Implement some way to save files to real file system in a HTML5 version of a Godot game.

[Ivoch]

Godot version: 3

OS/device including version: HTML5

Issue description: There is currently no way to save/download files from within a HTML5 version of a Godot game.

Steps to reproduce: Create a small project with one Button and one FileDialog that saves an arbitrary text file. Works as expected in all exported versions, except HTML5. There is currently no way to save a file locally from HTML5, for example save games. FileDialog shows some virtual file system, probably in the browser sandbox mode. The file is only saved there and is inaccessible from the local OS or the server backend.

Minimal reproduction project:
htmlsave.zip

[vnen]

I think this a browser limitation, it doesn't let JavaScript or WebAssembly mess with the host filesystem.

[akien-mga]

CC @eska014

[leonkrause]

We can open a native file dialog. But if I add a function for that to OS, it'll have to be implemented on all platforms

[Geequlim]

@eska014 Here is already implemented for Window, Linux (GTK+), and macOS
https://github.com/GodotExplorer/NativeDialogs

[Calinou]

Also see this thread on the Godot subreddit in which I suggested @Ivoch to open an issue here.

[Zylann]

Can't we use local storage? That's still not accessing the filesystem in general, but it does save stuff to a place on the filesystem.

[leonkrause]

Persistent user:// filesystem already works, using IndexedDB. This issue is about saving data from Godot as a file to the real file system.

The problem is, do we want to maintain functionality to open a native file dialog, e.g. OS.native_file_save()/_open(), for all platforms? This practically seems to be needed only for the HTML5 platform. On the other hand, I want to keep the number of HTML5-specific APIs as low as possible

[Zylann]

Or at least, we could use interoperability so that HTML5-specific stuff that we can't put in Godot can be done with JS?

[leonkrause]

You can already execute JavaScript in the browser using JavaScript.eval()

[raymoo]

On Android it would mean not having to make FileDialog touchscreen-friendly

[37TS]

What about an encrypted CSV with a dev-selected private key as an option to fill in the export settings?
The key could then be stored somewhere in the "project_name.pck" resource file and the CSV could be part of the project or the dev could prompt the user to save the CSV in a read/write folder of choice for saving purposes.

(mere guess, I mean, that's what I was planning for my own html5 projects, either we get such a functionality or not)

[Calinou]

Can JavaScript even force the browser to open a native file dialog? I don't remember seeing this, unless the browser is configured to ask for download location every time.

We can however force a download using JavaScript, including from a Base64 data URI or similar. Just make sure not to initiate several downloads at once, or the browser will prompt the user before starting (Firefox does this at least). The file location is chosen by the browser, there's no way to override it.

Maybe this can be implemented using the JavaScript singleton, in which case we could delegate this to a third-party asset.

[Calinou]

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