iOS自定义滑杆效果

UIImage *thumbWithLevel(float aLevel)
{
    float INSET_AMT = 1.5f;
    CGRect baseRect = CGRectMake(0, 0, 40, 100);
    CGRect thumbRect = CGRectMake(0, 40, 40, 20);
    
    UIGraphicsBeginImageContext(baseRect.size);
    CGContextRef context = UIGraphicsGetCurrentContext();
    
    [[UIColor darkGrayColor] setFill];
    CGContextAddRect(context, CGRectInset(thumbRect, INSET_AMT, INSET_AMT));
    CGContextFillPath(context);
    
    [[UIColor whiteColor] setStroke];
    CGContextSetLineWidth(context, 2);
    CGContextAddRect(context, CGRectInset(thumbRect, 2 * INSET_AMT, 2 * INSET_AMT));
    CGRect ellipseRect = CGRectMake(0, 0, 40, 40);
    [[UIColor colorWithWhite:aLevel alpha:1] setFill];
    CGContextAddEllipseInRect(context, ellipseRect);
    CGContextFillPath(context);
    
    NSString *numString = [NSString stringWithFormat:@"%0.2f",aLevel];
    UIColor *textColor = (aLevel > 0.5) ? [UIColor blackColor] : [UIColor whiteColor];
    UIFont *font = [UIFont fontWithName:@"Georgia" size:15];
    NSMutableParagraphStyle *style = [[NSMutableParagraphStyle alloc] init];
    style.lineBreakMode = NSLineBreakByCharWrapping;
    style.alignment = NSTextAlignmentCenter;
    NSDictionary *attr = @{NSFontAttributeName:font,NSParagraphStyleAttributeName:style,NSForegroundColorAttributeName:textColor};
    [numString drawInRect:CGRectInset(ellipseRect, 0, 6) withAttributes:attr];
    
    [[UIColor grayColor] setStroke];
    CGContextSetLineWidth(context, 3);
    CGContextAddEllipseInRect(context, CGRectInset(ellipseRect, 2, 2));
    CGContextStrokePath(context);
    
    UIImage *theImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    
    return  theImage;
}

- (void)updateThumb
{
    if ((self.value < 0.98) && (ABS(self.value - previousValue) < 0.1f)) {
        return;
    }
    
    UIImage *customImg = thumbWithLevel(self.value);
    [self setThumbImage:customImg forState:UIControlStateHighlighted];
    previousValue = self.value;
}

[self setThumbImage:simpleThumb() forState:UIControlStateNormal];
  [self addTarget:self action:@selector(startDrag:) forControlEvents:UIControlEventTouchDown];
  [self addTarget:self action:@selector(updateThumb) forControlEvents:UIControlEventValueChanged];
  [self addTarget:self action:@selector(endDrag:) forControlEvents:UIControlEventTouchUpOutside | UIControlEventTouchUpInside];