Scale UIButton Animation- Swift

All of the answers above are valid.
As a plus, with Swift I suggest to create an extension of UIView in order to "scale" any view you want.
You can take inspiration from this piece of code:

SWIFT 5.0

extension UIView {

/**
 Simply zooming in of a view: set view scale to 0 and zoom to Identity on 'duration' time interval.

 - parameter duration: animation duration
 */
func zoomIn(duration: TimeInterval = 0.2) {
    self.transform = CGAffineTransform(scaleX: 0.0, y: 0.0)
    UIView.animate(withDuration: duration, delay: 0.0, options: [.curveLinear], animations: { () -> Void in
        self.transform = .identity
        }) { (animationCompleted: Bool) -> Void in
    }
}

/**
 Simply zooming out of a view: set view scale to Identity and zoom out to 0 on 'duration' time interval.

 - parameter duration: animation duration
 */
func zoomOut(duration : TimeInterval = 0.2) {
    self.transform = .identity
    UIView.animate(withDuration: duration, delay: 0.0, options: [.curveLinear], animations: { () -> Void in
        self.transform = CGAffineTransform(scaleX: 0.0, y: 0.0)
        }) { (animationCompleted: Bool) -> Void in
    }
}

/**
 Zoom in any view with specified offset magnification.

 - parameter duration:     animation duration.
 - parameter easingOffset: easing offset.
 */
func zoomInWithEasing(duration: TimeInterval = 0.2, easingOffset: CGFloat = 0.2) {
    let easeScale = 1.0 + easingOffset
    let easingDuration = TimeInterval(easingOffset) * duration / TimeInterval(easeScale)
    let scalingDuration = duration - easingDuration
    UIView.animate(withDuration: scalingDuration, delay: 0.0, options: .curveEaseIn, animations: { () -> Void in
        self.transform = CGAffineTransform(scaleX: easeScale, y: easeScale)
        }, completion: { (completed: Bool) -> Void in
            UIView.animate(withDuration: easingDuration, delay: 0.0, options: .curveEaseOut, animations: { () -> Void in
                self.transform = .identity
                }, completion: { (completed: Bool) -> Void in
            })
    })
}

/**
 Zoom out any view with specified offset magnification.

 - parameter duration:     animation duration.
 - parameter easingOffset: easing offset.
 */
func zoomOutWithEasing(duration: TimeInterval = 0.2, easingOffset: CGFloat = 0.2) {
    let easeScale = 1.0 + easingOffset
    let easingDuration = TimeInterval(easingOffset) * duration / TimeInterval(easeScale)
    let scalingDuration = duration - easingDuration
    UIView.animate(withDuration: easingDuration, delay: 0.0, options: .curveEaseOut, animations: { () -> Void in
        self.transform = CGAffineTransform(scaleX: easeScale, y: easeScale)
        }, completion: { (completed: Bool) -> Void in
            UIView.animate(withDuration: scalingDuration, delay: 0.0, options: .curveEaseOut, animations: { () -> Void in
                self.transform = CGAffineTransform(scaleX: 0.0, y: 0.0)
                }, completion: { (completed: Bool) -> Void in
            })
    })
}

}

Usage is very simply:

let button = UIButton(frame: frame)
button.zoomIn() // here the magic

Swift 3 Version

extension UIView {

/**
 Simply zooming in of a view: set view scale to 0 and zoom to Identity on 'duration' time interval.

 - parameter duration: animation duration
 */
func zoomIn(duration: TimeInterval = 0.2) {
    self.transform = CGAffineTransform(scaleX: 0.0, y: 0.0)
    UIView.animate(withDuration: duration, delay: 0.0, options: [.curveLinear], animations: { () -> Void in
        self.transform = CGAffineTransform.identity
    }) { (animationCompleted: Bool) -> Void in
    }
}

/**
 Simply zooming out of a view: set view scale to Identity and zoom out to 0 on 'duration' time interval.

 - parameter duration: animation duration
 */
func zoomOut(duration: TimeInterval = 0.2) {
    self.transform = CGAffineTransform.identity
    UIView.animate(withDuration: duration, delay: 0.0, options: [.curveLinear], animations: { () -> Void in
        self.transform = CGAffineTransform(scaleX: 0.0, y: 0.0)
    }) { (animationCompleted: Bool) -> Void in
    }
}

/**
 Zoom in any view with specified offset magnification.

 - parameter duration:     animation duration.
 - parameter easingOffset: easing offset.
 */
func zoomInWithEasing(duration: TimeInterval = 0.2, easingOffset: CGFloat = 0.2) {
    let easeScale = 1.0 + easingOffset
    let easingDuration = TimeInterval(easingOffset) * duration / TimeInterval(easeScale)
    let scalingDuration = duration - easingDuration
    UIView.animate(withDuration: scalingDuration, delay: 0.0, options: .curveEaseIn, animations: { () -> Void in
        self.transform = CGAffineTransform(scaleX: easeScale, y: easeScale)
    }, completion: { (completed: Bool) -> Void in
        UIView.animate(withDuration: easingDuration, delay: 0.0, options: .curveEaseOut, animations: { () -> Void in
            self.transform = CGAffineTransform.identity
        }, completion: { (completed: Bool) -> Void in
        })
    })
}

/**
 Zoom out any view with specified offset magnification.

 - parameter duration:     animation duration.
 - parameter easingOffset: easing offset.
 */
func zoomOutWithEasing(duration: TimeInterval = 0.2, easingOffset: CGFloat = 0.2) {
    let easeScale = 1.0 + easingOffset
    let easingDuration = TimeInterval(easingOffset) * duration / TimeInterval(easeScale)
    let scalingDuration = duration - easingDuration
    UIView.animate(withDuration: easingDuration, delay: 0.0, options: .curveEaseOut, animations: { () -> Void in
        self.transform = CGAffineTransform(scaleX: easeScale, y: easeScale)
    }, completion: { (completed: Bool) -> Void in
        UIView.animate(withDuration: scalingDuration, delay: 0.0, options: .curveEaseOut, animations: { () -> Void in
            self.transform = CGAffineTransform(scaleX: 0.0, y: 0.0)
        }, completion: { (completed: Bool) -> Void in
        })
    })
}

}

Try this

UIView.animate(withDuration: 0.6,
    animations: {
        self.button.transform = CGAffineTransform(scaleX: 0.6, y: 0.6)
    },
    completion: { _ in
        UIView.animate(withDuration: 0.6) {
            self.button.transform = CGAffineTransform.identity
        }
    })

SWIFT 5 Code Update :I have animated button with a nice bouncing effect , with spring animation.

@IBOutlet weak var button: UIButton!

@IBAction func animateButton(sender: UIButton) {

    sender.transform = CGAffineTransform(scaleX: 0.6, y: 0.6)

    UIView.animate(withDuration: 2.0,
                               delay: 0,
                               usingSpringWithDamping: CGFloat(0.20),
                               initialSpringVelocity: CGFloat(6.0),
                               options: UIView.AnimationOptions.allowUserInteraction,
                               animations: {
                                sender.transform = CGAffineTransform.identity
        },
                               completion: { Void in()  }
    )
}