add event library

client/go/outline/event/event.go

@@ -0,0 +1,74 @@
+// Copyright 2025 The Outline Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package event provides a thread-safe mechanism for managing and triggering events.
+package event
+
+import (
+	"log/slog"
+	"slices"
+	"sync"
+
+	"github.com/Jigsaw-Code/outline-apps/client/go/outline/callback"
+)
+
+// EventName is a type alias for string that represents the name of an event.
+// Using a dedicated type improves type safety when working with event names.
+type EventName string
+
+var (
+	mu        sync.RWMutex
+	listeners = make(map[EventName][]callback.Token)
+)
+
+// AddListener adds a [callback.Callback] to a given [EventName].
+//
+// The provided callback will be called when the event is invoked, along with the event data.
+func AddListener(evt EventName, cb callback.Token) {
+	if evt == "" || cb == "" {
+		slog.Warn("empty event or callback are ignored")
+		return
+	}
+	mu.Lock()
+	defer mu.Unlock()
+
+	listeners[evt] = append(listeners[evt], cb)
+	slog.Debug("successfully subscribed to event", "event", evt, "callback", cb)
+}
+
+// RemoveListener removes a [callback.Callback] from the specified [EventName].
+//
+// Calling this function is safe even if the event has not been registered.
+func RemoveListener(evt EventName, cb callback.Token) {
+	mu.Lock()
+	defer mu.Unlock()
+
+	if cbs, ok := listeners[evt]; ok {
+		listeners[evt] = slices.DeleteFunc(cbs, func(t callback.Token) bool { return t == cb })
+	}
+	slog.Debug("successfully ubsubscribed from event", "event", evt, "callback", cb)
+}
+
+// Fire triggers the specified [EventName], invoking all associated callbacks with the given data.
+//
+// Calling this function is safe even if the event has not been registered.
+func Fire(evt EventName, data string) {
+	mu.RLock()
+	defer mu.RUnlock()
+
+	slog.Debug("firing event", "event", evt, "data", data)
+	for _, cb := range listeners[evt] {
+		callback.Call(cb, data)
+	}
+}