From dc91fdd86fbf9ba71356c9dede37f6f7c79cd50f Mon Sep 17 00:00:00 2001
From: Tim Ryan <tim@3drobotics.com>
Date: Thu, 16 Jul 2015 15:17:11 -0700
Subject: [PATCH 1/2] Adds test for #110.

---
 tests/sitl/test_110.py | 49 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 49 insertions(+)
 create mode 100644 tests/sitl/test_110.py

diff --git a/tests/sitl/test_110.py b/tests/sitl/test_110.py
new file mode 100644
index 000000000..96eea10c4
--- /dev/null
+++ b/tests/sitl/test_110.py
@@ -0,0 +1,49 @@
+from droneapi.lib import VehicleMode
+from pymavlink import mavutil
+import time
+import sys
+import os
+
+def test_110(local_connect):
+    api = local_connect()
+    v = api.get_vehicles()[0]
+    
+    # Change the vehicle into STABILIZE mode
+    v.mode = VehicleMode("STABILIZE")
+
+    # NOTE wait crudely for ACK on mode update
+    time.sleep(3)
+
+    # Define example callback for mode
+    allow_callback = True
+    bad_call = [False]
+    def armed_callback(attribute):
+        if not allow_callback:
+            bad_call[0] = True
+
+    # Only one of the same observer fn should be added.
+    v.add_attribute_observer('armed', armed_callback)
+    v.add_attribute_observer('armed', armed_callback)
+    v.add_attribute_observer('armed', armed_callback)
+    v.add_attribute_observer('armed', armed_callback)
+    allow_callback = True
+
+    # Disarm and see update.
+    v.armed = False
+    v.flush()
+
+    time.sleep(3)
+
+    # Rmove (all) observers.
+    v.remove_attribute_observer('armed', armed_callback)
+    v.remove_attribute_observer('armed', armed_callback)
+    allow_callback = False
+
+    # Re-arm and see update.
+    v.armed = True
+    v.flush()
+
+    time.sleep(3)
+
+    # Make sure no bad calls happened.
+    assert not bad_call[0], "Callback should not have been called once removed."

From de4b39b58ff7c40d3fde853719bcd4f3b9d61ba9 Mon Sep 17 00:00:00 2001
From: Tim Ryan <tim@3drobotics.com>
Date: Thu, 16 Jul 2015 15:18:59 -0700
Subject: [PATCH 2/2] Ensures observers can't be added twice. Fixes #110

---
 droneapi/lib/__init__.py |  3 ++-
 tests/sitl/test_110.py   | 31 +++++++++++++++++++------------
 2 files changed, 21 insertions(+), 13 deletions(-)

diff --git a/droneapi/lib/__init__.py b/droneapi/lib/__init__.py
index bcb7a37a6..87d7fa47d 100644
--- a/droneapi/lib/__init__.py
+++ b/droneapi/lib/__init__.py
@@ -351,7 +351,8 @@ def location_callback(location):
         if l is None:
             l = []
             self.__observers[attr_name] = l
-        l.append(observer)
+        if not observer in l:
+            l.append(observer)
 
     def remove_attribute_observer(self, attr_name, observer):
         """
diff --git a/tests/sitl/test_110.py b/tests/sitl/test_110.py
index 96eea10c4..4e220879d 100644
--- a/tests/sitl/test_110.py
+++ b/tests/sitl/test_110.py
@@ -3,6 +3,7 @@
 import time
 import sys
 import os
+from testlib import assert_equals
 
 def test_110(local_connect):
     api = local_connect()
@@ -15,35 +16,41 @@ def test_110(local_connect):
     time.sleep(3)
 
     # Define example callback for mode
-    allow_callback = True
-    bad_call = [False]
     def armed_callback(attribute):
-        if not allow_callback:
-            bad_call[0] = True
+        armed_callback.called += 1
+    armed_callback.called = 0
 
-    # Only one of the same observer fn should be added.
+    # When the same (event, callback) pair is passed to add_attribute_observer,
+    # only one instance of the observer callback should be added.
+    v.add_attribute_observer('armed', armed_callback)
     v.add_attribute_observer('armed', armed_callback)
     v.add_attribute_observer('armed', armed_callback)
     v.add_attribute_observer('armed', armed_callback)
     v.add_attribute_observer('armed', armed_callback)
-    allow_callback = True
 
     # Disarm and see update.
     v.armed = False
     v.flush()
-
+    # Wait for ACK.
     time.sleep(3)
 
-    # Rmove (all) observers.
+    # Ensure the callback was called.
+    assert armed_callback.called > 0, "Callback should have been called."
+
+    # Rmove all observers. The first call should remove all listeners
+    # we've added; the second call should be ignored and not throw.
+    # NOTE: We test if armed_callback were treating adding each additional callback
+    # and remove_attribute_observer were removing them one at a time; in this
+    # case, there would be three callbacks still attached.
     v.remove_attribute_observer('armed', armed_callback)
     v.remove_attribute_observer('armed', armed_callback)
-    allow_callback = False
+    callcount = armed_callback.called
 
     # Re-arm and see update.
     v.armed = True
     v.flush()
-
+    # Wait for ack
     time.sleep(3)
 
-    # Make sure no bad calls happened.
-    assert not bad_call[0], "Callback should not have been called once removed."
+    # Ensure the callback was called zero times.
+    assert_equals(armed_callback.called, callcount, "Callback should not have been called once removed.")