Implement Observer pattern

Author: sha1sum

Diff for: README.md

@@ -42,7 +42,7 @@ __Behavioral Patterns__:
 | TODO: [Command](behavioral/command.md) | Bundles a command and arguments to call later |
 | TODO: [Mediator](behavioral/mediator.md) | Connects objects and acts as a proxy |
 | TODO: [Memento](behavioral/memento.md) | Generate an opaque token that can be used to go back to a previous state |
-| TODO: [Observer](behavioral/observer.md) | Provide a callback for notification of events/changes to data |
+| [Observer](behavioral/observer.md) | Provide a callback for notification of events/changes to data |
 | TODO: [Registry](behavioral/registry.md) | Keep track of all subclasses of a given class |
 | TODO: [State](behavioral/state.md) | Encapsulates varying behavior for the same object based on its internal state |
 | [Strategy](behavioral/strategy.md) | Enables an algorithm's behavior to be selected at runtime |

Diff for: behavioral/observer.md

@@ -0,0 +1,47 @@
+# Observer Pattern
+
+The [observer pattern](https://en.wikipedia.org/wiki/Observer_pattern) allows a type instance to "publish" events to other type instances ("observers") who wish to be updated when a particular event occurs.
+
+## Implementation
+
+In long-running applications—such as webservers—instances can keep a collection of observers that will receive notification of triggered events.
+
+Implementations vary, but interfaces can be used to make standard observers and notifiers:
+
+```go
+type (
+	// Event defines an indication of a point-in-time occurrence.
+	Event struct {
+		// Data in this case is a simple int, but the actual
+		// implementation would depend on the application.
+		Data int64
+	}
+
+	// Observer defines a standard interface for instances that wish to list for
+	// the occurrence of a specific event.
+	Observer interface {
+		// OnNotify allows an event to be "published" to interface implementations.
+		// In the "real world", error handling would likely be implemented.
+		OnNotify(Event)
+	}
+
+	// Notifier is the instance being observed. Publisher is perhaps another decent
+	// name, but naming things is hard.
+	Notifier interface {
+		// Register allows an instance to register itself to listen/observe
+		// events.
+		Register(Observer)
+		// Deregister allows an instance to remove itself from the collection
+		// of observers/listeners.
+		Deregister(Observer)
+		// Notify publishes new events to listeners. The method is not
+		// absolutely necessary, as each implementation could define this itself
+		// without losing functionality.
+		Notify(Event)
+	}
+)
+```
+
+## Usage
+
+For usage, see [observer/main.go](observer/main.go) or [view in the Playground](https://play.golang.org/p/cr8jEmDmw0).

Diff for: behavioral/observer/main.go

@@ -0,0 +1,93 @@
+// Package main serves as an example application that makes use of the observer pattern.
+// Playground: https://play.golang.org/p/cr8jEmDmw0
+package main
+
+import (
+	"fmt"
+	"time"
+)
+
+type (
+	// Event defines an indication of a point-in-time occurrence.
+	Event struct {
+		// Data in this case is a simple int, but the actual
+		// implementation would depend on the application.
+		Data int64
+	}
+
+	// Observer defines a standard interface for instances that wish to list for
+	// the occurrence of a specific event.
+	Observer interface {
+		// OnNotify allows an event to be "published" to interface implementations.
+		// In the "real world", error handling would likely be implemented.
+		OnNotify(Event)
+	}
+
+	// Notifier is the instance being observed. Publisher is perhaps another decent
+	// name, but naming things is hard.
+	Notifier interface {
+		// Register allows an instance to register itself to listen/observe
+		// events.
+		Register(Observer)
+		// Deregister allows an instance to remove itself from the collection
+		// of observers/listeners.
+		Deregister(Observer)
+		// Notify publishes new events to listeners. The method is not
+		// absolutely necessary, as each implementation could define this itself
+		// without losing functionality.
+		Notify(Event)
+	}
+)
+
+type (
+	eventObserver struct{
+		id int
+	}
+
+	eventNotifier struct{
+		// Using a map with an empty struct allows us to keep the observers
+		// unique while still keeping memory usage relatively low.
+		observers map[Observer]struct{}
+	}
+)
+
+func (o *eventObserver) OnNotify(e Event) {
+	fmt.Printf("*** Observer %d received: %d\n", o.id, e.Data)
+}
+
+func (o *eventNotifier) Register(l Observer) {
+	o.observers[l] = struct{}{}
+}
+
+func (o *eventNotifier) Deregister(l Observer) {
+	delete(o.observers, l)
+}
+
+func (p *eventNotifier) Notify(e Event) {
+	for o := range p.observers {
+		o.OnNotify(e)
+	}
+}
+
+func main() {
+	// Initialize a new Notifier.
+	n := eventNotifier{
+		observers: map[Observer]struct{}{},
+	}
+
+	// Register a couple of observers.
+	n.Register(&eventObserver{id: 1})
+	n.Register(&eventObserver{id: 2})
+
+	// A simple loop publishing the current Unix timestamp to observers.
+	stop := time.NewTimer(10 * time.Second).C
+	tick := time.NewTicker(time.Second).C
+	for {
+		select {
+		case <- stop:
+			return
+		case t := <-tick:
+			n.Notify(Event{Data: t.UnixNano()})
+		}
+	}
+}