Add observable design pattern implementation

src/lib/observable.cr

@@ -0,0 +1,21 @@
+
+# Code from: https://github.com/crystal-community/crystal-patterns
+# Licence:   MIT
+
+module Observable(T)
+  getter observers
+
+  def add_observer(observer)
+    @observers ||= [] of T
+    @observers.not_nil! << observer
+  end
+
+  def delete_observer(observer)
+    @observers.try &.delete(observer)
+  end
+
+  def notify_observers
+    @observers.try &.each &.update self
+  end
+end
+