Add type hints to minimal publisher rclpy example

rclpy/topics/minimal_publisher/examples_rclpy_minimal_publisher/publisher_member_function.py

@@ -12,31 +12,35 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from typing import List, Optional
+
 import rclpy
 from rclpy.executors import ExternalShutdownException
 from rclpy.node import Node
+from rclpy.publisher import Publisher
+from rclpy.timer import Timer
 
 from std_msgs.msg import String
 
 
 class MinimalPublisher(Node):
 
-    def __init__(self):
+    def __init__(self) -> None:
         super().__init__('minimal_publisher')
-        self.publisher_ = self.create_publisher(String, 'topic', 10)
+        self.publisher_: Publisher[String] = self.create_publisher(String, 'topic', 10)
         timer_period = 0.5  # seconds
-        self.timer = self.create_timer(timer_period, self.timer_callback)
+        self.timer: Timer = self.create_timer(timer_period, self.timer_callback)
         self.i = 0
 
-    def timer_callback(self):
-        msg = String()
+    def timer_callback(self) -> None:
+        msg: String = String()
         msg.data = 'Hello World: %d' % self.i
         self.publisher_.publish(msg)
         self.get_logger().info('Publishing: "%s"' % msg.data)
         self.i += 1
 
 
-def main(args=None):
+def main(args: Optional[List[str]] = None) -> None:
     try:
         with rclpy.init(args=args):
             minimal_publisher = MinimalPublisher()