Introduce a singleton decorator (#34803)

homeassistant/helpers/singleton.py

@@ -0,0 +1,44 @@
+"""Helper to help coordinating calls."""
+import asyncio
+import functools
+from typing import Awaitable, Callable, TypeVar, cast
+
+from homeassistant.core import HomeAssistant
+
+T = TypeVar("T")
+
+FUNC = Callable[[HomeAssistant], Awaitable[T]]
+
+
+def singleton(data_key: str) -> Callable[[FUNC], FUNC]:
+    """Decorate a function that should be called once per instance.
+
+    Result will be cached and simultaneous calls will be handled.
+    """
+
+    def wrapper(func: FUNC) -> FUNC:
+        """Wrap a function with caching logic."""
+
+        @functools.wraps(func)
+        async def wrapped(hass: HomeAssistant) -> T:
+            obj_or_evt = hass.data.get(data_key)
+
+            if not obj_or_evt:
+                evt = hass.data[data_key] = asyncio.Event()
+
+                result = await func(hass)
+
+                hass.data[data_key] = result
+                evt.set()
+                return cast(T, result)
+
+            if isinstance(obj_or_evt, asyncio.Event):
+                evt = obj_or_evt
+                await evt.wait()
+                return cast(T, hass.data.get(data_key))
+
+            return cast(T, obj_or_evt)
+
+        return wrapped
+
+    return wrapper