UIButton won't go to Aspect Fit in iPhone

If you put the image in an UIImageView behind the button, you'll loose the built-in functionality of the UIButton class, such as adjustsImageWhenHighlighted and adjustsImageWhenDisabled, and of course the ability to set different images for different states (without the hazzle of doing this yourself).

If we want to have an image unstreched for all control states, one approuch is to get the image using imageWithCGImage:scale:orientation, as in the following method:

- (UIImage *) getScaledImage:(UIImage *)img insideButton:(UIButton *)btn {

    // Check which dimension (width or height) to pay respect to and
    // calculate the scale factor
    CGFloat imgRatio = img.size.width / img.size.height, 
            btnRatio = btn.frame.size.width / btn.frame.size.height,
            scaleFactor = (imgRatio > btnRatio 
                           ? img.size.width / btn.frame.size.width
                           : img.size.height / btn.frame.size.height;

    // Create image using scale factor
    UIImage *scaledImg = [UIImage imageWithCGImage:[img CGImage]
                                             scale:scaleFactor
                                       orientation:UIImageOrientationUp];
    return scaledImg;
}

To implement this we would write:

UIImage *scaledImg = [self getScaledImage:myBtnImg insideButton:myBtn];
[myBtn setImage:scaledImg forState:UIControlStateNormal];

This should prevent the image from stretching in all control states. It worked for me, but let me know if it doesn't!

NOTE: Here we are addressing a problem relating to UIButton, but the insideButton: might as well be insideView:, or whatever one would like to fit the image into.

Combining together a few different answers into one solution-- create a button with a custom type, set the button's imageView contentMode property, and set the image for the button (not the background image, which will still scale to fill).

//Objective-C:
UIImage *image = [UIImage imageNamed:"myImageName.png"];
UIButton *button = [UIButton buttonWithType:UIButtonTypeCustom];
[button setImage:image forState:UIControlStateNormal];
button.imageView.contentMode = UIViewContentModeScaleAspectFit;

//Swift:
let image = UIImage(named: "myImageName.png")
let button = UIButton(type: .custom)
button.imageView?.contentMode = .scaleAspectFit
button.setImage(image, for: .normal)

You can use imageWithCGImage as shown above (but without the missing parentheses).

Also... millions of non-4.0 phones won't work with that code at all.