在迷宫游戏中使用UIImage作为路径

Question

我正在尝试使用UIImage作为大理石游戏的路径。我正在使用CoreMotion和QuartzCore处理在屏幕上移动大理石。 UIImage是一个简单的黑白图像，其中路径为黑色，其余图像为白色背景。我有移动大理石的代码，但是当我尝试检查其下面的路径图像的颜色时，大理石上的动画停止并且它不再移动。如果我注释掉确定路径图像像素颜色的代码行，那么大理石就会移动得很好。当我检查像素的颜色时，我缺少什么来保持大理石移动？

- (void)viewDidLoad {
    [super viewDidLoad];

    // Movement of marble1
    self.lastUpdateTime = [[NSDate alloc] init];
    self.curPoint  = CGPointMake(443, 350);
    self.motionManager = [[CMMotionManager alloc]  init];
    self.queue         = [[NSOperationQueue alloc] init];
    self.motionManager.accelerometerUpdateInterval = kUpdateInterval;
    [self.motionManager startAccelerometerUpdatesToQueue:self.queue withHandler:
     ^(CMAccelerometerData *accelerometerData, NSError *error) {
         [(id) self setAcceleration:accelerometerData.acceleration];
         [self performSelectorOnMainThread:@selector(update) withObject:nil waitUntilDone:NO];
     }];
}

- (void)update {
    NSTimeInterval secondsSinceLastDraw = -([self.lastUpdateTime timeIntervalSinceNow]);
    self.marble1YVelocity = self.marble1YVelocity - (self.acceleration.x * secondsSinceLastDraw);
    self.marble1XVelocity = self.marble1XVelocity - (self.acceleration.y * secondsSinceLastDraw);
    CGFloat xDelta = secondsSinceLastDraw * self.marble1XVelocity * 500;
    CGFloat yDelta = secondsSinceLastDraw * self.marble1YVelocity * 500;
    self.curPoint = CGPointMake(self.curPoint.x + xDelta,
                                    self.curPoint.y + yDelta);
    [self moveMarble1];
    self.lastUpdateTime = [NSDate date];
}

- (void)moveMarble1 {
    [self collisionWithWalls];
    [self collisionWithBoundaries];
    self.prevPoint = self.curPoint;
    CGRect frame = self.marble1.frame;
    frame.origin.x = self.curPoint.x;
    frame.origin.y = self.curPoint.y;
    self.marble1.frame = frame;
}

- (void)collisionWithWalls {   
    CGRect frame = self.marble1.frame;
    frame.origin.x = self.curPoint.x;
    frame.origin.y = self.curPoint.y;
    NSLog(@"x=%f,y=%f",frame.origin.x,frame.origin.y);
    CALayer *pathLayer = [self.Path.layer presentationLayer];
    //if I comment out the following line the marble moves with no issues on the screen
    NSLog(@"Color of path is %@",[self pixelColorInImage:pathLayer atX:self.curPoint.x atY:self.curPoint.y]);  
}

- (UIColor*)pixelColorInImage:(CALayer*)layer atX:(int)x atY:(int)y {
    UIImage* image=[self imageFromLayer:layer];   
    CFDataRef pixelData = CGDataProviderCopyData(CGImageGetDataProvider(image.CGImage));
    const UInt8* data = CFDataGetBytePtr(pixelData);
    int pixelInfo = ((image.size.width  * y) + x ) * 4; // 4 bytes per pixel
    UInt8 red   = data[pixelInfo + 0];
    UInt8 green = data[pixelInfo + 1];
    UInt8 blue  = data[pixelInfo + 2];
    UInt8 alpha = data[pixelInfo + 3];
    CFRelease(pixelData);
    return [UIColor colorWithRed:red/255.0f
                           green:green/255.0f
                            blue:blue/255.0f
                           alpha:alpha/255.0f];
}

- (UIImage *)imageFromLayer:(CALayer *)layer{   
    if ([[UIScreen mainScreen] respondsToSelector:@selector(scale)])
        UIGraphicsBeginImageContextWithOptions([layer frame].size, NO, [UIScreen mainScreen].scale);
    else
        UIGraphicsBeginImageContext([layer frame].size);

    [layer renderInContext:UIGraphicsGetCurrentContext()];
    UIImage *outputImage = UIGraphicsGetImageFromCurrentImageContext();    
    UIGraphicsEndImageContext();
    return outputImage;
}