Allow passing local file path to ui.download

[rodja]

This PR enhances the ui.download element to also work with local file paths (like ui.image etc)

[falkoschindler]

I reviewed the PR:

• For consistency I chose to use helpers.is_file for checking if src is a local file.
• I moved the definition of DOWNLOAD_DIR out of test_helpers.py, because this file isn't meant to be a place for "test helpers", but rather a file testing the helpers.py file.
• I don't approve the PR, yet, because there is a potential memory leak: For every download there is a new route added to the FastAPI app. Now there are two cases:
  1. There are different file, e.g. captured images. Every download of the current image creates a new route which is never cleaned up, even if the image is eventually removed from memory.
  2. There is some button like a "Download the datasheet of our great product" triggering the download of a local PDF. Each client clicking the button will create another route with the same path. For a well visited website this might eventually be a significant leak.
     So I wonder if there's an alternative to avoid accumulating routes.

[rodja]

You are right. ui.download is not a normal element. We need to think a bit more about how we approach this.

[rodja]

Maybe we can create a unique route if the download is executed and remove it after the file has been served. Kind of a one-time-download-route. We do something similar in https://github.com/zauberzeug/nicegui/blob/main/examples/download_text_as_file/main.py.

[falkoschindler]

@rodja I implemented such single-use download routes. Have a look. 🙂

Test code:

ui.button('Download', on_click=lambda: ui.download('website/static/logo.png'))

l = ui.label()
ui.timer(1, lambda: l.set_text(len(app.routes)))

[rodja]

Super cool :-)