intercept scroll and intercept init

[croccio]

How can I intercept scroll? I can do it with this code
'''
{
window.PdfViewer.eventBus.on('pagechanging', function pagechange(evt) {
console.log(evt)
})
}
'''
but how can i intercept init of window.PdfViewer? wher i have to put that code? if i put it in click button it works, but i need to intercept when pdf and window.PdfViewer are loaded/setted/init