首先，需要使用Python中的os模块和AirFlow的BaseSensorOperator类来实现监视文件夹的任务。下面是代码示例：

import os
from airflow.sensors.base_sensor_operator import BaseSensorOperator
from airflow.utils.decorators import apply_defaults

class FolderSensor(BaseSensorOperator):
    @apply_defaults
    def __init__(
        self,
        folder_path,
        *args,
        **kwargs
    ):
        super(FolderSensor, self).__init__(*args, **kwargs)
        self.folder_path = folder_path

    def poke(self, context):
        self.log.info(f'Poking for file in folder {self.folder_path}')
        return any(
            f.startswith('input_')
            for f in os.listdir(self.folder_path)
        )

在上面的代码中，每次poke（轮询）任务运行时，文件夹中所有以“input_”开头的文件将被检查。如果输入文件被检测到，则任务将成功并继续下一步操作。

接下来，需要创建一个DAG，并使用上述类中定义的传感器operator来监测文件夹的输入文件。下面是一个示例DAG的代码：

from airflow import DAG
from airflow.operators.dummy_operator import DummyOperator
from datetime import datetime, timedelta
from airflow.operators.python_operator import PythonOperator

dag = DAG('folder_sensor_example', description='Example folder sensor DAG',
          schedule_interval=timedelta(days=1),
          start_date=datetime(2021, 1, 1),
          catchup=False)

start_task = DummyOperator(task_id='start_task', dag=dag)

folder_sensor_task = FolderSensor(
    task_id='folder_sensor_task',
    folder_path='/path/to/folder',
    dag=dag
)

next_task = PythonOperator(
    task_id='next_task',
    python_callable=lambda: print('Files detected!'),
    dag=dag
)

start_task >> folder_sensor_task >> next_task

在上述示例中，首先创建了一个运行间隔为1天