Detect retina screen/iPhone 4 in iPhone SDK

In my application I am downloading some images from the web (from my server to be precise), in order to save some bandwith and especially memory on the phone, I provid开发者_开发百科e them in two resolutions: 480x320 for the "old" iPhone series and in 960x640 for the iPhone 4 with the retina display. Now I need to be able to detect from within the app when it is running on a device that supports the retina screen. How could I do that?

I have been thinking about using the code snippet below which would return me a specific device identifier such as eg. "iPhone3", yet then I would be limiting the detection to the iPhone4 and would need to update that code for any subsequent device with a retina display.

size_t size;

// Set 'oldp' parameter to NULL to get the size of the data
// returned so we can allocate appropriate amount of space
sysctlbyname("hw.machine", NULL, &size, NULL, 0); 

// Allocate the space to store name
char *name = malloc(size);

// Get the platform name
sysctlbyname("hw.machine", name, &size, NULL, 0);

// Place name into a string
NSString *machine = [NSString stringWithCString:name];

Is there any better soution (maybe it is very obvious but I missed it)?

Just did some reading on the official Apple Developers Forums and the issues has been discussed in length there. The best way to me seems to be the use of the scale property of UIScreen. Although it is only available in iOS 4 and later, it will tell you everything you need to know and will most likely play an even more important role in the future (already noticed that the iPad's screen resolution of 1024x768 is exactly 32/15 * 480x320?).

UIScreen.mainScreen.scale

If you have yet another idea feel free to post it :)

Here is some code to do it the right way for both iOS 3.x and 4.x:

BOOL hasHighResScreen = NO;
if ([UIScreen instancesRespondToSelector:@selector(scale)]) {
    CGFloat scale = [[UIScreen mainScreen] scale];
    if (scale > 1.0) {
        hasHighResScreen = YES;
    }
}

A little update on answer by Scott Gustafson:

If we need to distinguish between iPad/Retina/iphone:

    if (UI_USER_INTERFACE_IDIOM() == UIUserInterfaceIdiomPad) 
    {
        CGFloat scale = [[UIScreen mainScreen] scale];

           if (scale > 1.0) 
           {
                //iPad retina screen
           }  
           else
           {
                //iPad screen
           }
    } 
    else
    {
         if ([UIScreen instancesRespondToSelector:@selector(scale)]) 
         {
               CGFloat scale = [[UIScreen mainScreen] scale];

               if (scale > 1.0) 
               {
                    if([[ UIScreen mainScreen ] bounds ].size.height == 568)
                    {
                        //iphone 5
                    }
                    else
                    {
                        //iphone retina screen
                    }
               }
               else
               {
                    //iphone screen
               }
         }
    }

I get real screen size (in pixels) by this way:

UIScreen *MainScreen = [UIScreen mainScreen];
UIScreenMode *ScreenMode = [MainScreen currentMode];
CGSize Size = [ScreenMode size]; // <--- Real screen size

- (BOOL)isRetina {

    BOOL isRetina = NO;

    if ([UIScreen instancesRespondToSelector:@selector(scale)]) {
        CGFloat scale = [[UIScreen mainScreen] scale];
        if (scale > 1.0) {
            isRetina = YES;
        }
    }

    return isRetina;
}


- (CGSize)sizeInPixels {

    CGRect appFrame = [[UIScreen mainScreen] applicationFrame];
    CGSize screenSize = CGSizeMake(appFrame.size.width, appFrame.size.height);

    return [self isRetina] ? CGSizeMake(screenSize.width * 2, screenSize.height * 2) : screenSize;
}

Go with Robin's answer. One other note: if you do need to check the device name, just use the methods on UIDevice.

[[UIDevice currentDevice] model];
[[UIDevice currentDevice] systemName];
[[UIDevice currentDevice] systemVersion];

UIScreen *MainScreen = [UIScreen mainScreen];
UIScreenMode *ScreenMode = [MainScreen currentMode];
CGSize Size = [ScreenMode size]; // <--- Real screen size

And for those who just want to copy/paste how to detect iphone/iphone_retina/ipad/ipad_retina, here's what I ended up doing after reading this thread. Heavily inspired by Guntis Treulands' contribution, who in turn expanded upon Scott Gustafsons answer.

- (NSString *) yesButWhichDeviceIsIt {    
    BOOL hasRetina = NO;
    if ([UIScreen instancesRespondToSelector:@selector(scale)]) {
        CGFloat scale = [[UIScreen mainScreen] scale];
        if (scale > 1.0) {
            hasRetina = YES;
        }
    }
    if (UI_USER_INTERFACE_IDIOM() == UIUserInterfaceIdiomPad) {
        if (hasRetina) {
            return @"iPad retina";
        } else {
            return @"iPad";
        }
    } else {
        if (hasRetina) {
            return @"iPhone retina";
        } else {
            return @"iPhone";
        }        
    }
}

If you're using Cocos2d, try the following:

[[CCDirector sharedDirector] winSizeInPixels];

It will return a CGSize with properties width and height.

+(BOOL)Retina{
        return ([[UIScreen mainScreen] respondsToSelector:@selector(displayLinkWithTarget:selector:)] && ([UIScreen mainScreen].scale == 2.0))?1:0; }

Although you've already selected an answer, there is a much easier way when specifically dealing with images so I'll mention it as well.

If you include two images in your bundle of the two sizes (320x480 and 640x960) and you append "@2x" at the end of the latter image's filename, [UIImage imageNamed:] will automagically pick the smaller image for older devices and the 2x for devices with a retina display, provided you leave off the image suffix. Ex.:

2 images named @"image.png" and @"image@2x.png", both included in the app bundle.

Then call:

[UIImage imageNamed:@"image"];

This is also how app icons and the loading screen work.