From 88f9878d0c2c9df83bd77e7c4ad152f9c80c2339 Mon Sep 17 00:00:00 2001
From: Jon Mease <jon.mease.dev@gmail.com>
Date: Sat, 15 Aug 2020 10:31:01 -0400
Subject: [PATCH] Add History stream and tests

---
 holoviews/streams.py           | 35 ++++++++++++++++++
 holoviews/tests/teststreams.py | 66 ++++++++++++++++++++++++++++++++++
 2 files changed, 101 insertions(+)

diff --git a/holoviews/streams.py b/holoviews/streams.py
index 3419c3c06a..d612165470 100644
--- a/holoviews/streams.py
+++ b/holoviews/streams.py
@@ -855,6 +855,41 @@ def __del__(self):
         self._unregister_input_streams()
 
 
+class History(Stream):
+    """
+    A Stream that maintains a history of the values of a single input stream
+    """
+    values = param.List(constant=True, doc="""
+        List containing the historical values of the input stream""")
+
+    def __init__(self, input_stream, **params):
+        super(History, self).__init__(**params)
+        self.input_stream = input_stream
+        self._register_input_stream()
+        # Trigger event on input stream after registering so that current value is
+        # added to our values list
+        self.input_stream.event()
+
+    def clear_history(self):
+        self.values.clear()
+        self.event()
+
+    def _register_input_stream(self):
+        """
+        Register callback on input_stream to watch for changes
+        """
+        def perform_update(**kwargs):
+            self.values.append(kwargs)
+            self.event()
+
+        self.input_stream.add_subscriber(perform_update)
+
+    def __del__(self):
+        self.input_stream.source = None
+        self.input_stream.clear()
+        self.values.clear()
+
+
 class SelectionExpr(Derived):
     selection_expr = param.Parameter(default=None, constant=True)
 
diff --git a/holoviews/tests/teststreams.py b/holoviews/tests/teststreams.py
index 9295efddcf..97a2b7201f 100644
--- a/holoviews/tests/teststreams.py
+++ b/holoviews/tests/teststreams.py
@@ -974,6 +974,72 @@ def test_exclusive_derived_stream(self):
         self.assertEqual(s0.v, -8.0)
 
 
+class TestHistoryStream(ComparisonTestCase):
+    def test_initial_history_stream_values(self):
+        # Check values list is initialized with initial contents of input stream
+        val = Val(v=1.0)
+        history = History(val)
+        self.assertEqual(history.contents, {"values": [val.contents]})
+
+    def test_history_stream_values_appended(self):
+        val = Val(v=1.0)
+        history = History(val)
+        # Perform a few updates on val stream
+        val.event(v=2.0)
+        val.event(v=3.0)
+        self.assertEqual(
+            history.contents,
+            {"values": [{"v": 1.0}, {"v": 2.0}, {"v": 3.0}]}
+        )
+
+        # clear
+        history.clear_history()
+        self.assertEqual(history.contents, {"values": []})
+
+    def test_history_stream_trigger_callbacks(self):
+        # Construct history stream
+        val = Val(v=1.0)
+        history = History(val)
+
+        # Register callback
+        callback_input = []
+        def cb(**kwargs):
+            callback_input.append(kwargs)
+        history.add_subscriber(cb)
+        self.assertEqual(callback_input, [])
+
+        # Perform updates on val stream and make sure history callback is triggered
+        callback_input.clear()
+        val.event(v=2.0)
+        self.assertEqual(
+            callback_input[0],
+            {"values": [{"v": 1.0}, {"v": 2.0}]}
+        )
+
+        callback_input.clear()
+        val.event(v=3.0)
+        self.assertEqual(
+            callback_input[0],
+            {"values": [{"v": 1.0}, {"v": 2.0}, {"v": 3.0}]}
+        )
+
+        # clearing history should trigger callback
+        callback_input.clear()
+        history.clear_history()
+        self.assertEqual(
+            callback_input[0],
+            {"values": []}
+        )
+
+        # Update after clearing
+        callback_input.clear()
+        val.event(v=4.0)
+        self.assertEqual(
+            callback_input[0],
+            {"values": [{"v": 4.0}]}
+        )
+
+
 class TestExprSelectionStream(ComparisonTestCase):
 
     def setUp(self):