1.版本判断使用API 新旧方法。

if __IPHONE_OS_VERSION_MIN_REQUIRED < 90000

    appearance = [UIActivityIndicatorView appearanceWhenContainedIn:[MBProgressHUD class], nil];

else

    // For iOS 9+
    appearance = [UIActivityIndicatorView appearanceWhenContainedInInstancesOfClasses:@[[MBProgressHUD class]]];

endif

2.[[[UIDevice currentDevice] systemVersion] floatValue] >= 9.0

大于9.0 小于8.0

3.BOOL iOS9OrLater = kCFCoreFoundationVersionNumber >= kCFCoreFoundationVersionNumber_iOS_9_0;
系统判断