Add example usage of outgoing webhooks (#171)

server/README.md

@@ -88,6 +88,17 @@ This demo implementation sends back whether or not the plugin hooks are currentl
 is used by the web app to recover from a network reconnection and synchronize the state of the
 plugin's hooks.
 
+It also implements a receiver for outgoing webhooks. To utilize that, create an Outgoing Webhook
+using the following configuration:
+- Title: Choose as you want
+- Content Type: `application/json`
+- Channel: Pick any channel
+- Callback URLs: `http://localhost:8065/plugins/com.mattermost.demo-plugin/webhook/outgoing`
+
+Leave the rest of the fields with their default value.
+
+Now post a message in the selected channel. You will see a webhook response, which contains the payload the plugin received.
+
 ## [message_hooks.go](message_hooks.go)
 
 ### MessageWillBePosted

server/http_hooks.go

@@ -32,6 +32,10 @@ func (p *Plugin) initializeAPI() {
 	router.HandleFunc("/dynamic_arg_test_url", p.handleDynamicArgTest)
 	router.HandleFunc("/check_auth_header", p.handleCheckAuthHeader)
 
+	webhook := router.PathPrefix("/webhook").Subrouter()
+	webhook.Use(p.withDelay)
+	webhook.HandleFunc("/outgoing", p.handleOutgoingWebhook).Methods(http.MethodPost)
+
 	interativeRouter := router.PathPrefix("/interactive").Subrouter()
 	interativeRouter.Use(p.withDelay)
 	interativeRouter.HandleFunc("/button/1", p.handleInteractiveAction)
@@ -103,6 +107,30 @@ func (p *Plugin) handleCheckAuthHeader(w http.ResponseWriter, r *http.Request) {
 	}
 }
 
+func (p *Plugin) handleOutgoingWebhook(w http.ResponseWriter, r *http.Request) {
+	var request model.OutgoingWebhookPayload
+	err := json.NewDecoder(r.Body).Decode(&request)
+	if err != nil {
+		p.API.LogError("Failed to decode OutgoingWebhookPayload", "err", err)
+		w.WriteHeader(http.StatusBadRequest)
+		return
+	}
+	defer r.Body.Close()
+
+	s, err := PrettyJSON(request)
+	if err != nil {
+		p.API.LogError("Failed to Marshal payload back to JSON", "err", err.Error())
+		return
+	}
+
+	text := "```\n" + s + "\n```"
+	resp := model.OutgoingWebhookResponse{
+		Text: &text,
+	}
+
+	p.writeJSON(w, resp)
+}
+
 func (p *Plugin) handleDialog1(w http.ResponseWriter, r *http.Request) {
 	var request model.SubmitDialogRequest
 	err := json.NewDecoder(r.Body).Decode(&request)

server/utils.go

@@ -0,0 +1,13 @@
+package main
+
+import (
+	"encoding/json"
+)
+
+func PrettyJSON(in interface{}) (string, error) {
+	bb, err := json.MarshalIndent(in, "", "  ")
+	if err != nil {
+		return "", err
+	}
+	return string(bb), nil
+}