[SPARK-24014][PYSPARK] Add onStreamingStarted method to StreamingListener

## What changes were proposed in this pull request?

The `StreamingListener` in PySpark side seems to be lack of `onStreamingStarted` method. This patch adds it and a test for it.

This patch also includes a trivial doc improvement for `createDirectStream`.

Original PR is apache#21057.

## How was this patch tested?

Added test.

Author: Liang-Chi Hsieh <[email protected]>

Closes apache#21098 from viirya/SPARK-24014.

Author: viirya

python/pyspark/streaming/kafka.py

@@ -104,7 +104,8 @@ def createDirectStream(ssc, topics, kafkaParams, fromOffsets=None,
         :param topics:  list of topic_name to consume.
         :param kafkaParams: Additional params for Kafka.
         :param fromOffsets: Per-topic/partition Kafka offsets defining the (inclusive) starting
-                            point of the stream.
+                            point of the stream (a dictionary mapping `TopicAndPartition` to
+                            integers).
         :param keyDecoder:  A function used to decode key (default is utf8_decoder).
         :param valueDecoder:  A function used to decode value (default is utf8_decoder).
         :param messageHandler: A function used to convert KafkaMessageAndMetadata. You can assess

python/pyspark/streaming/listener.py

@@ -23,6 +23,12 @@ class StreamingListener(object):
     def __init__(self):
         pass
 
+    def onStreamingStarted(self, streamingStarted):
+        """
+        Called when the streaming has been started.
+        """
+        pass
+
     def onReceiverStarted(self, receiverStarted):
         """
         Called when a receiver has been started

python/pyspark/streaming/tests.py

@@ -507,6 +507,10 @@ def __init__(self):
             self.batchInfosCompleted = []
             self.batchInfosStarted = []
             self.batchInfosSubmitted = []
+            self.streamingStartedTime = []
+
+        def onStreamingStarted(self, streamingStarted):
+            self.streamingStartedTime.append(streamingStarted.time)
 
         def onBatchSubmitted(self, batchSubmitted):
             self.batchInfosSubmitted.append(batchSubmitted.batchInfo())
@@ -530,9 +534,12 @@ def func(dstream):
         batchInfosSubmitted = batch_collector.batchInfosSubmitted
         batchInfosStarted = batch_collector.batchInfosStarted
         batchInfosCompleted = batch_collector.batchInfosCompleted
+        streamingStartedTime = batch_collector.streamingStartedTime
 
         self.wait_for(batchInfosCompleted, 4)
 
+        self.assertEqual(len(streamingStartedTime), 1)
+
         self.assertGreaterEqual(len(batchInfosSubmitted), 4)
         for info in batchInfosSubmitted:
             self.assertGreaterEqual(info.batchTime().milliseconds(), 0)