Resolve "Application that allows to access events through websocket"

Description

Related Issue

Motivation and Context

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

Closes #301

applications/ws-events/app.go

+package main
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"os"
+	"os/signal"
+	"syscall"
+	"time"
+
+	"code.fbi.h-da.de/danet/gosdn/application-framework/event"
+	"github.com/lesismal/nbio/nbhttp"
+	"github.com/sirupsen/logrus"
+)
+
+// Application is an example for a sdn application.
+type Application struct {
+	eventService event.ServiceInterface
+	stopChannel  chan os.Signal
+}
+
+// Run runs the application.
+func (a *Application) Run() error {
+	signal.Notify(a.stopChannel, os.Interrupt, syscall.SIGTERM)
+
+	// subscribe to Add,Delete and Update event types.
+	a.eventService.SubscribeToEventType([]event.TypeToCallbackTuple{
+		{Type: event.Add, Callback: a.callback},
+		{Type: event.Delete, Callback: a.callback},
+		{Type: event.Update, Callback: a.callback},
+	})
+	a.eventService.SetupEventReciever(a.stopChannel)
+
+	mux := &http.ServeMux{}
+	mux.HandleFunc("/events", onWebsocket)
+
+	svr := nbhttp.NewServer(nbhttp.Config{
+		Network: "tcp",
+		Addrs:   []string{"localhost:4000"},
+		Handler: mux,
+	})
+
+	err := svr.Start()
+	if err != nil {
+		return fmt.Errorf("Server start failed: %w\n", err)
+	}
+
+	<-a.stopChannel
+	ctx, cancel := context.WithTimeout(context.Background(), time.Second*5)
+	defer cancel()
+	return svr.Shutdown(ctx)
+}
+
+// callback handles an incoming event from the event system. The event is
+// turned into JSON encoding and published to all websocket clients through the
+// ClientManager.
+func (a *Application) callback(event *event.Event) {
+	b, err := json.Marshal(event)
+	if err != nil {
+		logrus.Error("Failed marshal of event")
+	}
+	clientManager.Publish(b)
+}