This is the recommanded way clear an image for reuse (as a UITableViewCell for instance). Calling setImageWithURL:
with nil URL will have the effect to cancel an eventual currently in progress download of a thumbnail for this UIImageView.
Affecting the image property directly won't have this effect and may lead to image being re-affected once download is completed.
The helper (now called manager) is now handling the mapping between the UIImageView and its downloader.
This way we don't polute the UIImageView, and don't remove its capability to have subviews.
This change removes the automatic handling of image placeholder. The placeholder image can
be passed as second argument of setImageWithURL:placeholderImage:
The manager now handle duplicate downloads for the same URL gracefuly by sharing the same downloader
for all requestors.
Finaly, the manager handles URLs which can't create an image (HTTP error or invalid format) by flagging
them so it won't retry to download them again and again.
Olivier Poitrey