Added functionality to read only changed data.

CHANGES.txt

@@ -1,3 +1,36 @@
+================================================================================
+OpenOPC for Python 1.4.0
+RELEASE NOTES
+
+Added functionality to read only changed data (without breaking the current API).
+
+Here's a snippet to illustrate usage:
+
+read_type = 'CD'        # 'SR'=SyncRead, 'AR'=AsyncRefresh, 'CD'=ChangedData (new functionality)
+if read_type == 'SR':
+    print("\n...Initial read (sync):\n\n" + str(opc.read(taglist, group=tag_group, sync=True, include_error=True)))   # include_error=True also forces sync
+elif read_type == 'AR':
+    print("\n...Initial read (async/refresh):\n\n" + str(opc.read(taglist, group=tag_group, sync=False, include_error=False)))
+elif read_type == 'CD':
+    print("\n...Initial read (on data change):\n\n" + str(opc.read(taglist, group=tag_group, sync=False, include_error=False, update=500)))   # note: initial read performs an AsyncRefresh
+else:
+    print("\n\n*********** Invalid read_type ***********\n\n")
+print("\n...Next reads... by group:")    
+for i in range(4):
+    print("\nIteration: " + str(i))
+    loop = True
+    while loop:
+        print('')
+        tag_iter = opc.iread(group=tag_group, sync=(read_type=='SR'), update=(1 if read_type=='CD' else -1))    # although true update value will be from initial group read, the update needs to be specified as greater than 0 in order to only read changed data
+        loop = False
+        for (name, val, qual, time) in tag_iter:
+            if read_type=='CD': loop = True
+            if val != None:      # val==None means data didn't change
+                print(name, val, qual, time)
+    pytime.sleep(2)
+
+
+================================================================================
 OpenOPC for Python 1.3.1
 Copyright (c) 2008-2015 by Barry Barnreiter (barrybb@gmail.com)