From ff04be396bf5176715c2796672df6424130700d6 Mon Sep 17 00:00:00 2001
From: Jae Kwon <jkwon.work@gmail.com>
Date: Tue, 31 Mar 2015 21:11:40 -0700
Subject: [PATCH] First draft of events handling

---
 events/events.go | 189 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 189 insertions(+)
 create mode 100644 events/events.go

diff --git a/events/events.go b/events/events.go
new file mode 100644
index 000000000..05c6f663f
--- /dev/null
+++ b/events/events.go
@@ -0,0 +1,189 @@
+package events
+
+import (
+	"sync"
+	"sync/atomic"
+)
+
+type EventSwitch struct {
+	mtx        sync.RWMutex
+	eventCells map[string]*eventCell
+	listeners  map[string]*eventListener
+	running    uint32
+	quit       chan struct{}
+}
+
+func (evsw *EventSwitch) Start() {
+	if atomic.CompareAndSwapUint32(&evsw.running, 0, 1) {
+		evsw.eventCells = make(map[string]*eventCell)
+		evsw.listeners = make(map[string]*eventListener)
+		evsw.quit = make(chan struct{})
+	}
+}
+
+func (evsw *EventSwitch) Stop() {
+	if atomic.CompareAndSwapUint32(&evsw.running, 1, 0) {
+		evsw.eventCells = nil
+		evsw.listeners = nil
+		close(evsw.quit)
+	}
+}
+
+func (evsw *EventSwitch) AddListenerForEvent(listenerId, event string, cb eventCallback) {
+	// Get/Create eventCell and listener
+	evsw.mtx.Lock()
+	eventCell := evsw.eventCells[event]
+	if eventCell == nil {
+		eventCell = newEventCell()
+		evsw.eventCells[event] = eventCell
+	}
+	listener := evsw.listeners[listenerId]
+	if listener == nil {
+		listener = newEventListener(listenerId)
+		evsw.listeners[listenerId] = listener
+	}
+	evsw.mtx.Unlock()
+
+	// Add event and listener
+	eventCell.AddListener(listenerId, cb)
+	listener.AddEvent(event)
+}
+
+func (evsw *EventSwitch) RemoveListener(listenerId string) {
+	// Get and remove listener
+	evsw.mtx.RLock()
+	listener := evsw.listeners[listenerId]
+	delete(evsw.listeners, listenerId)
+	evsw.mtx.RUnlock()
+
+	if listener == nil {
+		return
+	}
+
+	// Remove callback for each event.
+	listener.SetRemoved()
+	for _, event := range listener.GetEvents() {
+		evsw.RemoveListenerForEvent(event, listenerId)
+	}
+}
+
+func (evsw *EventSwitch) RemoveListenerForEvent(event string, listenerId string) {
+	// Get eventCell
+	evsw.mtx.Lock()
+	eventCell := evsw.eventCells[event]
+	evsw.mtx.Unlock()
+
+	if eventCell == nil {
+		return
+	}
+
+	// Remove listenerId from eventCell
+	numListeners := eventCell.RemoveListener(listenerId)
+
+	// Maybe garbage collect eventCell.
+	if numListeners == 0 {
+		// Lock again and double check.
+		evsw.mtx.Lock()      // OUTER LOCK
+		eventCell.mtx.Lock() // INNER LOCK
+		if len(eventCell.listeners) == 0 {
+			delete(evsw.eventCells, event)
+		}
+		eventCell.mtx.Unlock() // INNER LOCK
+		evsw.mtx.Unlock()      // OUTER LOCK
+	}
+}
+
+func (evsw *EventSwitch) FireEvent(event string, msg interface{}) {
+	// Get the eventCell
+	evsw.mtx.RLock()
+	eventCell := evsw.eventCells[event]
+	evsw.mtx.RUnlock()
+
+	if eventCell == nil {
+		return
+	}
+
+	// Fire event for all listeners in eventCell
+	eventCell.FireEvent(msg)
+}
+
+//-----------------------------------------------------------------------------
+
+// eventCell handles keeping track of listener callbacks for a given event.
+type eventCell struct {
+	mtx       sync.RWMutex
+	listeners map[string]eventCallback
+}
+
+func newEventCell() *eventCell {
+	return &eventCell{
+		listeners: make(map[string]eventCallback),
+	}
+}
+
+func (cell *eventCell) AddListener(listenerId string, cb eventCallback) {
+	cell.mtx.Lock()
+	cell.listeners[listenerId] = cb
+	cell.mtx.Unlock()
+}
+
+func (cell *eventCell) RemoveListener(listenerId string) int {
+	cell.mtx.Lock()
+	delete(cell.listeners, listenerId)
+	numListeners := len(cell.listeners)
+	cell.mtx.Unlock()
+	return numListeners
+}
+
+func (cell *eventCell) FireEvent(msg interface{}) {
+	cell.mtx.RLock()
+	for _, listener := range cell.listeners {
+		listener(msg)
+	}
+	cell.mtx.RUnlock()
+}
+
+//-----------------------------------------------------------------------------
+
+type eventCallback func(msg interface{})
+
+type eventListener struct {
+	id string
+
+	mtx     sync.RWMutex
+	removed bool
+	events  []string
+}
+
+func newEventListener(id string) *eventListener {
+	return &eventListener{
+		id:      id,
+		removed: false,
+		events:  nil,
+	}
+}
+
+func (evl *eventListener) AddEvent(event string) {
+	evl.mtx.Lock()
+	defer evl.mtx.Unlock()
+
+	if evl.removed {
+		return
+	}
+	evl.events = append(evl.events, event)
+}
+
+func (evl *eventListener) GetEvents() []string {
+	evl.mtx.RLock()
+	defer evl.mtx.RUnlock()
+
+	events := make([]string, len(evl.events))
+	copy(events, evl.events)
+	return events
+}
+
+func (evl *eventListener) SetRemoved() {
+	evl.mtx.Lock()
+	defer evl.mtx.Unlock()
+	evl.removed = true
+}