lemonbar: Breakout condition logic to Shelf class

.i3/lemonbar/i3_lemonbar_common.py

@@ -1,3 +1,4 @@
+import threading
 import subprocess, os
 import re # regexp
 from enum import Enum
@@ -48,3 +49,28 @@ def create_new_fifo(fifo_file):
         logger.error('''Failed, couldn't create fifo file''')
         os.remove(config.pid_file) # Clean up own PID file
         sys.exit(0)
+
+class Shelf():
+    # A one-slot queue implemented with locks and condition objects
+    def __init__(self, item = None):
+        self.lock            = threading.Lock()
+        self.item_ready = threading.Condition(self.lock)
+        self.item       = item
+
+    def get(self):
+        # Consumes item
+        self.item_ready.acquire()
+        if self.item is None:
+            # Wait until becomes ready
+            self.item_ready.wait()
+        ret = self.item
+        self.item = None
+        self.item_ready.release()
+        return ret
+
+    def put(self, item):
+        # Discards previous item
+        self.item_ready.acquire()
+        self.item = item
+        self.item_ready.notify()
+        self.item_ready.release()