animateAndDraw.swift

//MARK: - common init
private func commonInit(){
    shapeLayer.fillColor = setFillColor.cgColor
    shapeLayer.actions = ["path" : NSNull(), "position" : NSNull(), "bounds" : NSNull()]
    layer.addSublayer(shapeLayer)
    shapeLayer.masksToBounds = true

    circleLayer.lineWidth = 4
    circleLayer.strokeColor = setRefreshCircleColor.cgColor
    circleLayer.fillColor = UIColor.clear.cgColor
    circleLayer.actions = ["path" : NSNull(), "position" : NSNull(), "bounds" : NSNull()]
    layer.addSublayer(circleLayer)
}

//MARK: - draw rect
override func draw(_ rect: CGRect) {

    guard let _ = containerScrollView else {return}
    calculate(rect)

    leftTop = CGPoint(x: rect.minX, y: rect.minY)
    rightTop = CGPoint(x: rect.maxX, y: rect.minY)
    leftBottom = CGPoint(x: rect.minX, y: edgeBottomPointYOffset)
    rightBottom = CGPoint(x: rect.maxX, y: edgeBottomPointYOffset)
    midBottom = CGPoint(x: xPositionOfPan, y: middleBottomPointYOffset)

    ///border path of refresh control
    let path = CGMutablePath()
    path.move(to: leftTop)
    path.addLine(to: leftBottom)
    path.addLine(to: midBottom)
    path.addLine(to: rightBottom)
    path.addLine(to: rightTop)
    path.closeSubpath()

    shapeLayer.path = path

    if !refreshingStatus {
        ///circle refresh path
        let draggedFractionCompleted = edgeBottomPointYOffset / thresholdDrag
        let circlePath = UIBezierPath(arcCenter: CGPoint(x: 0, y: 0), radius: refreshCircleSize.rawValue, startAngle: getStartAngle(draggedFractionCompleted), endAngle: getStartAngle(draggedFractionCompleted + 0.85), clockwise: true)
        circleLayer.path = circlePath.cgPath
    }
}

//MARK: - calculation
/// calculates all required dynamic variables and sets frame of layers
/// - Parameter rect: CGRect of view's frame
private func calculate(_ rect : CGRect){

    //guard "scrollViewContentYOffset" to be greater than zero
    //i.e. user is dragging scroll view downward such that actual content offset of scroll view is negative
    guard scrollViewContentYOffset >= 0 else {
        middleBottomPointYOffset = 0
        edgeBottomPointYOffset = 0
        return

    }

    //calculating y offsets of points
    //if refreshing status is false then we have to draw V shape at bottom
    if !refreshingStatus{
        middleBottomPointYOffset = min(scrollViewContentYOffset, maxHeightOfRefreshControl)
        edgeBottomPointYOffset = max((middleBottomPointYOffset - 20),0)
    }else{
        //else if refreshing status is true then --- straight line at bottom
        middleBottomPointYOffset = min(scrollViewContentYOffset, thresholdDrag)
        edgeBottomPointYOffset = middleBottomPointYOffset

        //then set scroll view's content inset
        containerScrollView?.contentInset.top = middleBottomPointYOffset
    }

    //calculating frame of layer
    shapeLayer.frame = CGRect(x: 0, y: 0, width: rect.width, height: middleBottomPointYOffset)

    //calculate center of circle
    centerForCircle = CGPoint(x: rect.midX, y: edgeBottomPointYOffset - (thresholdDrag / 2))
    circleLayer.frame = CGRect(x: centerForCircle.x, y: centerForCircle.y, width: 0, height: 0)
    //wondering why are we providing frame with height and width zero and origin to center of circle??
    //since animating a layer about z axis by default rotates whole frame of that layer about its origin
    //so setting origin of frame of circle layer to center of circle with height and width as zero(basically a pin point \ (•◡•) /)
    //then applying rotation about z-axis will rotate our circle in desired way
    // kinda hack you want to use if you are ever stuck in these rotation stuff ¯\_(ツ)_/¯

}


/// calculating starting angle to draw circle according to fraction of drag completed
/// - Parameter fractionCompleted: fraction of drag completed
private func getStartAngle(_ fractionCompleted : CGFloat) -> CGFloat{
    return ((2 * CGFloat.pi) * (fractionCompleted))
}

//MARK: - animation
/// animates refresh circle
private func animateRefreshCircle(){
    let animation = CABasicAnimation(keyPath: "transform.rotation.z")
    animation.fromValue = 0.0
    animation.toValue = CGFloat.pi * CGFloat(2.0)
    animation.duration = 1.5
    animation.repeatCount = .infinity
    animation.fillMode = .forwards
    animation.isRemovedOnCompletion = false

    circleLayer.add(animation, forKey: "rotate")
}