add documentation for registerProtocolHandler

main/register-protocol-handler.js

@@ -1,9 +1,29 @@
 module.exports = function () {
   if (process.platform === 'win32') {
-    registerProtocolHandler('magnet', 'URL:BitTorrent Magnet URL', process.execPath)
+    registerProtocolHandler('magnet', 'BitTorrent Magnet URL', process.execPath)
   }
 }
 
+/**
+ * To add a protocol handler on Windows, the following keys must be added to the Windows
+ * registry:
+ *
+ * HKEY_CLASSES_ROOT
+ *    $PROTOCOL
+ *       (Default) = "URL:$NAME"
+ *       URL Protocol = ""
+ *       shell
+ *          open
+ *             command
+ *                (Default) = "$COMMAND" "%1"
+ *
+ * Source: https://msdn.microsoft.com/en-us/library/aa767914.aspx
+ *
+ * However, the "HKEY_CLASSES_ROOT" key can only be written by the Administrator user.
+ * So, we instead write to "HKEY_CURRENT_USER\Software\Classes", which is inherited by
+ * "HKEY_CLASSES_ROOT" anyway, and can be written by unprivileged users.
+ */
+
 function registerProtocolHandler (protocol, name, command) {
   var Registry = require('winreg')
 
@@ -11,7 +31,7 @@ function registerProtocolHandler (protocol, name, command) {
     hive: Registry.HKCU, // HKEY_CURRENT_USER
     key: '\\Software\\Classes\\' + protocol
   })
-  protocolKey.set('', Registry.REG_SZ, name, callback)
+  protocolKey.set('', Registry.REG_SZ, 'URL:' + name, callback)
   protocolKey.set('URL Protocol', Registry.REG_SZ, '', callback)
 
   var commandKey = new Registry({