Move to a callback for notifications.

Rather than using a channel for notifictions, use a callback instead.
There are several issues that arise with async notifications via a
channel, so use a callback instead.  The caller can always make the
notifications async by using channels in the provided callback if
needed.

chain.go

@@ -144,7 +144,7 @@ func removeChildNode(children []*blockNode, node *blockNode) []*blockNode {
 type BlockChain struct {
 	db            btcdb.Db
 	btcnet        btcwire.BitcoinNet
-	notifications chan *Notification
+	notifications NotificationCallback
 	root          *blockNode
 	bestChain     *blockNode
 	index         map[btcwire.ShaHash]*blockNode
@@ -960,12 +960,12 @@ func (b *BlockChain) connectBestChain(node *blockNode, block *btcutil.Block) err
 }
 
 // New returns a BlockChain instance for the passed bitcoin network using the
-// provided backing database.  It accepts a channel on which asynchronous
+// provided backing database.  It accepts a callback on which notifications
-// notifications will be sent when various events take place.  See the
+// will be sent when various events take place.  See the documentation for
-// documentation for Notification and NotificationType for details on the
+// Notification and NotificationType for details on the types and contents of
-// types and contents of notifications.  The provided channel can be nil if the
+// notifications.  The provided callback can be nil if the caller is not
-// caller is not interested in receiving notifications.
+// interested in receiving notifications.
-func New(db btcdb.Db, btcnet btcwire.BitcoinNet, c chan *Notification) *BlockChain {
+func New(db btcdb.Db, btcnet btcwire.BitcoinNet, c NotificationCallback) *BlockChain {
 	b := BlockChain{
 		db:            db,
 		btcnet:        btcnet,

notifications.go

@@ -11,6 +11,10 @@ import (
 // NotificationType represents the type of a notification message.
 type NotificationType int
 
+// NotificationCallback is used for a caller to provide a callback for
+// notifications about various chain events.
+type NotificationCallback func(*Notification)
+
 // Constants for the type of a notification message.
 const (
 	// NTOrphanBlock indicates an orphan block was processed and the
@@ -50,10 +54,9 @@ func (n NotificationType) String() string {
 	return fmt.Sprintf("Unknown Notification Type (%d)", int(n))
 }
 
-// Notification defines an asynchronous notification that is sent to the caller
+// Notification defines notification that is sent to the caller via the callback
-// over the notification channel provided during the call to New and consists
+// function provided during the call to New and consists of a notification type
-// of a notification type as well as associated data that depends on the type as
+// as well as associated data that depends on the type as follows:
-// follows:
 // 	- NTOrphanBlock:       *btcwire.ShaHash
 // 	- NTBlockAccepted:     *btcutil.Block
 // 	- NTBlockConnected:    *btcutil.Block
@@ -64,7 +67,8 @@ type Notification struct {
 }
 
 // sendNotification sends a notification with the passed type and data if the
-// caller requested notifications by providing a channel in the call to New.
+// caller requested notifications by providing a callback function in the call
+// to New.
 func (b *BlockChain) sendNotification(typ NotificationType, data interface{}) {
 	// Ignore it if the caller didn't request notifications.
 	if b.notifications == nil {
@@ -73,5 +77,5 @@ func (b *BlockChain) sendNotification(typ NotificationType, data interface{}) {
 
 	// Generate and send the notification.
 	n := Notification{Type: typ, Data: data}
-	b.notifications <- &n
+	b.notifications(&n)
 }