EPhotoSource is an interface used to extend the functionality of
EPhotoCache. You can add an object implementing EPhotoSource to an
EPhotoCache with e_photo_cache_add_photo_source() and remove it with
e_photo_cache_remove_photo_source(). When EPhotoCache needs a photo
for an email address, it will invoke e_photo_source_get_photo() on all
available EPhotoSource objects simultaneously and select one photo.
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