UISlider that snaps to a fixed number of steps (like Text Size in the iOS 7 Settings app)

I'm trying to create a UISlider that lets you choose from an array of numbers. Each slider position should be equidistant and the slider should snap to each position, rather than smoothly slide between them. (This is the behavior of the slider in Settings > General > Text Size, which was introduced in iOS 7.)

The numbers I want to choose from are: -3, 0, 2, 4, 7, 10, and 12.

(I'm very new to Objective-C, so a complete code example would be much more helpful than a code snippet. =)

Some of the other answers work, but this will give you the same fixed space between every position in your slider. In this example you treat the slider positions as indexes to an array which contains the actual numeric values you are interested in.

@interface MyViewController : UIViewController {
    UISlider *slider;
    NSArray *numbers;
}
@end

@implementation MyViewController
- (void)viewDidLoad {
    [super viewDidLoad];
    slider = [[UISlider alloc] initWithFrame:self.view.bounds];
    [self.view addSubview:slider];

    // These number values represent each slider position
    numbers = @[@(-3), @(0), @(2), @(4), @(7), @(10), @(12)];
    // slider values go from 0 to the number of values in your numbers array
    NSInteger numberOfSteps = ((float)[numbers count] - 1);
    slider.maximumValue = numberOfSteps;
    slider.minimumValue = 0;

    // As the slider moves it will continously call the -valueChanged: 
    slider.continuous = YES; // NO makes it call only once you let go
    [slider addTarget:self
               action:@selector(valueChanged:)
     forControlEvents:UIControlEventValueChanged];
}
- (void)valueChanged:(UISlider *)sender {
    // round the slider position to the nearest index of the numbers array
    NSUInteger index = (NSUInteger)(slider.value + 0.5);
    [slider setValue:index animated:NO];
    NSNumber *number = numbers[index]; // <-- This numeric value you want
    NSLog(@"sliderIndex: %i", (int)index);
    NSLog(@"number: %@", number);
}

I hope that helps, good luck.

Edit: Here's a version in Swift 4 that subclasses UISlider with callbacks.

class MySliderStepper: UISlider {
    private let values: [Float]
    private var lastIndex: Int? = nil
    let callback: (Float) -> Void
    
    init(frame: CGRect, values: [Float], callback: @escaping (_ newValue: Float) -> Void) {
        self.values = values
        self.callback = callback
        super.init(frame: frame)
        self.addTarget(self, action: #selector(handleValueChange(sender:)), for: .valueChanged)
        
        let steps = values.count - 1
        self.minimumValue = 0
        self.maximumValue = Float(steps)
    }
    
    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    @objc func handleValueChange(sender: UISlider) {
        let newIndex = Int(sender.value + 0.5) // round up to next index
        self.setValue(Float(newIndex), animated: false) // snap to increments
        let didChange = lastIndex == nil || newIndex != lastIndex!
        if didChange {
            lastIndex = newIndex
            let actualValue = self.values[newIndex]
            self.callback(actualValue)
        }
    }
}

If you have regular spaces between steps, you can use it like this for 15 value:

@IBAction func timeSliderChanged(sender: UISlider) {
    
    let newValue = Int(sender.value/15) * 15
    sender.setValue(Float(newValue), animated: false)
}

This worked for my particular case, using @IBDesignable:

Requires even, integer intervals:

@IBDesignable
class SnappableSlider: UISlider {

    @IBInspectable
    var interval: Int = 1

    override init(frame: CGRect) {
        super.init(frame: frame)
        setUpSlider()
    }

    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        setUpSlider()
    }

    private func setUpSlider() {
        addTarget(self, action: #selector(handleValueChange(sender:)), for: .valueChanged)
    }

    @objc func handleValueChange(sender: UISlider) {
        let newValue =  (sender.value / Float(interval)).rounded() * Float(interval)
        setValue(Float(newValue), animated: false)
    }
}

The answer is essentially the same at this answer to https://stackoverflow.com/questions/2519460/how-to-uislider-with-increments-of-5

To modify it to work for your case, you'll need to create a rounding function that returns only the values you want. For example, you could do something simple (though hacky) like this:

-(int)roundSliderValue:(float)x {
    if (x < -1.5) {
        return -3;
    } else if (x < 1.0) {
        return 0;
    } else if (x < 3.0) {
        return 2;
    } else if (x < 5.5) {
        return 4;
    } else if (x < 8.5) {
        return 7;
    } else if (x < 11.0) {
        return 10;
    } else {
        return 12;
    }
}

Now use the answer from the previous post to round the value.

slider.continuous = YES;
[slider addTarget:self
      action:@selector(valueChanged:) 
      forControlEvents:UIControlEventValueChanged];

Finally, implement the changes:

-(void)valueChanged:(id)slider {
    [slider setValue:[self roundSliderValue:slider.value] animated:NO];
}

If anyone also needs snaping animation then can do the following:

1. Uncheck the continuous update of the slider from the storyboard.

You can do the same from swift slider.isContinuous = false

2. Add the following @IBAction in your ViewController:

   @IBAction func sliderMoved(_ slider: UISlider){
        
        let stepCount = 10
        
        let roundedCurrent = (slider.value/Float(stepCount)).rounded()
        let newValue = Int(roundedCurrent) * stepCount
        
        slider.setValue(Float(newValue), animated: true)
    }

I was inspired by this answer

Similar approach as PengOne, but I did manual rounding to make it more clear what was happening.

- (IBAction)sliderDidEndEditing:(UISlider *)slider {
    // default value slider will start at
    float newValue = 0.0;

    if (slider.value < -1.5) {
        newValue = -3;
    } else if (slider.value < 1) {
        newValue = 0;
    } else if (slider.value < 3) {
        newValue = 2;
    } else if (slider.value < 5.5) {
        newValue = 4;
    } else if (slider.value < 8.5) {
        newValue = 7;
    } else if (slider.value < 11) {
        newValue = 10;
    } else {
        newValue = 12;
    }
    slider.value = newValue;
}

- (IBAction)sliderValueChanged:(id)sender {
    UISlider *slider = sender;
    float newValue = 0.0;

    if (slider.value < -1.5) {
        newValue = -3;
    } else if (slider.value < 1) {
        newValue = 0;
    } else if (slider.value < 3) {
        newValue = 2;
    } else if (slider.value < 5.5) {
        newValue = 4;
    } else if (slider.value < 8.5) {
        newValue = 7;
    } else if (slider.value < 11) {
        newValue = 10;
    } else {
        newValue = 12;
    }
    // and if you have a label displaying the slider value, set it
    [yourLabel].text = [NSString stringWithFormat:@"%d", (int)newValue];
}