新手问题：拖动图片后View上会留有图片划过的残影

悬赏园豆：20 [待解决问题]

一个图标拖动的小程序，拖动图片后界面上会留有图片划过的残影，不知道如何解决，请各位大侠指教

- (void)drawRect:(CGRect)rect {
       [imageBox drawInRect:CGRectMake(currentX, 329, imageBox.size.width, imageBox.size.height)];
}


-(void) touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
    UITouch *t = [touches anyObject];
    CGPoint where = [t locationInView:self];
    
    CGRect rcBox = CGRectMake(currentX, 329, imageBox.size.width, imageBox.size.height);

    if(CGRectContainsPoint(rcBox, where))
    {
        touchBox = YES;
        currentX = where.x;
        [self setNeedsDisplay];
    }
}

-(void) touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
{
    if(touchBox)
    {
        UITouch *t = [touches anyObject];
        
        CGPoint where = [t locationInView:self];
        currentX = where.x;
        [self setNeedsDisplay];
    }
}

-(void) touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event
{
    touchBox = NO;
    [self setNeedsDisplay];
}