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update DESIGN.md

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Michael Hudson-Doyle 2019-12-03 13:12:39 +13:00
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DESIGN.md
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@ -146,22 +146,23 @@ controllers don't have a corresponding model class.
### Doing things in the background ### Doing things in the background
If the UI does not block, as promised above, then there needs to be a way of If the UI does not block, as promised above, then there needs to be a way of
running things in the background. The main way this is done is the `run_in_bg` running things in the background. Subiquity uses a few different ways to do
function, which takes two functions: one that takes no arguments and is called this but new code should use
in a background thread and a callback that takes one argument, and is called in [asyncio](https://docs.python.org/3/library/asyncio.html). `subiquity.async_helpers`
the main/UI thread with a `concurrent.futures.Future` representing the result defines two useful helper functions `run_in_thread` (just a nicer wrapper
of calling the first function. This is a fairly clunky interface that can lead around `run_in_executor`) and `schedule_task` (a wrapper around `create_task`
to some slightly baffling control flow but, well, it works. I've tried a few that works before the event loop is
abstractions that make things clearer -- `subiquity.tasksequence.TaskSequence` started). [trio](https://trio.readthedocs.io/en/stable/) has nicer APIs but is
being the older one that doesn't quite cover all interesting cases and a bit too new for now.
`subiquity.controllers.installprogress.StateMachine` being the newer one that I
hope can be generalized to something useful in many places.
Currently subiquity just uses urwid's default event loop (`SelectEventLoop`). The older approach which is still present in the codebase is the `run_in_bg`
Switching to `AsyncioEventLoop` would probably be an improvement and might let function, which takes two functions: one that takes no arguments and is called
us not use a background thread for absolutely every action we want to run in in a background thread and a callback that takes one argument, and is called
the background (there's no real reason to tie up a thread waiting for a in the main/UI thread with a `concurrent.futures.Future` representing the
subprocess to exit, for example). result of calling the first function. I tried a few abstractions to try to
make things clearer -- `subiquity.tasksequence.TaskSequence` and
`subiquity.controllers.installprogress.StateMachine` being two -- but they
didn't really help all that much in the end.
A cast-iron rule: Only touch the UI from the main thread. A cast-iron rule: Only touch the UI from the main thread.