Re: Proposal for a page visibility API

On Wed, Jan 19, 2011 at 9:22 PM, Alex Komoroske <komoroske@chromium.org>wrote:
>
> Use cases
>
> * A page wants to detect when it is being prerendered so it can behave
> appropriately.
> * A puzzle game has a timer that keeps track of how long the user has taken
> to solve the puzzle.  It wants to pause the timer when the user has hidden
> the tab.
> * A web app that uses polling to fetch dynamic content can pause polling
> when it knows the page is hidden from the user.
> * A streaming video site doesn’t want to start the video until the user
> actually views the tab for the first time (i.e. video shouldn’t start
> automatically if a user opens the tab in the background).
>

Another example of this use case: a browser session is restored with many
tabs.  As with loading a link in a background tab, video shouldn't start
playing.  I've lost count of the number of times I've reloaded my browser
and had a YouTube video or three start playing (somewhere among my three
browser windows and around 60-80 tabs).

Note that the Flash videos on http://www.ted.com do load paused if they're
loaded in the background, which is very helpful; I can middle-click lots of
videos in the video index into background tabs, and then view them one at a
time.  It should be possible to implement this for HTML5 video.

The same applies to anything that plays sound, like games--they shouldn't
start immediately when they're in a background tab on load.

* Values returned by all conforming implementations
>     * “visible” : the full-size page content may be at least partially
> visible on at least one screen.
>

More simply, "the full-size page content may be at least partially visible".

    * “hidden” : the full-size page content is not visible to the user at
> all.
> * Additional values potentially returned by some implementations in some
> cases
>     * “prerender” : the page is currently being loaded off-screen and might
> never be shown to the user.
>     * “cache” : the page is currently “frozen” in a cache and not displayed
> on screen (e.g. the back-forward cache).
>     * “preview” : the page is currently visible only in a lower-resolution
> thumbnail.
>

I think that at least visible and hidden should be explicitly defined as
having document.visible true and false, not technically left
implementation-defined.  Defining "prerender" and "cache" as false is
probably good too, if their descriptions specifically say "off-screen".

If you intend "preview" to include large but smaller-than-full-size
previews, eg. scaled to 50%, I'd recommend avoiding the word "thumbnail"; I
think most people wouldn't consider that a thumbnail.  (I could probably
come up with a reasonable UI where a preview is at 100%, too...)

> visibilitychange
>
> A simple event, fired at the document object immediately after
document.visibilityState transitions between visibility states.  The event
has a property, fromState, that is set to the value of
document.visibilityState just before it was changed to the current value.
 Note that visibility has nothing to do with whether the document’s contents
have fully loaded or not, which implies that for any given visibility
transition event, onload may or may not have already fired.

This should also include the old document.visibility value.

Thoughts and comments are welcome.
>

I think is very important information to make available to scripts.

-- 
Glenn Maynard