Composing draft messages

In a Google Workspace Add-on you can create widgets that have linked actions. You can use an action to compose new email drafts, optionally filling them using information entered into the add-on UI or information from an open message. For example, you can have a button in your add-on's message UI that creates a reply to the currently opened message prepopulated with information from the add-on.

When an action that builds messages is triggered, Gmail executes a callback function to build and return the draft. Gmail then displays that draft in its UI in a standard email compose window, which the user can then edit and send as needed.

Configuring an action to build a draft message

To configure a widget to start a draft-building action when selected, you must do the following:

1. Make sure your manifest includes the action.compose scope:

   https://www.googleapis.com/auth/gmail.addons.current.action.compose

   You can use more a permissive scope instead, but should only do so if that scope is absolutely necessary.

2. Create an Action object and associate it with a callback function you define.

3. Call the widget's setComposeAction() widget handler function, providing it the Action object and specifying the ComposeEmailType.

4. Implement the callback function that executes the draft-building action. This function is given an event object as an argument. The callback function must do the following:

   1. Create a GmailDraft object.
   2. Build a ComposeActionResponse object using the ComposeActionResponseBuilder class and the GmailDraft object.
   3. Return the built ComposeActionResponse.

You can prefill the GmailDraft you create in the callback function with recipients, a subject, a message body, and attachments. To fill in the draft, data can come from any source, but typically it derives from information provided to the add-on itself, information in the open message, or information gathered from a third-party service. The event object passed to the callback function contains the open message ID and other add-on information you can use to prefill the draft.

You can create the draft as a new standalone message or a reply to an existing message. This is controlled by the ComposeEmailType enum given to the setComposeAction(). You can create reply drafts as single replies or 'reply-all' messages.

Standalone drafts

A standalone draft starts a new thread and isn't a reply to any existing message. You can create a standalone draft with one of the following Gmail service functions:

• GmailApp.createDraft(recipient, subject, body)
• GmailApp.createDraft(recipient, subject, body, options)

Reply drafts

A reply draft is part of an existing message thread. Reply drafts are either single replies that only get sent to the sender of a message or "reply all" drafts that get sent to everyone who received that message. You can create a reply draft with one of these Gmail service functions:

• GmailMessage.createDraftReply(body)
• GmailMessage.createDraftReply(body, options)
• GmailMessage.createDraftReplyAll(body)
• GmailMessage.createDraftReplyAll(body, options)
• GmailThread.createDraftReply(body)
• GmailThread.createDraftReply(body, options)
• GmailThread.createDraftReplyAll(body)
• GmailThread.createDraftReplyAll(body, options)

Example

The following code snippet shows how to assign an action that builds a reply draft to a button.

  var composeAction = CardService.newAction()
      .setFunctionName('createReplyDraft');
  var composeButton = CardService.newTextButton()
      .setText('Compose Reply')
      .setComposeAction(
          composeAction,
          CardService.ComposedEmailType.REPLY_AS_DRAFT);

  // ...

  /**
   *  Creates a draft email (with an attachment and inline image)
   *  as a reply to an existing message.
   *  @param {Object} e An event object passed by the action.
   *  @return {ComposeActionResponse}
   */
  function createReplyDraft(e) {
    // Activate temporary Gmail scopes, in this case to allow
    // a reply to be drafted.
    var accessToken = e.gmail.accessToken;
    GmailApp.setCurrentMessageAccessToken(accessToken);

    // Creates a draft reply.
    var messageId = e.gmail.messageId;
    var message = GmailApp.getMessageById(messageId);
    var draft = message.createDraftReply('',
        {
            htmlBody: "Kitten! <img src='cid:kitten'/>",
            attachments: [
              UrlFetchApp.fetch('https://example.com/images/myDog.jpg')
                  .getBlob()
            ],
            inlineImages: {
              "kitten": UrlFetchApp.fetch('https://example.com/images/myKitten.jpg')
                           .getBlob()
            }
        }
    );

    // Return a built draft response. This causes Gmail to present a
    // compose window to the user, pre-filled with the content specified
    // above.
    return CardService.newComposeActionResponseBuilder()
        .setGmailDraft(draft).build();
  }