利用基本動畫實現圖標旋轉

private func  startAnimation() {

        let anim = CABasicAnimation(keyPath: "transform.rotation")

        anim.toValue = 2 * M_PI

        anim.repeatCount = MAXFLOAT

        anim.duration = 3

        // 防止切換界面以後動畫中止.

        anim.removedOnCompletion = false 

        homeIconView.layer.addAnimation(anim, forKey:nil)

    }