怎么判断IOS设备的版本

有两个storyboard分别制定给iPhone5和iPhone4的尺寸。如何让代码检测ios版本？

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{

    // 1
    UIStoryboard * mainStoryboard = nil ;
    if ( SYSTEM_VERSION_GREATER_THAN_OR_EQUAL_TO ( @ "6.0" ) ) {
        mainStoryboard = [ UIStoryboard storyboardWithName : @ "iPhones6.0" bundle : nil ] ;
    } else {
        mainStoryboard = [ UIStoryboard storyboardWithName : @ "iPhones3-5" bundle : nil ] ;
    }
    // 2
    self.window = [ [ UIWindow alloc ] initWithFrame : [ [ UIScreen mainScreen ] bounds ] ] ;
    self.window.rootViewController = [ mainStoryboard instantiateInitialViewController ] ;
    [ self.window makeKeyAndVisible ] ;
    return YES;
}

判断是否为iphone5的宏

#define IPHONE5 ([UIScreen instancesRespondToSelector:@selector(currentMode)] ? CGSizeEqualToSize(CGSizeMake(640, 1136), [[UIScreen mainScreen] currentMode].size) : 0)

这个不对吧，版本代表不了手机型号，为何不去获取物理屏幕大小好了呢？

其实一个storyboard就可以适配iphone5,iphone4的尺寸。在storyboard中可以设置UI元素的约束来定位位置，这里需要用到NSLayoutConstrant 技术