Add events (#777)

Co-authored-by: kolaente <k@knt.li>
Reviewed-on: https://kolaente.dev/vikunja/api/pulls/777
Co-authored-by: konrad <konrad@kola-entertainments.de>
Co-committed-by: konrad <konrad@kola-entertainments.de>

pkg/events/listeners.go

@@ -0,0 +1,36 @@
+// Vikunja is a to-do list application to facilitate your life.
+// Copyright 2018-2021 Vikunja and contributors. All rights reserved.
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public Licensee as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public Licensee for more details.
+//
+// You should have received a copy of the GNU Affero General Public Licensee
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+package events
+
+import "github.com/ThreeDotsLabs/watermill/message"
+
+// Listener represents something that listens to events
+type Listener interface {
+	Handle(payload message.Payload) error
+	Name() string
+}
+
+var listeners map[string][]Listener
+
+func init() {
+	listeners = make(map[string][]Listener)
+}
+
+// RegisterListener is used to register a listener when a specific event happens
+func RegisterListener(name string, listener Listener) {
+	listeners[name] = append(listeners[name], listener)
+}