获取当前iOS版本

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