Help appreciated: shared projects and file format


I am a student who would like to use PXT to let users program an industrial robot. There is additional (non-web) software running on the server that can parse and execute instructions. Currently I am trying to get familiar with but cannot solve the following issues:

  1. How can load/store PXT projects server-sided (shared projects)? I found a “cloud” option in pxttarget.json but no helpful information on what it actually does. What would be needed to implement such cloud?
  2. The pxt-sample project saves .mkcd files to the client disk. However, the file magic does not match the UF2 specification. How can parse this format, or replace it with my own?
  1. Share links are not supported for static-pkgs, as they are part of the backend server. Our backend implementation is unfortunately not public so you’d have to implement a version of your own to support this; that is, implementing a api/user/project/share post endpoint which stores the project and gives back the proejct id, as was as an endpoint to display the shared project.
  2. the .mkcd file it’s giving you is an LZMA compressed json blob of the file; that is, something like
    "main.blocks": "<block stuff goes here",
    "main.ts": "function myCode() {...}",
    "pxt.json": "{...}"

we can decompress it in the webapp with pxt.lzmaDecompressAsync, or you can find another format as needed. That save button goes through this path (assuming you don’t turn on the flag for saving as a png, which would be harder to match the parsing for):

Note that this is the ‘file save’ portion rather than the hardware support, since you mention targeting a device / uf2 output you might want to take a look at pxt-maker here to see a bit on how it’s done as there’s a bit more involved than I can describe without more context. the supported variants section in pxtarget.json might be a good place to start, as that describes the different chips supported – e.g. here’s one of the prs that added rp2040 in

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Thank you for this incredible fast answer!

Yesterday I spent a few hours trying to figure out how this share feature works in order to implement the required API/endpoint. Unfortunately I am a novice when it comes to web development, which might be the reason why I am somewhat overwhelmed by the complexity of PXT.

It is however good to know that the mkcd file could be parsed after downloading to e.g. a shared drive, as an alternative to simply exchange project files between the dedicated robot control logic and the PXT GUI. I do not need UF2 output - it just happens to be the only documentation about saved files that I could find. As long as I can somehow convert my blocks into custom code (and back) it’s fine.

In the meantime I also experimented with Blockly which does not look as good but has a great documentation and is noticeably faster to compile/run. Since there’s less API to get familiar with, I might get to my goals quicker by writing my own code than to understand and modify the PXT structure.

By using Blockly I quickly reached some progress without digging through the source code too often. If that approach turns out to be too limited for my use-case, I might return with follow-up questions but as for now it seems to be “good enough” for my requirements.