Force saving of PDF annotations

I am using a modified ViewSDKClient.es6 and have registered a save callback. This works perfectly when the user presses the Save button and also every 5 seconds. It also gives a warning when I try to close the tab before the PDF has been saved. I need to close one last loophole for losing changes: when I programmatically create a new ViewSDKClient for a different PDF file in the same tab, in the same space.

I could call my save callback, but I don't know how to access the fileContent manually. 

Alternatively, is there some way to have ViewSDKClient call the save function before it is garbage collected  (an equivalent of the "componentWillUnmount" function from React) ? 

Thanks a lot!