官方推薦的用於版本判斷的代碼

NSUInteger DeviceSystemMajorVersion();
NSUInteger DeviceSystemMajorVersion() {
    static NSUInteger _deviceSystemMajorVersion = -1;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        _deviceSystemMajorVersion = [[[[[UIDevice currentDevice] systemVersion]
  componentsSeparatedByString:@"."] objectAtIndex:0] intValue];
    });
    return _deviceSystemMajorVersion;
}
#define IOS7_OR_LATER (DeviceSystemMajorVersion() >= 7)