ElementReferenceException when method chaining with Navigator

[Mark]

This code is a pared-down version of some web scraping to isolate errors I'm experiencing in a minimal implementation.  I've narrowed it down to the difference between doing Navigator.$(selector)  and  $(full selector).   

import geb.Browser
System.setProperty("webdriver.chrome.driver", 'c:\\webdrivers\\chromedriver.exe')
Browser.drive {
  go "https://www.uline.com/Product/GuidedNav?t=184360&dup=over" // direct to Shipping Boxes
  assert title == 'ULINE - Shipping Boxes'
  while (true) {
    def table = $('table.GNItemTable')
    assert table!=null && table.size()>0    
    // def theader = table.$('thead tr th')  // usually fails after one page
    def theader = $('table.GNItemTable thead tr th')  // never fails
    def next = $('a.EnabledLink', text: 'Next>')
    if (next == null || next.size() == 0) break;
    next[0].click()
  }
}

When I use method chaining; table.$('thead tr th') the code will fail with an ElementReferenceException usually on loading 2nd page.  When I use the full selector $('table.GNItemTable thead tr th') it runs consistently flawless.

[Mark]

It turns out a better work-around, since I only want to reference the first table, is to use

('table.GNItemTable',0 ).$('thead tr th')

which is a better fit for my needs, as I want to reference only the first table.

Still, any reason why the chaining form table.$('thead tr th') doesn't work?

[Alexander Kriegisch]

You are having a timing issue. 'next[0].click()' is an asynchronous action changing the page content. You should use 'waitFor' in order to make sure that the page was actually updated as expected, e.g. like this:

go "https://www.uline.com/Product/GuidedNav?t=184360&dup=over" // direct to Shipping Boxes
assert title == 'ULINE - Shipping Boxes'

def currentPage = 1
while (true) {
waitFor { $("span.GNPaging a.DisabledLink")*.text().contains(currentPage.toString()) }

def table = $('table.GNItemTable')
assert table != null && table.size() > 0

def theader = table.$('thead tr th')

def next = $('a.EnabledLink', text: 'Next>')
if (next == null || next.size() == 0) break
next[0].click()

currentPage++
}

For me this works and fixes the problem.
--
Alexander Kriegisch
https://scrum-master.de

[Mark]

Thanks for the help!   You waitFor() approach which also confirms the expected page is loaded is great!

I had previously tried someone else's suggestion, which didn't help, which was to wrap the click in a waitFor

waitFor { next[0].click() }

[Alexander Kriegisch]

Yes, it does not make sense to wait for an asynchronous action. ;-)

-- 

Alexander Kriegisch

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