diff --git a/playground/Playground.cs b/playground/Playground.cs
index 497a760..509fd40 100644
--- a/playground/Playground.cs
+++ b/playground/Playground.cs
@@ -1,34 +1,45 @@
 using System;
 using System.Diagnostics;
 using System.Threading.Tasks;
+using System.Timers;
 using TinkState;
 
 class Playground
 {
-	static async Task Main()
+	static void Main()
 	{
-		var stateA = Observable.State("hello");
-		var stateB = Observable.State("world");
+		var timer = new Timer(1000);
+		ElapsedEventHandler timerElapsedHandler = null;
 
-		var o = Observable.Auto(async () =>
-		{
-			Console.WriteLine("computing");
-			var a = stateA.Value;
-			await Task.Delay(1000);
-			var b = stateB.Value;
-			return a + " " + b;
-		});
+		var time = Observable.External(
+			() => DateTime.UtcNow,
+			invalidate =>
+			{
+				Console.WriteLine("wakeup");
+				timerElapsedHandler = (_, _) =>
+				{
+					Console.WriteLine("tick");
+					invalidate();
+				};
+				timer.Elapsed += timerElapsedHandler;
+				timer.Start();
+			},
+			() =>
+			{
+				Console.WriteLine("sleep");
+				timer.Elapsed -= timerElapsedHandler;
+				timer.Stop();
+			}
+		);
 
-		o.Bind(result => Console.WriteLine(result.Status switch
+		var counter = 0;
+		IDisposable binding = null;
+		binding = time.Bind(v =>
 		{
-			AsyncComputeStatus.Loading => "Loading...",
-			AsyncComputeStatus.Done => "Done: " + result.Result,
-			AsyncComputeStatus.Failed => "Failed: " + result.Exception,
-		}));
-
-		await Task.Delay(1500);
-
-		stateB.Value = "Dan";
+			Console.WriteLine($"Current time is: {v}");
+			counter++;
+			if (counter >= 5) binding.Dispose();
+		});
 
 		Process.GetCurrentProcess().WaitForExit();
 	}
diff --git a/src/TinkState/Runtime/ExternalObservable.cs b/src/TinkState/Runtime/ExternalObservable.cs
new file mode 100644
index 0000000..a6e49d3
--- /dev/null
+++ b/src/TinkState/Runtime/ExternalObservable.cs
@@ -0,0 +1,90 @@
+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+
+namespace TinkState.Internal
+{
+	class ExternalObservable<T> : Dispatcher, Observable<T>, DispatchingObservable<T>
+	{
+		[DebuggerBrowsable(DebuggerBrowsableState.Never)]
+		public T Value => AutoObservable.Track(this);
+
+		readonly Func<T> getter;
+		readonly Action<Action> wakeup;
+		readonly Action sleep;
+		readonly IEqualityComparer<T> comparer;
+
+		T last;
+		bool isDirty;
+		bool isSubscribedTo;
+
+		public ExternalObservable(Func<T> getter, Action<Action> wakeup, Action sleep, IEqualityComparer<T> comparer)
+		{
+			this.getter = getter;
+			this.wakeup = wakeup;
+			this.sleep = sleep;
+			this.comparer = comparer ?? EqualityComparer<T>.Default;
+			isDirty = true;
+		}
+
+		public IDisposable Bind(Action<T> callback, IEqualityComparer<T> comparer = null, Scheduler scheduler = null)
+		{
+			return Binding<T>.Create(this, callback, comparer, scheduler);
+		}
+
+		public Observable<TOut> Map<TOut>(Func<T, TOut> transform, IEqualityComparer<TOut> comparer = null)
+		{
+			return TransformObservable.Create(this, transform, comparer);
+		}
+
+		public IEqualityComparer<T> GetComparer()
+		{
+			return comparer;
+		}
+
+		public long GetRevision()
+		{
+			return revision;
+		}
+
+		public T GetCurrentValue()
+		{
+			if (isDirty || !isSubscribedTo) RecalculateCurrentValue();
+			return last;
+		}
+
+		void RecalculateCurrentValue()
+		{
+			last = getter();
+			isDirty = false;
+		}
+
+		protected override void OnStatusChange(bool active)
+		{
+			if (active) WakeUp();
+			else Sleep();
+		}
+
+		void WakeUp()
+		{
+			wakeup(Notify);
+			isSubscribedTo = true;
+			RecalculateCurrentValue();
+		}
+
+		void Sleep()
+		{
+			sleep();
+			isSubscribedTo = false;
+		}
+
+		void Notify()
+		{
+			if (!isDirty)
+			{
+				isDirty = true;
+				Fire();
+			}
+		}
+	}
+}
diff --git a/src/TinkState/Runtime/Observable.cs b/src/TinkState/Runtime/Observable.cs
index 36fb641..7ab2613 100644
--- a/src/TinkState/Runtime/Observable.cs
+++ b/src/TinkState/Runtime/Observable.cs
@@ -99,6 +99,11 @@ public static Observable<T> Auto<T>(Func<T> compute, IEqualityComparer<T> compar
 			return new AutoObservable<T>(new SyncComputation<T>(compute), comparer);
 		}
 
+		public static Observable<T> External<T>(Func<T> getter, Action<Action> wakeup, Action sleep, IEqualityComparer<T> comparer = null)
+		{
+			return new ExternalObservable<T>(getter, wakeup, sleep, comparer);
+		}
+
 		/// <summary>
 		///     <para>
 		///     Create an observable containing data computed using given <paramref name="compute"/> function.
@@ -220,4 +225,4 @@ public static ObservableDictionary<TKey, TValue> Dictionary<TKey, TValue>()
 		/// </remarks>
 		public static Scheduler Scheduler = Scheduler.Direct;
 	}
-}
\ No newline at end of file
+}