event: implement pollEvent and tryEvent

Implement a way for an application to poll the event loop in a blocking
way without popping an event. Implement a non-blocking pop. These
together enable an application to poll the event loop and then drain it.
This is useful when lots of events are delivered in a short amount of
time so an application can batch process the events and then render.

Signed-off-by: Tim Culverhouse <tim@timculverhouse.com>

src/queue.zig

@@ -87,6 +87,16 @@ pub fn Queue(
             return self.pop();
         }
 
+        /// Poll the queue. This call blocks until events are in the queue
+        pub fn poll(self: *Self) void {
+            self.mutex.lock();
+            defer self.mutex.unlock();
+            while (self.isEmptyLH()) {
+                self.not_empty.wait(&self.mutex);
+            }
+            std.debug.assert(!self.isEmptyLH());
+        }
+
         fn isEmptyLH(self: Self) bool {
             return self.write_index == self.read_index;
         }

src/vaxis.zig

@@ -146,6 +146,17 @@ pub fn Vaxis(comptime T: type) type {
             return self.queue.pop();
         }
 
+        /// blocks until an event is available. Useful when your application is
+        /// operating on a poll + drain architecture (see tryEvent)
+        pub fn pollEvent(self: *Self) void {
+            self.queue.poll();
+        }
+
+        /// returns an event if one is available, otherwise null. Non-blocking.
+        pub fn tryEvent(self: *Self) ?Event {
+            return self.queue.tryPop();
+        }
+
         /// posts an event into the event queue. Will block if there is not
         /// capacity for the event
         pub fn postEvent(self: *Self, event: T) void {