AnimatingConstraints3.swift

// Add a completion block to the UIViewPropertyAnimator 
linesConvergingAnimation.addCompletion({ (position) in
    // since this is the reversed animation's completion block, the
    // "final position" is the initial animation's target-start-position.
    if position == .start {
        self.linesAreX = false
        // reset constraints back to normal  
        self.firstLine.snp.remakeConstraints({ (make) in
            make.height.equalTo(self.lineHeight)
            make.width.equalTo(self.containerView.snp.width)
            make.centerX.equalTo(self.containerView.snp.centerX)
            make.centerY.equalTo(0 + self.lineHeight/2)
        })
        self.thirdLine.snp.remakeConstraints({ (make) in
            make.height.equalTo(self.lineHeight)
            make.width.equalTo(self.containerView.snp.width)
            make.centerX.equalTo(self.containerView.snp.centerX)
            make.centerY.equalTo(self.containerHeight - self.lineHeight/2)
        })
        // notice I'm not calling .layoutIfNeeded() because there's nothing new to draw
    }
})