How to write custom metadata to PNG images in iOS

Question

My application should be able to write custom metadata entries to PNG images for export to the UIPasteboard.

By piecing together various posts on the subject, I\'ve been

Answer 1

If you will try to save your image with modified metadata

[data writeToFile:[NSTemporaryDirectory() stringByAppendingPathComponent:@"test.png"]   
       atomically:YES];

And than view it properties in Finder. You will see that kCGImagePropertyPNGDescription field was setted up successfully.

But if you will try read metadata of this new file, kCGImagePropertyPNGDescription will be lost.

ColorModel = RGB;
Depth = 8;
PixelHeight = 1136;
PixelWidth = 640;
"{PNG}" =     {
    InterlaceType = 0;
};

After some research I found that PNG doesn't contain metadata. But it may contain XMP metadata. However seems like ImageIO didn't work with XMP.
Maybe you can try to use ImageMagic or libexif.

Useful links:
PNG Specification
Reading/Writing image XMP on iPhone / Objective-c
Does PNG support metadata fields like Author, Camera Model, etc?
Does PNG contain EXIF data like JPG?
libexif.sourceforge.net