How to get the center of the thumb image of UISlider

I'm creating a custom UISlider to test out some interface ideas. Mostly based around making the thumb image larger.

I found out how to do that, like so:

UIImage *thumb = [UIImage imageNamed:@"newThumbImage_64px.png"];  
[self.slider setThumbImage:thumb forState:UIControlStateNormal];  
[self.slider setThumbImage:thumb forState:UIControlStateHighlighted];  
[thumb release];  

To calculate a related value I need to know where the center point of the thumb image falls when it's being manipulated. And the point should be in it's superview's coordinates.

Looking at the UISlider docs, I didn't see any property that tracked this.

Is there some easy way to calculate this or can it be derived from some existing value(s)?

Solution 1:

This will return the correct X position of center of thumb image of UISlider in view coordinates:

- (float)xPositionFromSliderValue:(UISlider *)aSlider {
     float sliderRange = aSlider.frame.size.width - aSlider.currentThumbImage.size.width;
     float sliderOrigin = aSlider.frame.origin.x + (aSlider.currentThumbImage.size.width / 2.0);

     float sliderValueToPixels = (((aSlider.value - aSlider.minimumValue)/(aSlider.maximumValue - aSlider.minimumValue)) * sliderRange) + sliderOrigin;

     return sliderValueToPixels;
}

Put it in your view controller and use it like this: (assumes property named slider)

float x = [self xPositionFromSliderValue:self.slider];

Solution 2:

I tried this after reading the above suggestion -

yourLabel = [[UILabel alloc]initWithFrame:....];

//Call this method on Slider value change event

-(void)sliderValueChanged{
    CGRect trackRect = [self.slider trackRectForBounds:self.slider.bounds];
    CGRect thumbRect = [self.slider thumbRectForBounds:self.slider.bounds
                               trackRect:trackRect
                                   value:self.slider.value];

    yourLabel.center = CGPointMake(thumbRect.origin.x + self.slider.frame.origin.x,  self.slider.frame.origin.y - 20);
}

For Swift version

func sliderValueChanged() -> Void {
        let trackRect =  self.slider.trackRect(forBounds: self.slider.bounds)
        let thumbRect = self.slider.thumbRect(forBounds: self.slider.bounds, trackRect: trackRect, value: self.slider.value)
        yourLabel.center = CGPoint(x: thumbRect.origin.x + self.slider.frame.origin.x + 30, y: self.slider.frame.origin.y - 60)
    }

I could get most accurate value by using this snippet.

Solution 3:

Swift 3

extension UISlider {
    var thumbCenterX: CGFloat {
        let trackRect = self.trackRect(forBounds: frame)
        let thumbRect = self.thumbRect(forBounds: bounds, trackRect: trackRect, value: value)
        return thumbRect.midX
    }
}

Solution 4:

I would like to know why none of you provide the simplest answer which consist in reading the manual. You can compute all these values accurately and also MAKING SURE THEY STAY THAT WAY, by simply using the methods:

- (CGRect)trackRectForBounds:(CGRect)bounds
- (CGRect)thumbRectForBounds:(CGRect)bounds trackRect:(CGRect)rect value:(float)value

which you can easily find in the developer documentation.

If thumb image changes and you want to change how it's positioned, you subclass and override these methods. The first one gives you the rectangle in which the thumb can move the second one the position of the thumb itself.