Allow standalone system simulation component

examples/configs/standalone-system-sim-counter.yaml

@@ -0,0 +1,11 @@
+- tickit.core.components.system_simulation.SystemSimulation:
+    name: internal_tickit
+    inputs: {}
+    components:
+      - examples.devices.counter.Counter:
+          name: counter  
+          inputs: {}
+      - tickit.devices.sink.Sink:
+          name: counter_sink
+          inputs:
+            input: counter:value

examples/configs/standalone-system-sim.yaml

@@ -0,0 +1,12 @@
+- tickit.core.components.system_simulation.SystemSimulation:
+    name: internal_tickit
+    inputs: {}
+    components:
+      - tickit.devices.source.Source:
+          name: internal_source
+          inputs: {}
+          value: 42
+      - tickit.devices.sink.Sink:
+          name: internal_sink
+          inputs:
+            input: internal_source:value

examples/configs/test.yaml

@@ -0,0 +1,8 @@
+- tickit.devices.source.Source:
+    name: internal_source
+    inputs: {}
+    value: 42
+- tickit.devices.sink.Sink:
+    name: internal_sink
+    inputs:
+      input: internal_source:value

src/tickit/core/components/system_simulation.py

@@ -102,10 +102,8 @@ async def stop_component(self) -> None:
 class SystemSimulation(ComponentConfig):
     """Simulation of a nested set of components."""
 
-    name: ComponentID
-    inputs: Dict[PortID, ComponentPort]
     components: List[ComponentConfig]
-    expose: Dict[PortID, ComponentPort]
+    expose: Dict[PortID, ComponentPort] = field(default_factory=dict)
 
     def __call__(self) -> Component:  # noqa: D102
         return SystemSimulationComponent(

src/tickit/core/management/schedulers/slave.py

@@ -1,3 +1,4 @@
+import asyncio
 import logging
 from typing import Awaitable, Callable, Dict, Optional, Set, Tuple, Type, Union
 
@@ -48,10 +49,22 @@ def __init__(
         wiring = self.add_exposing_wiring(wiring, expose)
         super().__init__(wiring, state_consumer, state_producer)
 
+        reads_from_external: bool = False
+        for component in self._wiring:
+            input_components = [
+                p.component for p in self._wiring[component].values()  # type: ignore
+            ]
+            if "external" in input_components:
+                reads_from_external = True
+                break
+
+        self.reads_from_external = reads_from_external
         self.raise_interrupt = raise_interrupt
         self.interrupts: Set[ComponentID] = set()
         self.component_error: ComponentException
 
+        self.first_tick: asyncio.Event = asyncio.Event()
+
     @staticmethod
     def add_exposing_wiring(
         wiring: Union[Wiring, InverseWiring],
@@ -127,17 +140,20 @@ async def on_tick(
         wakeup_components = {
             component for component, when in self.wakeups.items() if when <= time
         }
-        root_components: Set[ComponentID] = {
-            *self.interrupts,
-            *wakeup_components,
-            ComponentID("external"),
-        }
+
+        root_components: Set[ComponentID] = {*self.interrupts, *wakeup_components}
+
+        if self.reads_from_external:
+            root_components.update([ComponentID("external")])
+
         for component in wakeup_components:
             del self.wakeups[component]
         self.interrupts.clear()
 
         self.input_changes = changes
         self.output_changes = Changes(Map())
+
+        await asyncio.wait_for(self.first_tick.wait(), timeout=30)
         await self.ticker(time, root_components)
 
         _, call_at = self.get_first_wakeups()
@@ -146,6 +162,11 @@ async def on_tick(
     async def run_forever(self) -> None:
         """Delegates to setup which instantiates the ticker and state interfaces."""
         await self.setup()
+        await self.ticker(
+            SimTime(0),
+            self.ticker.components,
+        )
+        self.first_tick.set()
 
     async def schedule_interrupt(self, source: ComponentID) -> None:
         """Schedules the interrupt of a component immediately.

tests/core/management/schedulers/test_slave_scheduler.py

@@ -179,6 +179,8 @@ async def test_slave_scheduler_run_forever_method(slave_scheduler: SlaveSchedule
 async def test_slave_scheduler_on_tick_method(
     slave_scheduler: SlaveScheduler, mock_ticker: Mock
 ):
+    await slave_scheduler.run_forever()
+
     changes = Changes(Map({PortID("67"): 67}))
     slave_scheduler.ticker = mock_ticker
     output_changes, call_at = await slave_scheduler.on_tick(SimTime(8), changes)
@@ -190,6 +192,7 @@ async def test_slave_scheduler_on_tick_method(
 async def test_slave_scheduler_on_tick_method_with_wakeups(
     slave_scheduler: SlaveScheduler, mock_ticker: Mock
 ):
+    await slave_scheduler.run_forever()
     changes = Changes(Map({PortID("67"): 67}))
     slave_scheduler.ticker = mock_ticker
     slave_scheduler.wakeups = {