Device Collector Event-Loop Choice#

In a sync context, the ophyd-async DeviceCollector requires the bluesky event-loop to connect to devices. In an async context, it does not.

Sync Context#

In a sync context the run-engine must be initialized prior to connecting to devices. We enfore usage of the bluesky event-loop in this context.

The following will fail if RE = RunEngine() has not been called already:

with DeviceCollector():
    device1 = Device1(prefix)
    device2 = Device2(prefix)
    device3 = Device3(prefix)

The DeviceCollector connects to devices in the event-loop created in the run-engine.

Async Context#

In an async context device connection is decoupled from the run-engine. The following attempts connection to all the devices in the DeviceCollector before or after run-engine initialization.

async def connection_function() :
    async with DeviceCollector():
        device1 = Device1(prefix)
        device2 = Device2(prefix)
        device3 = Device3(prefix)

asyncio.run(connection_function())

The devices will be unable to be used in the run-engine unless they share the same event-loop. When the run-engine is initialised it will create a new background event-loop to use if one is not passed in with RunEngine(loop=loop).

If the user wants to use devices in the async DeviceCollector within the run-engine they can either:

Run the DeviceCollector first and pass the event-loop into the run-engine.
Initialize the run-engine first and run the DeviceCollector using the bluesky event-loop.