You need to implement a function that determines if a sequence of tasks can be completed based on given dependencies between them. Each task is represented by a unique integer ID. You must return a valid order of tasks if possible, or an empty list if not.

``` def task_order(tasks, dependencies): from collections import defaultdict, deque indegree = {task: 0 for task in tasks} graph = defaultdict(list) for a, b in dependencies: graph[a].append(b) indegree[b] += 1 queue = deque([task for task in tasks if indegree[task] == 0]) order = [] while queue: curr = queue.popleft() order.append(curr) for neighbor in graph[curr]: indegree[neighbor] -= 1 if indegree[neighbor] == 0: queue.append(neighbor) return order if len(order) == len(tasks) else [] ```