This repository contains research into ways of providing web apps with means of exporting data as files in way which is user-friendly for users of iOS Safari.
iOS (as of version 12, version 13 might change things) doesn't offer a convenient way to manage general files. The Files app is available, but is not installed by default. Moreover, not all file types can be downloaded there and it's still very unwieldy to use as it lacks good file and folder management capabilities.
Safari will offer you a Save to Files option for some files in the Share button menu when navigated to them directly, but not all. It will not do it for the file types it can display (HTML, TXT, but it will do it for SVG even though it can display it) but will for unknown files (and SVG). This is okay but is not great for sharing or subsequent import to restore (need to navigate to the Files app.)
Another option is the Photos app. Throughtout my research I have verified that
the Photos app on iOS leaves PNG metadata intact and I have managed to store up
to 18 MB in a tEXt chunk of a PNG image that I was able to restore, too.
The theoretical maximum for the tEXt chunk size is 2 GB:
The text string may be of any length from zero bytes up to the maximum permissible chunk size less the length of the keyword and null character separator.
https://www.w3.org/TR/PNG#11tEXt
Length: A four-byte unsigned integer giving the number of bytes in the chunk's data field. The length counts only the data field, not itself, the chunk type, or the CRC. Zero is a valid length. Although encoders and decoders should treat the length as unsigned, its value shall not exceed 2^31-1 bytes.
https://www.w3.org/TR/PNG#5Chunk-layout
Right now I have a PNG generator which is capable of inserting a tEXt chunk
before the IEND chunk and infrastructure that allows the user to reupload
the downloaded image for checking if the contents of the chuck got disrupted or
not. Using binary search, the user is able to zero in on the approximate limit:
So far the largest payload I was able to generate and restore was 18 MB. 19 MB manages to generate but crashes during validation. 20 MB struggles to generate at all. This is not super consistent as I expect it depends on available memory, power saver etc. Safe value might be around 15 MB.
When the image is transfered using WhatsApp, it is converted to JPG so all PNG metadata is lost.
iOS 13 is out and it is now supported in Safari
Like the program name, or none/empty - will it have the same behavior or will it be treated differently by iOS?
Arbitrary downloads can be saved to and read from the Files app rendering this whole idea obsolete.