add event signals for video frames

Author: miguelgrinberg

base_camera.py

@@ -1,11 +1,61 @@
 import time
 import threading
+try:
+    from greenlet import getcurrent as get_ident
+except ImportError:
+    try:
+        from thread import get_ident
+    except ImportError:
+        from _thread import get_ident
+
+
+class CameraEvent(object):
+    """An Event-like class that signals all active clients when a new frame is
+    available.
+    """
+    def __init__(self):
+        self.events = {}
+
+    def wait(self):
+        """Invoked from each client's thread to wait for the next frame."""
+        ident = get_ident()
+        if ident not in self.events:
+            # this is a new client
+            # add an entry for it in the self.events dict
+            # each entry has two elements, a threading.Event() and a timestamp
+            self.events[ident] = [threading.Event(), time.time()]
+        return self.events[ident][0].wait()
+
+    def set(self):
+        """Invoked by the camera thread when a new frame is available."""
+        now = time.time()
+        remove = None
+        for ident, event in self.events.items():
+            if not event[0].isSet():
+                # if this client's event is not set, then set it
+                # also update the last set timestamp to now
+                event[0].set()
+                event[1] = now
+            else:
+                # if the client's event is already set, it means the client
+                # did not process a previous frame
+                # if the event stays set for more than 5 seconds, then assume
+                # the client is gone and remove it
+                if now - event[1] > 5:
+                    remove = ident
+        if remove:
+            del self.events[remove]
+
+    def clear(self):
+        """Invoked from each client's thread after a frame was processed."""
+        self.events[get_ident()][0].clear()
 
 
 class BaseCamera(object):
     thread = None  # background thread that reads frames from camera
     frame = None  # current frame is stored here by background thread
     last_access = 0  # time of last client access to the camera
+    event = CameraEvent()
 
     def __init__(self):
         """Start the background camera thread if it isn't running yet."""
@@ -21,6 +71,11 @@ def __init__(self):
     def get_frame(self):
         """Return the current camera frame."""
         BaseCamera.last_access = time.time()
+
+        # wait for a signal from the camera thread
+        BaseCamera.event.wait()
+        BaseCamera.event.clear()
+
         return BaseCamera.frame
 
     @staticmethod
@@ -35,6 +90,7 @@ def _thread(cls):
         frames_iterator = cls.frames()
         for frame in frames_iterator:
             BaseCamera.frame = frame
+            BaseCamera.event.set()  # send signal to clients
 
             # if there hasn't been any clients asking for frames in
             # the last 10 seconds then stop the thread