iPhone图像交换屏幕未更新

@implementation draw2D


- (id)initWithFrame:(CGRect)frame {
    if ((self = [super initWithFrame:frame])) {
        // Initialization code
        [self myTimer];
    }
    return self;
}


// Only override drawRect: if you perform custom drawing.
// An empty implementation adversely affects performance during animation.
- (void)drawRect:(CGRect)rect {
// Drawing code
    [self updateBackground];
}

NSTimer *timer = nil;
int count = 0;

- (void)myTimer
{
    timer = [NSTimer scheduledTimerWithTimeInterval:0.5 target:self selector:@selector(updateBackground) userInfo:nil repeats:NO];
    if (count < 1) {
        count++;
    } else {
        count = 0;
    }


}

- (void)updateBackground:
{
    if(count == 1)
    {
        UIImage *myImage = [UIImage imageNamed:@"bg.png"];
        CGRect imageRect = CGRectMake(0, 0, 320, 480);
        [myImage drawInRect:imageRect];
        [myImage release];
    //  count++;
    } 
    else 
        if (count == 0) 
        {
            UIImage *myImage = [UIImage imageNamed:@"SCENE_002.png"];
            CGRect imageRect = CGRectMake(0, 0, 320, 480);
            [myImage drawInRect:imageRect];
            [myImage release];
    //      count = 0;
        }
    [self myTimer];
}
@end

编辑:它显示第一个图像,但从不切换到第二个图像。