Mailspring/app/internal_packages/send-later/lib/main.es6

import {
  ComponentRegistry,
  DatabaseStore,
  SyncbackMetadataTask,
  Message,
  SendActionsStore,
  Actions,
} from 'nylas-exports';
import {HasTutorialTip} from 'nylas-component-kit';

import SendLaterButton from './send-later-button';
import SendLaterStatus from './send-later-status';
import {PLUGIN_ID} from './send-later-constants'

const SendLaterButtonWithTip = HasTutorialTip(SendLaterButton, {
  title: "Send on your own schedule",
  instructions: "Schedule this message to send at the ideal time. N1 makes it easy to control the fabric of spacetime!",
});

let unlisten = null;

export function activate() {
  ComponentRegistry.register(SendLaterButtonWithTip, {role: 'Composer:ActionButton'})
  ComponentRegistry.register(SendLaterStatus, {role: 'DraftList:DraftStatus'})

  unlisten = DatabaseStore.listen((change) => {
    if (change.type !== 'metadata-expiration' || change.objectClass !== Message.name) {
      return;
    }
    for (const message of change.objects) {
      if (!message.draft) {
        continue;
      }
      const metadata = message.metadataForPluginId(PLUGIN_ID);
      if (!metadata || !metadata.expiration || metadata.expiration > new Date()) {
        continue;
      }

      // clear the metadata
      Actions.queueTask(new SyncbackMetadataTask({
        model: message,
        pluginId: PLUGIN_ID,
        value: {
          expiration: null,
        },
      }));

      // send the draft
      Actions.sendDraft(message.headerMessageId, SendActionsStore.DefaultSendActionKey)
    }
  });
}

export function deactivate() {
  ComponentRegistry.unregister(SendLaterButtonWithTip)
  ComponentRegistry.unregister(SendLaterStatus)
  unlisten();
}

export function serialize() {

}