swift之弹出一个背景半透明的UIViewController

let viewController=storyboard.instantiateViewControllerWithIdentifier("对应要启动的ViewController名字") as 对应要启动的ViewController名字
viewController.view.backgroundColor = UIColor(red: 0, green: 0, blue: 0, alpha: 0.5)
//viewController.view.backgroundColor = UIColor(white: 1, alpha: 0.9)
self.presentViewController(vPersonalizedSettingsViewController, animated: 动不动画随便你, completion: nil)

这里要注意的是，要启动的viewController必须要在这里设置半透明，如果在对应的UIViewController viewDidLoad里面设置是没有半透明的！

然后要启动的UIViewController viewDidLoad的时候设置

self.modalPresentationStyle = .Custom