iOS button的image旋转

    UIView.animate(withDuration: 0.25) { [weak self] in
        if self?.explainButton.isSelected {
            self?.explainButton.imageView?.transform = CGAffineTransform(rotationAngle: CGFloat.pi)
        }else {
            self?.explainButton.imageView?.transform = CGAffineTransform.identity
        }
    }