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).
