dispatch_after vs performSelector afterDelay

Question

我正在寫滑塊遊戲。爲了從已解決的狀態中洗牌遊戲，我想定期撥打pushRandomPiece以視覺上洗牌遊戲。

我想首先使用dispatch_after但我必須與發射日期的問題：

這工作：

-(void)shuffle { 
    for (int i=0; i<50;i++) 
    [self performSelector:@selector(pushRandomPiece) withObject:nil afterDelay:i*0.50*2]; 
} 

在pushRandomPiece兩個連續調用之間的差異幾乎不斷等於一秒。

但是，這並不工作：

-(void)shuffle { 
    for (int i=0; i<50;i++) 
//  [self performSelector:@selector(pushRandomPiece) withObject:nil afterDelay:i*0.50*2]; 
     dispatch_after(dispatch_time(DISPATCH_TIME_NOW, i*NSEC_PER_SEC*0.5*2), dispatch_get_main_queue(), ^{ [self pushRandomPiece]; }); 
} 

這是連續調用之間的時間差：

2013-10-01 11:02:53.147 SlidingPuzzle[2006:60b] diff= 1.077376 
2013-10-01 11:02:54.262 SlidingPuzzle[2006:60b] diff= 1.111863 
2013-10-01 11:02:55.335 SlidingPuzzle[2006:60b] diff= 1.070456 
2013-10-01 11:02:56.455 SlidingPuzzle[2006:60b] diff= 1.117381 
2013-10-01 11:02:57.542 SlidingPuzzle[2006:60b] diff= 1.084070 
2013-10-01 11:02:58.655 SlidingPuzzle[2006:60b] diff= 1.110574 
2013-10-01 11:02:59.757 SlidingPuzzle[2006:60b] diff= 1.098654 
2013-10-01 11:03:00.862 SlidingPuzzle[2006:60b] diff= 1.103132 
2013-10-01 11:03:01.956 SlidingPuzzle[2006:60b] diff= 1.091535 
2013-10-01 11:03:03.050 SlidingPuzzle[2006:60b] diff= 1.090532 
2013-10-01 11:03:04.160 SlidingPuzzle[2006:60b] diff= 1.107981 
2013-10-01 11:03:04.164 SlidingPuzzle[2006:60b] diff= 0.000982 
2013-10-01 11:03:06.354 SlidingPuzzle[2006:60b] diff= 2.187945 
2013-10-01 11:03:06.357 SlidingPuzzle[2006:60b] diff= 0.000862 
2013-10-01 11:03:08.498 SlidingPuzzle[2006:60b] diff= 2.139442 
2013-10-01 11:03:08.501 SlidingPuzzle[2006:60b] diff= 0.000805 
2013-10-01 11:03:10.750 SlidingPuzzle[2006:60b] diff= 2.246749 
2013-10-01 11:03:10.753 SlidingPuzzle[2006:60b] diff= 0.000839 

這裏是使得該方法將塊移動：

-(void) pushRandomPiece { 
    NSSet * s = [self freeBlocks]; 
    int n = [s count]; 
    int piece = arc4random_uniform(n); 
    Piece * p = [[s allObjects] objectAtIndex:piece]; 
    dispatch_async(dispatch_get_main_queue(), ^{ 
     [self pushPiece:p]; 
    }); 
} 

Answer 1

dispatch_after只會在指定的時間後將您的塊添加到隊列中。如果隊列爲空，則可能會在將塊添加到隊列後立即運行該塊。否則，在運行之前它可能會排隊等待一段時間 - 等待其他任務完成。