objective-c - 用于在 NSMutableArray 崩溃中删除项目的快速枚举

Question

我有一个奇怪的问题，如果我在 forin 枚举中删除我的项目，它会崩溃，就像这样:

for (Obstacle *obstacleToTrack in _obstaclesToAnimate) {
    //this if else not so important for happening crash
    if(obstacleToTrack.distance > 0){
        obstacleToTrack.distance -= _playerSpeed * _elapsed;
    }else{
        if (obstacleToTrack.watchOut) {
            obstacleToTrack.watchOut = NO;
        }
        obstacleToTrack.x -= (_playerSpeed + obstacleToTrack.speed) * _elapsed;
    }
    if (obstacleToTrack.x < -obstacleToTrack.width || _gameState == GS_OVER) {
        [self removeChild:obstacleToTrack];
        //this line makes crash happen , if remove this line code work fine
        [_obstaclesToAnimate removeObject:obstacleToTrack];
    }
}

如果我将代码更改为

NSMutableArray *forRemoving = [[NSMutableArray alloc]init];
for (Obstacle *obstacleToTrack in _obstaclesToAnimate) {
    //this if else not so important for happening crash
    if(obstacleToTrack.distance > 0){
        obstacleToTrack.distance -= _playerSpeed * _elapsed;
    }else{
        if (obstacleToTrack.watchOut) {
            obstacleToTrack.watchOut = NO;
        }
        obstacleToTrack.x -= (_playerSpeed + obstacleToTrack.speed) * _elapsed;
    }
    if (obstacleToTrack.x < -obstacleToTrack.width || _gameState == GS_OVER) {
        // code change here
        [self removeChild:obstacleToTrack];
        [forRemoving addObject:obstacleToTrack];
    }
}
for(Obstacle *obstacleToTrack in forRemoving){
    [_obstaclesToAnimate removeObject:obstacleToTrack];
    [forRemoving removeObject:obstacleToTrack];
}
[forRemoving release];

这会很完美，有人能告诉我为什么吗？

Answer 1

答案是，如果删除一个对象，则该数组中的其他对象会移动数组中的位置，因为删除了一个项目。

例如，我们有一个包含 4 个项目的数组，如果我们删除第一个项目(项目 0)，以前位于索引 1 的项目现在位于索引 0，位于 2 的项目现在位于 1。
因此枚举中断。

您可以通过从计数到 0 循环遍历数组来解决此问题:

for (int i = [array count]-1; i >= 0; i--) {
    id object = [array objectAtIndex:i];

    if (some check) {
       [array removeObjectAtIndex:i];
    }
}