The method I use is to have a Boolean field and a observeField that monitors for changes.
For example, I have a field for In app purchases. When the store cycle completes it pushes a "True" value to the field, then the observe Field runs a callback function to send that data to Google Analytics.
Admitedly, I'm not a "Pro" B/S developer, so there may be a better way, but it works for me.
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