Fix timer not removing listeners on tick

shared/src/main/scala/async/Timer.scala

@@ -13,19 +13,26 @@ import scala.annotation.tailrec
 import java.util.concurrent.CancellationException
 
 /** 
- * Timer exposes a steady Async.Source of ticks that happens every `tickDuration` milliseconds.
+ * Timer exposes a steady [[Async.Source]] of ticks that happens every `tickDuration` milliseconds.
  * Note that the timer does not start ticking until `start` is called (which is a blocking operation, until the timer is cancelled).
  * 
- * You might want to manually `cancel` the timer, so that it gets garbage collected (before the enclosing `Async` scope ends).
+ * You might want to manually `cancel` the timer, so that it gets garbage collected (before the enclosing [[Async]] scope ends).
 */
 class Timer(tickDuration: Duration) extends Cancellable {
-  var isCancelled = false
+  enum TimerEvent:
+    case Tick
+    case Cancelled
 
-  private class Source extends Async.OriginalSource[this.TimerEvent] {
+  private var isCancelled = false
+
+  private object Source extends Async.OriginalSource[this.TimerEvent] {
+    val listeners = mutable.Set[Listener[TimerEvent]]()
     def tick() = synchronized {
-      listeners.foreach(_.completeNow(TimerEvent.Tick, this))
+      listeners.filterInPlace(l =>
+        l.completeNow(TimerEvent.Tick, this)
+        false
+      )
     }
-    val listeners = mutable.Set[Listener[TimerEvent]]()
     override def poll(k: Listener[TimerEvent]): Boolean = 
       if isCancelled then k.completeNow(TimerEvent.Cancelled, this) else false // subscribing to a timer always takes you to the next tick
     override def dropListener(k: Listener[TimerEvent]): Unit = listeners -= k
@@ -36,36 +43,30 @@ class Timer(tickDuration: Duration) extends Cancellable {
           if isCancelled then k.completeNow(TimerEvent.Cancelled, this)
           else listeners += k
   }
-  private val _src = Source()
 
   /** Ticks of the timer are delivered through this source. Note that ticks are ephemeral. */
-  val src: Async.Source[this.TimerEvent] = _src
+  inline final def src: Async.Source[this.TimerEvent] = Source
 
+  /** Starts the timer. Suspends until the timer is cancelled. */
   def start()(using Async, AsyncOperations): Unit =
     cancellationScope(this):
       loop()
 
   @tailrec private def loop()(using Async, AsyncOperations): Unit =
     if !isCancelled then
-      try
-        // println(s"Sleeping at ${new java.util.Date()}, ${isCancelled}, ${this}")
-        sleep(tickDuration.toMillis)
+      try sleep(tickDuration.toMillis)
       catch
         case _: CancellationException => cancel()
     if !isCancelled then
-      _src.tick()
+      Source.tick()
       loop()
 
 
   override def cancel(): Unit = 
     synchronized { isCancelled = true }
     src.synchronized {
-      _src.listeners.foreach(_.completeNow(TimerEvent.Cancelled, src))
-      _src.listeners.clear()
+      Source.listeners.foreach(_.completeNow(TimerEvent.Cancelled, src))
+      Source.listeners.clear()
     }
-
-  enum TimerEvent:
-    case Tick
-    case Cancelled
 }