Computed properties

[rwb27]

This is the start of computed properties. Computed properties are like functional properties, but they keep track of attribute access. This should then allow us to update them whenever their inputs change.

Currently, an error is thrown if anything other than an observable property is accessed.

Once this is better-tested, we need logic to:

• Call the computed property at the right time, to initialise its dependencies.
• Automatically recompute it when those dependencies change.
• Clean everything up so the server shuts down nicely.

My current thinking is that we'll use the MessageBroker to subscribe to changes in our dependencies. That way, there's no need to add any more notification logic.

Using the message broker will require us to have a coroutine running that receives from a stream and recomputes the property. At the moment I think it's probably best to recompute the property in a worker thread, because it will cause confusion to run Thing code in the event loop. That might be a higher-performance alternative we can introduce in the future.

There are two concurrency challenges here:

• if a dependency changes while the property is being recomputed, we might miss it. That's hard to fix without blocking the event loop (or at least blocking notifications of events), which we don't want to do.
• we would need to signal the coroutine responsible for triggering recomputation to shut down when the server stops. Currently, that's tricky. This could use the same solution as the MJPEG stream in OpenFlexure. Might be a good time to finally merge that in to this repo.

[barecheck]

Barecheck - Code coverage report

Total: 97.42%

Your code coverage diff: -0.01% ▾

Uncovered files and lines

File	Lines
src/labthings_fastapi/computed_properties.py	53