply測試[3060:171850] 0: a
     2016-02-25 19:49:53.189 dispatch_apply測試[3060:171856] 4: e
     2016-02-25 19:49:53.189 dispatch_apply測試[3060:171852] 5: f
     2016-02-25 19:49:53.190 dispatch_apply測試[3060:171853] 6: g
     2016-02-25 19:49:53.190 dispatch_apply測試[3060:171850] 7: h
     2016-02-25 19:49:53.190 dispatch_apply測試[3060:171852] 9: j
     2016-02-25 19:49:53.190 dispatch_apply測試[3060:171856] 8: i
     2016-02-25 19:49:53.218 dispatch_apply測試[3060:171760] 回到主線程執行用戶界面更新等操作
     *
     */
}