RotatableImageView.swift

/// 中心に画像を描画する
class RotatableImageView: UIView {
    var image: UIImage? {
        didSet {
            setNeedsDisplay()
        }
    }
    var angle: CGFloat = 0.0
    var scale: CGFloat = 1.0
    var translateX: CGFloat = 0.0
    var translateY: CGFloat = 0.0
    
    var contentSize: CGSize {
        return image?.size ?? .zero
    }
    
    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        _commonInit()
    }
    
    override init(frame: CGRect) {
        super.init(frame: frame)
        _commonInit()
    }
    
    private func _commonInit() {
        self.backgroundColor = .clear
    }
    
    override func sizeToFit() {
        self.frame = CGRect(origin: self.frame.origin, size: contentSize)
    }
    
    func getImage(of frame: CGRect) -> UIImage? {
        UIGraphicsBeginImageContext(bounds.size)
        defer {
            UIGraphicsEndImageContext()
        }
        guard let context = _draw() else {
            return nil
        }
        return context.makeImage()?.cropping(to: frame).map { UIImage(cgImage: $0) }
    }
    
    @discardableResult
    private func _draw() -> CGContext? {
        guard let context = UIGraphicsGetCurrentContext() else {
            return nil
        }
        
        guard let image = image else {
            return context
        }
        
        // clear
        context.clear(bounds)
        
        // background
        context.setFillColor((backgroundColor ?? UIColor.clear).cgColor)
        context.fill(bounds)
        
        // 座標系を一時退避
        context.saveGState()
        
        // 座標系を中心に
        context.translateBy(x: bounds.midX + translateX, y: bounds.midY + translateY)
        
        let imageFrame = CGRect(origin: .zero, size: image.size)
        
        // rotate
        context.rotate(by: angle)
        
        // scale
        context.scaleBy(x: scale, y: scale)
        
        // draw
        image.draw(in: imageFrame.offsetBy(dx: -imageFrame.midX, dy: -imageFrame.midY))
        
        // 座標系を戻す
        context.restoreGState()
        
        return context
    }
    
    override func draw(_ rect: CGRect) {
        _draw()
    }
}