add once option to amp-position-observer

[aghassemi]

There are cases where we want amp-position-observer to only run once (e.g. fade in images). Support that option.

[ampprojectbot]

This issue seems to be in Pending Triage for awhile. @aghassemi Please triage this to an appropriate milestone.

[mustafaj17]

would really be useful, anyone found any work arounds ?

[aghassemi]

@mustafaj17 are you using it for scroll-bound animations or just triggering animations when something comes into the view? If the latter, you can work around this by not providing a first keyframe for your animation, this way the second time it plays, it would try to go from the current state (which would be the last keyframe of previous run) to last keyframe again (which would be the same) and effectively it won't do anything.