A while ago I wrote a small post on how to add alternate state support to your extension: https://extendingqlik.upper88.com/adding-alternate-state-support-to-your-qlik-sense-extension/
I am happy to tell you that with Qlik Sense november 2018 this is now obsolete. In most cases you don’t need to do anything to have alternate state support in november 2018, it will work out of the box. For example the wordcloud extension:
You will get a new section in the property panel where you can set the state to use. The default setting, ‘inherited’ will mean that the chart will inherit its state from it’s parent (currently the sheet), but you can also specify what state you want.
November 2018 also includes support for managing alternate states and the selection toolbar shows selections for all