Swift 不通过注册通知获取键盘高度

func getVisiableKeyboardHeight() -> CGFloat {
    
    var keyboardWindow : UIWindow?
    for window : UIWindow in UIApplication.shared.windows {
        
        if !window.isKind(of: UIWindow.self) {
            keyboardWindow = window
        }
    }
    if keyboardWindow == nil { return 0 }
    
    var inputView : UIView?
    for testWindow : UIView in keyboardWindow!.subviews {

        if testWindow.isKind(of: NSClassFromString("UIInputSetContainerView")!) {

            inputView = testWindow;
            break;
        }
    }
    if inputView == nil { return 0 }
    
    for possibleKeyboard : UIView  in inputView!.subviews {

        if possibleKeyboard.isKind(of: NSClassFromString("UIInputSetHostView")!) || possibleKeyboard.isKind(of: NSClassFromString("UIKeyboard")!) {
            
            return possibleKeyboard.bounds.size.height;
        }
    }
    
    return 0
}