Retina display and [UIImage initWithData]

I need to initi开发者_StackOverflow社区alise images from raw data downloaded from a server which delivers the correct size of image based on the type of iPhone client.

I know that I should be setting the scale value to be 2.0 on the 640x960 display, however this is a readonly property and cannot be set during the init when using initWithData.

Any ideas?

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I'm not aware of anything you can embed in the image data itself to tell the phone that it's a @2x image, but something like this should work:

UIImage * img = ...;
img = [UIImage imageWithCGImage:img.CGImage scale:2 orientation:img.imageOrientation];

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Since iOS 6.0 UIImage has method + imageWithData:scale:, you can pass 2.0 as scale for retina.

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You can pass [[UIScreen mainScreen] scale] as the scale parameter instead of 2.0f.

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Swift3, 4 version

let image = UIImage(data: imageData, scale: UIScreen.main.scale)

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put this in your .m if you want or on an imported class (the syntax of c is nicer when calling the function IMAO)

BOOL isRetina(){
    if ([[UIScreen mainScreen] respondsToSelector:@selector(scale)]) {
        return [[UIScreen mainScreen] scale] == 2.0;
    }
    return NO;
}

Then when creating the image with the server data:

[UIImage imageWithData:dataFromServer scale:isRetina()?2:1];

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AFAIK you don't need to set the scale value yourself. The OS will handle the points to pixel translation for you.