An iOS version of the tabs used with Android's view pager, based on a UISegmentedControl

ViewPagerTabs.swift

//
//  ViewPagerTabs.swift
//
//  Created by jesse calkin on 3/14/16.
//  Copyright © 2016 Shoshin Boogie. All rights reserved.
//

import UIKit

class ViewPager: UISegmentedControl {

    //MARK: - Properties

    //MARK: Internal

    @IBInspectable var highlightColor: UIColor?
    @IBInspectable var activeTextColor: UIColor = UIColor.whiteColor() {
        didSet {
            setTitleTextAttributes([NSForegroundColorAttributeName: activeTextColor], forState: UIControlState.Selected)
        }
    }
    @IBInspectable var inactiveTextColor: UIColor = UIColor.lightGrayColor() {
        didSet {
            setTitleTextAttributes([NSForegroundColorAttributeName: inactiveTextColor], forState: UIControlState.Normal)
        }
    }

    @IBInspectable var highlightHeight: CGFloat = 2.0
    @IBInspectable var hightlightInset: CGFloat = 0.0

    var segmentSpacing: CGFloat = 1.0

    override var selectedSegmentIndex: Int {
        didSet {
            animateSelection()
        }
    }

    //MARK: Private

    private let lineLayer = CAShapeLayer()
    private var lastPath: CGPath?

    //MARK: - View Lifecycle

    override init(items: [AnyObject]?) {
        super.init(items: items)
        setupStyle()
    }

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

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

    override func touchesEnded(touches: Set<UITouch>, withEvent event: UIEvent?) {
        super.touchesEnded(touches, withEvent: event)

        animateSelection()

        sendActionsForControlEvents(UIControlEvents.TouchUpInside)
    }

    override func drawRect(rect: CGRect) {
        super.drawRect(rect)
        animateSelection()
    }

    override func prepareForInterfaceBuilder() {
        super.prepareForInterfaceBuilder()
        setupStyle()
        animateSelection()
    }

    //MARK: - Private

    private func animateSelection() {
        let width = widthForSegmentAtIndex(selectedSegmentIndex) > 0.0 ? widthForSegmentAtIndex(selectedSegmentIndex) : bounds.width / CGFloat(numberOfSegments)

        let startPoint = CGPoint(x: width * CGFloat(selectedSegmentIndex) + hightlightInset, y: bounds.maxY - 2)
        let endPoint = CGPoint(x: width * CGFloat(selectedSegmentIndex) + (width - hightlightInset), y: bounds.maxY - 2)

        let path = UIBezierPath()
        path.moveToPoint(startPoint)
        path.addLineToPoint(endPoint)

        lineLayer.strokeColor = highlightColor?.CGColor ?? tintColor.CGColor
        lineLayer.lineWidth = highlightHeight
        lineLayer.path = path.CGPath

        let lineAnimation = CABasicAnimation(keyPath: "path")
        lineAnimation.fromValue = lastPath
        lineAnimation.toValue = path.CGPath
        lineAnimation.duration = AnimationTime.Default
        lineAnimation.fillMode = kCAFillModeForwards
        lineAnimation.timingFunction = CAMediaTimingFunction(name: kCAMediaTimingFunctionEaseInEaseOut)

        lineLayer.addAnimation(lineAnimation, forKey: "ViewPagerLineAnimation")

        lastPath = lineLayer.path
    }

    private func setupStyle() {
        layer.addSublayer(lineLayer)

        let colorImage = UIImage.imageWithColor(UIColor.clearColor())

        momentary = false

        setDividerImage(colorImage, forLeftSegmentState: UIControlState.Normal, rightSegmentState: UIControlState.Normal, barMetrics: UIBarMetrics.Default)
        setBackgroundImage(colorImage, forState: .Normal, barMetrics: .Default)
        setBackgroundImage(colorImage, forState: .Selected, barMetrics: .Default)
        setBackgroundImage(colorImage, forState: .Highlighted, barMetrics: .Default)

        setTitleTextAttributes([NSForegroundColorAttributeName: inactiveTextColor], forState: UIControlState.Normal)
        setTitleTextAttributes([NSForegroundColorAttributeName: activeTextColor], forState: UIControlState.Selected)
    }
}

//MARK: - UIImage Extension

extension UIImage {
    class func imageWithColor(color: UIColor, frame: CGRect = CGRect(x: 0.0, y: 0.0, width: 1.0, height: 1.0)) -> UIImage {
        UIGraphicsBeginImageContextWithOptions(frame.size, false, 0.0)

        let context = UIGraphicsGetCurrentContext()

        CGContextSetFillColorWithColor(context, color.CGColor)
        CGContextFillRect(context, frame)

        let image = UIGraphicsGetImageFromCurrentImageContext()

        UIGraphicsEndImageContext()
        return image
    }
}