Getting mail sender in a Thunderbird extension

I am working on a very simple Thunderbird extension, which is supposed to alert the name of the sender along with the names of the recipients whenever a mail is sent. The problem is that gMsgCompose.compFields.from field is empty in the below snippet (the .to field works as expected), which handles the开发者_运维百科 "compose-send-message" event. What am I missing here?

function send_event_handler( evt ) {
var msgcomposeWindow = document.getElementById( "msgcomposeWindow" );
var msg_type = msgcomposeWindow.getAttribute( "msgtype" );

// do not continue unless this is an actual send event
if( !(msg_type == nsIMsgCompDeliverMode.Now || msg_type == nsIMsgCompDeliverMode.Later) )
  return;

var promptService = Components.classes["@mozilla.org/embedcomp/prompt-service;1"].getService(Components.interfaces.nsIPromptService);
promptService.alert(window, "From", gMsgCompose.compFields.from);
promptService.alert(window, "To", gMsgCompose.compFields.to);
}

window.addEventListener( "compose-send-message", send_event_handler, true );

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You can get the sending identity through the msgIdentity widget:

var identityWidget = document.getElementById('msgIdentity');
var fromIdentityKey= self.identityWidget.value;

Then, lookup identity information using the account manager:

var acctMgr = Components.classes["@mozilla.org/messenger/account-manager;1"]
                      .getService(Components.interfaces.nsIMsgAccountManager);
var accounts = acctMgr.accounts;
for (var i = 0; i < accounts.Count(); i++) {
  var account = accounts.QueryElementAt(i, Components.interfaces.nsIMsgAccount);

  var accountIdentities = account.identities;

  for(var identCount = 0; identCount < accountIdentities.Count(); identCount++) {
    var identity = accountIdentities.QueryElementAt(identCount,
                                                 Components.interfaces.nsIMsgIdentity);

    if(identity.key == fromIdentityKey) {
       // Identity found
       alert(identity.email);
    }
  }
}

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Or in TB 7.0+ more conveniently & simply as:

alert(gCurrentIdentity.email);

No need to getService() and iterate, it's already set by TB, see https://dxr.mozilla.org/comm-central/source/mail/components/compose/content/MsgComposeCommands.js#71