If a cache check is initiated, then canceled with cancelForDelegate: before a new cache check
is started with a different URL but the same delegate, the ongoing disk cache check would still
message the delegate despite being canceled by cancelForDelegate:. This is because it only
checked to see if the delegate was in the cacheDelegates array; it had been removed by
cancelForDelegate:, but added back by the new unrelated cache check. cacheURLs keeps track
of which specific URLs are actually requested by the delegate. If the URL from a completed
disk cache check does not match the delegate in cacheDelegates, the delegate is not messaged
(since we know that request was canceled).
The -cancelForDelegate: method was working for downloads but not local cache checks. Add some new machinery to keep track of pending cache requests and only message the delegate if it hasn't since requested cancellation.
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.
Adam Ernst
Olivier Poitrey
Jeppe Vesterbæk
Jamie Pinkham
Elio Gonzalez