如何解决错误＆＃34; CCNode，选择器xxx已经安排好了

Question

我正在尝试构建模仿康威生命游戏的简单游戏（在线教程的帮助下）。我把网格放在屏幕上。

当用户点击网格时，它将禁用/启用点，当用户点击播放按钮时，网格上的点将根据康威的生命游戏规则发展。

然而，在完成所有代码之后出现问题，我尝试运行模拟（我在网格上启用了一些点）。

然后点击播放按钮，什么也没发生，我点击播放按钮两次，Xcode注销了这条消息：

GameOfLife2[823:43706] -[CCNode schedule:interval:repeat:delay:] : Selector 'step1' was already scheduled on <MainScene = 0x7fca01e69590 | Name = >

有关我的一些与此错误相关的方法的详细信息（发布在Mainscene.m上）

#import "MainScene.h"
#import "Grid.h"

@implementation MainScene {
    Grid *_grid;
    CCTimer *_timer;
    CCLabelTTF *_generationLabel;
    CCLabelTTF *_populationLabel;
}

- (id)init
{
    self = [super init];

    if (self) {
        _timer = [[CCTimer alloc] init];
    }

    return self;
}

- (void)play
{

     [self schedule:@selector(step1) interval:0.05f];}

- (void)pause
{
    [self unschedule:@selector(step1)];
}


- (void)step1
{
    [_grid evolveStep];
    _generationLabel.string = [NSString stringWithFormat:@"%d", _grid.generation];
    _populationLabel.string = [NSString stringWithFormat:@"%d", _grid.totalAlive];
}

@end

在Grid.m上

 -(void)evolveStep{

    [self countNeighbors];

    [self updateCreatures];

    _generation++;
}

-(void)countNeighbors
{

    for (int i = 0; i < [_gridArray count]; i++)
    {


        for (int j = 0; j < [_gridArray[i] count]; j++)
        {


            Creature *currentCreature = _gridArray[i][j];


            currentCreature.livingNeighbors = 0;

            for (int x = (i-1) ; x <= (i+1); x++)
            {
                for (int y = (j-1); y <= (j+1); y++)
                {

                    BOOL isIndexValid;

                    isIndexValid = [self isIndexValidForX:x andY:y];


                    if (!((x == i) && (y == j)) && isIndexValid)
                    {
                        Creature *neighbor = _gridArray[x][y];

                        if (neighbor.isAlive)
                        {
                            currentCreature.livingNeighbors +=1;
                        }
                    }
                }
            }

        }
    }
}

-(void)updateCreatures{

    _totalAlive = 0;

    for (int l = 0; l <= [_gridArray count]; l++) {

        for (int k = 0; [_gridArray[l] count]; k++) {
            Creature *currentCreature = _gridArray[l][k];

            if (currentCreature.livingNeighbors == 3) {
                currentCreature.isAlive = YES;
            } else if ( (currentCreature.livingNeighbors <= 1) || (currentCreature.livingNeighbors >= 4) ) {
                currentCreature.isAlive = NO;
            }
            if (currentCreature.isAlive) {
                _totalAlive++;
            }
        }
    }

}

尝试谷歌解决这个问题，但还没有解决方案。