屏幕底部安全距离(工具栏底部距离屏幕底部的距离通过safeAreaInsets获取)

    CGFloat safeNum = 0;
    //判断版本
    if (@available(iOS 11.0, *)) {
        //通过系统方法keyWindow来获取safeAreaInsets
        UIEdgeInsets safeArea = [[UIApplication sharedApplication] keyWindow].safeAreaInsets;
        safeNum = safeArea.bottom;
    }

//如果是iPhoneX 之类刘海屏底部的工具栏距离屏幕底部(底部安全距离) safeNum =34
//如果是iPhone8 之类的工具栏距离屏幕底部(底部安全距离) safeNum = 0