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Re: [NavigationTiming] few comments

From: Olli Pettay <Olli.Pettay@helsinki.fi>
Date: Sat, 19 Mar 2011 22:53:59 +0200
Message-ID: <4D8517E7.3000207@helsinki.fi>
To: public-web-perf@w3.org
Some more comments


"Some user agents maintain the DOM structure of the document in memory 
during navigation operations such as forward and backward. In those 
cases, the window.performance.timing and window.performance.navigation 
objects must not be altered during the navigation."
What does this actually mean?

If bfcache (which keeps DOM in memory for session history) is enabled
and page A is first loaded normally and the window has
window.performance.<a> values. Then page B is loaded and window gets 
window.performance.<b>. Then page C is loaded and window gets
values window.performance.<c>.
Then 'back' button is pressed so that B is again the active document.
Does "the window.performance.timing and window.performance.navigation 
objects must not be altered during the navigation" mean that
window.performance.* point to <c> or <b>?
I assume <b>. And <b>'s unload event timing refers to the
unload event of page A.
(window.performance.navigation.type would never be TYPE_BACK_FORWARD in 
this scenario.)
But it would be better to clarify this all.

If bfcache is disabled and 'back' button is pressed,
window.performance.* will get values <b'>, the navigation.type
is TYPE_BACK_FORWARD and <b'>'s unload event timing refers to the
unload event of page C. So the window.performance.* will be different
depending on whether DOM was cached. And at least in case
of Gecko web page cannot know whether some page will
be bfcached, since it depends not only beforeunload/unload event 
handlers, but also possibly open network connections and also
timing (cached DOM will be evicted after X minutes)


So this all needs clarifications. Otherwise writing any cross-browser
code for window.performance.* is difficult.
And even writing reliable tests for this all isn't possible.



-Olli
Received on Saturday, 19 March 2011 20:54:33 GMT

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