UISlider with ProgressView combined

Here’s a simple version of what you’re describing.

It is “simple” in the sense that I didn’t bother trying to add the shading and other subtleties. But it’s easy to construct and you can tweak it to draw in a more subtle way if you like. For example, you could make your own image and use it as the slider’s thumb.

This is actually a UISlider subclass lying on top of a UIView subclass (MyTherm) that draws the thermometer, plus two UILabels that draw the numbers.

The UISlider subclass eliminates the built-in track, so that the thermometer behind it shows through. But the UISlider’s thumb (knob) is still draggable in the normal way, and you can set it to a custom image, get the Value Changed event when the user drags it, and so on. Here is the code for the UISlider subclass that eliminates its own track:

- (CGRect)trackRectForBounds:(CGRect)bounds {
    CGRect result = [super trackRectForBounds:bounds];
    result.size.height = 0;
    return result;
}

The thermometer is an instance of a custom UIView subclass, MyTherm. I instantiated it in the nib and unchecked its Opaque and gave it a background color of Clear Color. It has a value property so it knows how much to fill the thermometer. Here’s its drawRect: code:

- (void)drawRect:(CGRect)rect {
    CGContextRef c = UIGraphicsGetCurrentContext();
    [[UIColor whiteColor] set];
    CGFloat ins = 2.0;
    CGRect r = CGRectInset(self.bounds, ins, ins);
    CGFloat radius = r.size.height / 2.0;
    CGMutablePathRef path = CGPathCreateMutable();
    CGPathMoveToPoint(path, NULL, CGRectGetMaxX(r) - radius, ins);
    CGPathAddArc(path, NULL, radius+ins, radius+ins, radius, -M_PI/2.0, M_PI/2.0, true);
    CGPathAddArc(path, NULL, CGRectGetMaxX(r) - radius, radius+ins, radius, M_PI/2.0, -M_PI/2.0, true);
    CGPathCloseSubpath(path);
    CGContextAddPath(c, path);
    CGContextSetLineWidth(c, 2);
    CGContextStrokePath(c);
    CGContextAddPath(c, path);
    CGContextClip(c);
    CGContextFillRect(c, CGRectMake(r.origin.x, r.origin.y, r.size.width * self.value, r.size.height));
}

To change the thermometer value, change the MyTherm instance’s value to a number between 0 and 1, and tell it to redraw itself with setNeedsDisplay.