diff --git a/plugboard-schemas/plugboard_schemas/_validation.py b/plugboard-schemas/plugboard_schemas/_validation.py
index 98eb886c..e7b6bf93 100644
--- a/plugboard-schemas/plugboard_schemas/_validation.py
+++ b/plugboard-schemas/plugboard_schemas/_validation.py
@@ -18,6 +18,9 @@
 from ._validator_registry import validator
 
 
+_SYSTEM_STOP_EVENT = "system_stop"
+
+
 def _build_component_graph(
     connectors: dict[str, dict[str, _t.Any]],
 ) -> dict[str, set[str]]:
@@ -100,6 +103,9 @@ def validate_all_inputs_connected(
         all_inputs = set(io.get("inputs", []))
         connected = connected_inputs.get(comp_name, set())
         unconnected = all_inputs - connected
+        if unconnected:
+            event_covered_fields = set().union(*io.get("event_field_coverage", {}).values())
+            unconnected -= event_covered_fields
         if unconnected:
             errors.append(f"Component '{comp_name}' has unconnected inputs: {sorted(unconnected)}")
     return errors
diff --git a/plugboard/component/component.py b/plugboard/component/component.py
index 6fe0ad20..608689b1 100644
--- a/plugboard/component/component.py
+++ b/plugboard/component/component.py
@@ -95,6 +95,7 @@ def __init__(
             initial_values=self._initial_values,
             input_events=self.__class__.io.input_events,
             output_events=self.__class__.io.output_events,
+            event_field_coverage=self.__class__.io.event_field_coverage,
             namespace=self.name,
             component=self,
         )
@@ -356,7 +357,7 @@ async def _wrapper() -> None:
                         raise e
                 self._bind_outputs()
                 await self.io.write()
-                self._field_inputs_ready = False
+                self._reset_input_trackers()
                 await self._set_status(Status.WAITING, publish=not self._is_running)
 
         return _wrapper
@@ -365,6 +366,11 @@ async def _wrapper() -> None:
     def _has_field_inputs(self) -> bool:
         return len(self.io.inputs) > 0
 
+    @property
+    def _has_connected_field_inputs(self) -> bool:
+        """Whether any declared field inputs are connected via input channels."""
+        return self.io.has_connected_field_inputs
+
     @cached_property
     def _has_event_inputs(self) -> bool:
         input_events = set([evt.safe_type() for evt in self.io.input_events])
@@ -409,7 +415,7 @@ async def _io_read_with_status_check(self) -> None:
             task.cancel()
         for task in done:
             exc = task.exception()
-            if isinstance(exc, EventStreamClosedError) and len(self.io.inputs) == 0:
+            if isinstance(exc, EventStreamClosedError) and not self._has_connected_field_inputs:
                 await self.io.close()  # Call close for final wait and flush event buffer
             elif exc is not None:
                 raise exc
@@ -422,7 +428,7 @@ async def _periodic_status_check(self) -> None:
             # TODO : Eventually producer graph update will be event driven. For now,
             #      : the update is performed periodically, so it's called here along
             #      : with the status check.
-            if len(self.io.inputs) == 0:
+            if not self._has_connected_field_inputs:
                 await self._update_producer_graph()
 
     async def _status_check(self) -> None:
@@ -455,8 +461,11 @@ def _bind_inputs(self) -> None:
         for field in self.io.inputs:
             field_default = getattr(self, field, None)
             value = self._field_inputs.get(field, field_default)
-            setattr(self, field, value)
+            super().__setattr__(field, value)
+
+    def _reset_input_trackers(self) -> None:
         self._field_inputs = {}
+        self._field_inputs_ready = False
 
     def _bind_outputs(self) -> None:
         """Binds component fields to output fields."""
diff --git a/plugboard/component/io_controller.py b/plugboard/component/io_controller.py
index 7500aee2..870c49c6 100644
--- a/plugboard/component/io_controller.py
+++ b/plugboard/component/io_controller.py
@@ -38,6 +38,7 @@ def __init__(
         initial_values: _t.Optional[dict[str, _t.Iterable]] = None,
         input_events: _t.Optional[list[_t.Type[Event]]] = None,
         output_events: _t.Optional[list[_t.Type[Event]]] = None,
+        event_field_coverage: _t.Optional[dict[str, list[str]]] = None,
         namespace: str = IO_NS_UNSET,
         component: _t.Optional[Component] = None,
     ) -> None:
@@ -47,6 +48,7 @@ def __init__(
         self.initial_values = initial_values or {}
         self.input_events = input_events or []
         self.output_events = output_events or []
+        self.event_field_coverage = event_field_coverage or {}
         if set(self.initial_values.keys()) - set(self.inputs):
             raise ValueError("Initial values must be for input fields only.")
         self._component = component
@@ -86,8 +88,9 @@ def is_closed(self) -> bool:
         """Returns `True` if the `IOController` is closed, `False` otherwise."""
         return self._is_closed
 
-    @cached_property
-    def _has_field_inputs(self) -> bool:
+    @property
+    def has_connected_field_inputs(self) -> bool:
+        """Returns whether any field inputs are connected via channels."""
         return len(self._input_channels) > 0
 
     @cached_property
@@ -96,7 +99,7 @@ def _has_event_inputs(self) -> bool:
 
     @cached_property
     def _has_inputs(self) -> bool:
-        return self._has_field_inputs or self._has_event_inputs
+        return self.has_connected_field_inputs or self._has_event_inputs
 
     async def read(self, timeout: float | None = None) -> None:
         """Reads data and/or events from input channels.
@@ -139,7 +142,7 @@ async def read(self, timeout: float | None = None) -> None:
 
     def _set_read_tasks(self) -> list[asyncio.Task]:
         read_tasks: list[asyncio.Task] = []
-        if self._has_field_inputs:
+        if self.has_connected_field_inputs:
             if _fields_read_task not in self._read_tasks:
                 read_fields_task = asyncio.create_task(self._read_fields(), name=_fields_read_task)
                 self._read_tasks[_fields_read_task] = read_fields_task
@@ -374,7 +377,7 @@ def _add_channel_for_event(
 
     def _create_input_field_group_tasks(self) -> None:
         """Groups input field channels by field name and launches read tasks for group inputs."""
-        if not self._has_field_inputs:
+        if not self.has_connected_field_inputs:
             return
         field_channels: dict[str, list[tuple[_t_field_key, Channel]]] = defaultdict(list)
         for key, chan in self._input_channels.items():
@@ -410,6 +413,7 @@ def dict(self) -> dict[str, _t.Any]:  # noqa: D102
             "input_events": [e.safe_type() for e in self.input_events],
             "output_events": [e.safe_type() for e in self.output_events],
             "initial_values": {k: list(v) for k, v in self._initial_values.items()},
+            "event_field_coverage": {k: list(v) for k, v in self.event_field_coverage.items()},
         }
 
 
diff --git a/plugboard/events/event.py b/plugboard/events/event.py
index 2becb56d..858e26a5 100644
--- a/plugboard/events/event.py
+++ b/plugboard/events/event.py
@@ -75,9 +75,28 @@ def safe_type(cls, event_type: _t.Optional[str] = None) -> str:
         """Returns a safe event type string for use in broker topic strings."""
         return (event_type or cls.type).replace(".", "_").replace("-", "_")
 
+    @_t.overload
     @classmethod
-    def handler(cls, method: AsyncCallable) -> AsyncCallable:
+    def handler(cls, method: AsyncCallable) -> AsyncCallable: ...
+
+    @_t.overload
+    @classmethod
+    def handler(
+        cls, *, populates_fields: _t.Optional[list[str]] = None
+    ) -> _t.Callable[[AsyncCallable], AsyncCallable]: ...
+
+    @classmethod
+    def handler(
+        cls,
+        method: _t.Optional[AsyncCallable] = None,
+        *,
+        populates_fields: _t.Optional[list[str]] = None,
+    ) -> _t.Union[AsyncCallable, _t.Callable[[AsyncCallable], AsyncCallable]]:
         """Registers a class method as an event handler."""
+        if method is None:
+            # Invoked as @Event.handler(populates_fields=[...])
+            return EventHandlers.add(cls, populates_fields=populates_fields)
+        # Invoked as @Event.handler
         return EventHandlers.add(cls)(method)
 
 
diff --git a/plugboard/events/event_handlers.py b/plugboard/events/event_handlers.py
index 344522ce..3eb8223d 100644
--- a/plugboard/events/event_handlers.py
+++ b/plugboard/events/event_handlers.py
@@ -18,11 +18,16 @@ class EventHandlers:  # pragma: no cover
     _handlers: _t.ClassVar[dict[str, dict[str, AsyncCallable]]] = defaultdict(dict)
 
     @classmethod
-    def add(cls, event: _t.Type[Event] | Event) -> _t.Callable[[AsyncCallable], AsyncCallable]:
+    def add(
+        cls,
+        event: _t.Type[Event] | Event,
+        populates_fields: _t.Optional[list[str]] = None,
+    ) -> _t.Callable[[AsyncCallable], AsyncCallable]:
         """Decorator that registers class methods as handlers for specific event types.
 
         Args:
             event: Event class this handler processes
+            populates_fields: Optional list of fields that the handler populates
 
         Returns:
             Callable: Decorated method
@@ -31,6 +36,13 @@ def add(cls, event: _t.Type[Event] | Event) -> _t.Callable[[AsyncCallable], Asyn
         def decorator(method: AsyncCallable) -> AsyncCallable:
             class_path = cls._get_class_path_for_method(method)
             cls._handlers[class_path][event.type] = method
+            if populates_fields is not None:
+                comp_cls = method.__self__.__class__
+                if not hasattr(comp_cls, "io"):
+                    raise ValueError(
+                        "populates_fields must be specified on method of Component subclass."
+                    )
+                comp_cls.io.event_field_coverage[event.type] = populates_fields
             return method
 
         return decorator
@@ -43,6 +55,12 @@ def _get_class_path_for_method(method: AsyncCallable) -> str:
         class_name = qualname_parts[-2]  # Last part is the method name
         return f"{module_name}.{class_name}"
 
+    @staticmethod
+    def _iter_mro(_class: _t.Type) -> _t.Iterator[str]:
+        """Iterate over class MRO, yielding fully qualified class paths."""
+        for base_class in _class.__mro__:
+            yield f"{base_class.__module__}.{base_class.__name__}"
+
     @classmethod
     def get(cls, _class: _t.Type, event: _t.Type[Event] | Event) -> AsyncCallable:
         """Retrieve a handler for a specific class and event type.
@@ -57,10 +75,11 @@ def get(cls, _class: _t.Type, event: _t.Type[Event] | Event) -> AsyncCallable:
         Raises:
             KeyError: If no handler found for class or event type
         """
+        store = cls._handlers
         for base_class in _class.__mro__:
             base_path = f"{base_class.__module__}.{base_class.__name__}"
-            if base_path in cls._handlers and event.type in cls._handlers[base_path]:
-                return cls._handlers[base_path][event.type]
+            if base_path in store and event.type in store[base_path]:
+                return store[base_path][event.type]
         raise KeyError(
             f"No handler found for class '{_class.__name__}' and event type '{event.type}'"
         )
diff --git a/plugboard/library/data_writer.py b/plugboard/library/data_writer.py
index 96d14538..f281a531 100644
--- a/plugboard/library/data_writer.py
+++ b/plugboard/library/data_writer.py
@@ -43,6 +43,7 @@ def __init__(
             **kwargs: Additional keyword arguments for [`Component`][plugboard.component.Component].
         """
         super().__init__(**kwargs)
+        # Use a single buffer to track everything
         self._buffer: dict[str, deque] = defaultdict(deque)
         self._chunk_size = chunk_size
         self.io = IOController(
@@ -76,18 +77,39 @@ async def _convert(self, data: dict[str, deque]) -> _t.Any:
     def _bind_inputs(self) -> None:
         """Binds input fields to component fields and append to internal buffer."""
         super()._bind_inputs()
-        for field in self.io.inputs:
+        for field in self._field_inputs:
             value = getattr(self, field, None)
             self._buffer[field].append(value)
 
+    @property
+    def _completed_rows(self) -> int:
+        """Calculates how many fully formed rows exist in the buffer."""
+        if not self.io.inputs:
+            return 0
+        return min((len(self._buffer[f]) for f in self.io.inputs), default=0)
+
+    @property
+    def _can_step(self) -> bool:
+        """We can step if we have at least one fully formed row."""
+        return self._completed_rows > 0
+
     async def _save_chunk(self) -> None:
-        """Write data from the buffer."""
+        """Write completed data rows from the buffer."""
+        completed_rows = self._completed_rows
+        if completed_rows == 0:
+            return
+
         if self._task is not None:
             await self._task
-        # Create task to save next chunk of data
-        chunk = await self._convert(self._buffer)
+
+        # Extract only the completed rows into a new chunk
+        chunk_data = {
+            field: deque([self._buffer[field].popleft() for _ in range(completed_rows)])
+            for field in self.io.inputs
+        }
+
+        chunk = await self._convert(chunk_data)
         self._task = asyncio.create_task(self._save(chunk))
-        self._buffer = defaultdict(deque)
 
     async def step(self) -> None:
         """Trigger save when buffer is at target size."""
diff --git a/tests/integration/test_process_with_components_run.py b/tests/integration/test_process_with_components_run.py
index fe047ae8..2a16c41f 100644
--- a/tests/integration/test_process_with_components_run.py
+++ b/tests/integration/test_process_with_components_run.py
@@ -23,6 +23,7 @@
 )
 from plugboard.events import Event
 from plugboard.exceptions import ConstraintError, NotInitialisedError, ProcessStatusError
+from plugboard.library import FileWriter
 from plugboard.process import LocalProcess, Process, RayProcess
 from plugboard.schemas import ConnectorSpec, Status
 from tests.conftest import ComponentTestHelper, zmq_connector_cls
@@ -459,6 +460,99 @@ async def test_event_driven_process_shutdown(
     await process.destroy()
 
 
+class MessageEventData(BaseModel):
+    """Data for a message event."""
+
+    message: str
+
+
+class MessageEvent(Event):
+    """Event carrying a file-writer message."""
+
+    type: _t.ClassVar[str] = "message_event"
+    data: MessageEventData
+
+
+class MessageEventGenerator(ComponentTestHelper):
+    """Produces a fixed number of message events."""
+
+    io = IO(output_events=[MessageEvent])
+
+    def __init__(
+        self,
+        iters: int,
+        *args: _t.Any,
+        delay: float = 0.0,
+        start: int = 0,
+        stride: int = 1,
+        **kwargs: _t.Any,
+    ) -> None:
+        super().__init__(*args, **kwargs)
+        self._iters = iters
+        self._delay = delay
+        self._start = start
+        self._stride = stride
+
+    async def init(self) -> None:
+        await super().init()
+        self._seq = iter(range(self._start, self._start + self._iters * self._stride, self._stride))
+
+    async def step(self) -> None:
+        # Optional delay to simulate staggered event arrival
+        if self._delay > 0.0:
+            await asyncio.sleep(self._delay)
+        try:
+            idx = next(self._seq)
+        except StopIteration:
+            await self.io.close()
+        else:
+            evt = MessageEvent(
+                source=self.name,
+                data=MessageEventData(message=f"Message {idx}"),
+            )
+            self.io.queue_event(evt)
+            await super().step()
+
+
+class EventReaderFileWriter(FileWriter):
+    """`FileWriter` variant that adds event handling instead of a connector for `message`."""
+
+    io = IO(input_events=[MessageEvent])
+
+    @MessageEvent.handler
+    async def handle_message(self, event: MessageEvent) -> None:
+        self.message = event.data.message
+
+
+@pytest.mark.asyncio
+async def test_event_driven_file_writer_reuse(tmp_path: Path) -> None:
+    """Test that field-input components can be reused in event-driven processes."""
+    output_path = tmp_path / "output_messages.csv"
+    components = [
+        MessageEventGenerator(iters=3, name="message_event_generator"),
+        EventReaderFileWriter(
+            path=output_path,
+            name="event_reader_file_writer",
+            field_names=["message"],
+        ),
+    ]
+    event_connectors = AsyncioConnector.builder().build_event_connectors(components)
+    process = LocalProcess(components=components, connectors=event_connectors)
+
+    await process.init()
+    await process.run()
+
+    assert process.status == Status.COMPLETED
+    assert output_path.read_text().splitlines() == [
+        "message",
+        "Message 0",
+        "Message 1",
+        "Message 2",
+    ]
+
+    await process.destroy()
+
+
 _SHORT_TIMEOUT = 0.1
 
 
@@ -536,3 +630,88 @@ async def test_constraint_error_stops_background_status_check() -> None:
         )
 
         await process.destroy()
+
+
+class StaggeredEventFileWriter(FileWriter):
+    """`FileWriter` variant that adds event handling instead of a connector for `message`."""
+
+    io = IO(input_events=[MessageEvent])
+
+    def __init__(self, *args: _t.Any, field_names: list[str], **kwargs: _t.Any) -> None:
+        super().__init__(*args, field_names=field_names, **kwargs)
+        self.step_count: int = 0
+        self.step_for_message: dict[str, int] = {}
+
+    @MessageEvent.handler
+    async def handle_message(self, event: MessageEvent) -> None:
+        msg = event.data.message
+        match event.source:
+            case "mg1":
+                self.mg1 = msg
+            case "mg2":
+                self.mg2 = msg
+            case "mg3":
+                self.mg3 = msg
+            case _:
+                raise ValueError(f"Unexpected event source: {event.source}")
+        self.step_for_message[msg] = self.step_count
+        self.step_count += 1
+
+
+@pytest.mark.asyncio
+@pytest_cases.parametrize(
+    "process_cls, connector_cls",
+    [
+        (LocalProcess, AsyncioConnector),
+    ],
+)
+async def test_data_writer_handles_staggered_input_events(
+    process_cls: type[Process], connector_cls: type[Connector], tmp_path: Path, ray_ctx: None
+) -> None:
+    """Test that a FileWriter can handle input events arriving in different steps.
+
+    Input messages with data for different fields may arrive in different steps. The FileWriter
+    should write out a new row only when all required fields have received data, and should not
+    overwrite field values if only a subset of fields receive new data in a step.
+    """
+    output_path = tmp_path / "staggered_output_messages.csv"
+
+    writer = StaggeredEventFileWriter(
+        path=output_path, field_names=["mg1", "mg2", "mg3"], name="writer"
+    )
+    components = [
+        # 3 inputs with different delays
+        MessageEventGenerator(iters=10, delay=0.005, start=0, stride=3, name="mg1"),
+        MessageEventGenerator(iters=10, delay=0.010, start=1, stride=3, name="mg2"),
+        MessageEventGenerator(iters=10, delay=0.020, start=2, stride=3, name="mg3"),
+        writer,
+    ]
+
+    async with process_cls(
+        components=components,
+        connectors=AsyncioConnector.builder().build_event_connectors(components),
+    ) as process:
+        await process.run()
+
+    with output_path.open() as f:
+        content = f.read().splitlines()
+
+    assert len(content) == 11  # header + 10 rows of data
+    assert content[0] == "mg1,mg2,mg3"
+    assert content[1] == "Message 0,Message 1,Message 2"
+    assert content[2] == "Message 3,Message 4,Message 5"
+    assert content[3] == "Message 6,Message 7,Message 8"
+    assert content[4] == "Message 9,Message 10,Message 11"
+    assert content[5] == "Message 12,Message 13,Message 14"
+    assert content[6] == "Message 15,Message 16,Message 17"
+    assert content[7] == "Message 18,Message 19,Message 20"
+    assert content[8] == "Message 21,Message 22,Message 23"
+    assert content[9] == "Message 24,Message 25,Message 26"
+    assert content[10] == "Message 27,Message 28,Message 29"
+
+    # Verify that messages from different generators were received in different steps
+    assert writer.step_count == 30
+    assert len(writer.step_for_message) == 30
+    assert len(set(writer.step_for_message.values())) == 30, (
+        "Expected each message to be received in a different step"
+    )
diff --git a/tests/unit/test_process_validation.py b/tests/unit/test_process_validation.py
index 02e0a4d2..df20132e 100644
--- a/tests/unit/test_process_validation.py
+++ b/tests/unit/test_process_validation.py
@@ -95,6 +95,7 @@ def _make_component(
     outputs: list[str] | None = None,
     input_events: list[str] | None = None,
     output_events: list[str] | None = None,
+    event_field_coverage: dict[str, list[str]] | None = None,
     initial_values: dict[str, _t.Any] | None = None,
 ) -> dict[str, _t.Any]:
     """Build a component dict matching process.dict() format."""
@@ -108,6 +109,7 @@ def _make_component(
             "outputs": outputs or [],
             "input_events": input_events or [],
             "output_events": output_events or [],
+            "event_field_coverage": event_field_coverage or {},
             "initial_values": initial_values or {},
         },
     }
@@ -303,6 +305,22 @@ def test_no_inputs_no_errors(self) -> None:
         errors = validate_all_inputs_connected(pd)
         assert errors == []
 
+    def test_event_covered_fields(self) -> None:
+        """Unconnected inputs are allowed when non-system input events can populate them."""
+        pd = _make_process_dict(
+            components={
+                "producer": _make_component("producer", output_events=["message_event"]),
+                "writer": _make_component(
+                    "writer",
+                    inputs=["message"],
+                    input_events=["system_stop", "message_event"],
+                    event_field_coverage={"message_event": ["message"]},
+                ),
+            },
+        )
+        errors = validate_all_inputs_connected(pd)
+        assert errors == []
+
 
 # ---------------------------------------------------------------------------
 # Tests for validate_input_events