Meteor Packages Asset all Load callback (Flow Router)

Hi I am new here in this forum,

I have packages where it need to load asset file like font (that are not available as cdn). This is my code:

api.addAssets([ 'lib/fonts/ek4gzZ-GeXAPcSbHtCeQI_esZW2xOQ-xsNqO47m55DA.woff2', .. ],["client", "server"]);

There is no problem with this code actually. However, I want to make a loader by myself (I know there are many Loader package out there). The problem is the page is display before my assets in this package are display. If i have icon fonts, the fonts still not display when the first time page are load. To make it clear, you can see my dev web How can I make sure all assets are load ? Is there any callbacks function that i need to know?