Common Background Task(s) for APIRouter(s) #1576

from fastapi import APIRouter, BackgroundTasks, Depends, Request
from fastapi.routing import APIRoute


def get_router_tasks(request: Request) -> BackgroundTasks:
    return request.scope["router_bg"]


class RouterBackgroundRoute(APIRoute):
    def get_route_handler(self):
        original = super().get_route_handler()

        async def handler(request: Request):
            tasks = BackgroundTasks()
            request.scope["router_bg"] = tasks
            response = await original(request)
            existing = response.background
            response.background = tasks
            if isinstance(existing, BackgroundTasks):
                tasks.tasks.extend(existing.tasks)   # keep the endpoint's own tasks
            elif existing is not None:
                tasks.add_task(existing)
            return response

        return handler


router = APIRouter(route_class=RouterBackgroundRoute)


@router.get("/items")
async def list_items(bg: BackgroundTasks = Depends(get_router_tasks)):
    bg.add_task(clean_expired_cache)   # your router-wide cleanup
    return {"ok": True}


app.include_router(router)

from starlette.middleware.base import BaseHTTPMiddleware


class RouterTasksMiddleware(BaseHTTPMiddleware):
    async def dispatch(self, request, call_next):
        tasks = BackgroundTasks()
        request.scope["router_bg"] = tasks
        response = await call_next(request)
        existing = response.background
        response.background = tasks
        if isinstance(existing, BackgroundTasks):
            tasks.tasks.extend(existing.tasks)
        elif existing is not None:
            tasks.add_task(existing)
        return response