Provide a simple prefetcher implementation.

[krider2010]

This functionality is available in SDWebImage which influenced this
library, and was something we needed in migrating over.

It is a simple implementation, using default managers etc. but could be
expanded upon pretty easily to use different managers, downloaders, and
queues etc.

A utility method to check for cached images (useful when deciding
whether to prefetch or not) has also been added to KingfisherManager.

All new code is tested.

[codecov-io]

Current coverage is 78.38%

Merging #249 into master will increase coverage by +0.98% as of bf24397

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+ Hit            973    1030    +57
  Partial          0       0       
  Missed         284     284       

Review entire Coverage Diff as of bf24397

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[onevcat]

Although it could be useful and a quick way to check an image cached or not, I don't think it's a good idea to have a method for it here.

I believe it should only in the ImageCache. Furthermore, since cache of KingfisherManager could be set by the users, it is not clear and might be confusing which cache is used for the checking.

Generally speaking, I cannot agree to add this here. We could use the same methods in ImageCache directly. Would you mind to remove this before this p-r could be merged?

[krider2010]

I'll happily move it to ImageCache but I feel it is important for our use case with prefetching as we do not care what the cache is doing. We let the library do that!

[onevcat]

Hi, @krider2010

Thanks for the pull request. I just took a look at it and I believe it is a cool feature that could boost the speed.

Except the comment above for the implementation detail, I have several concern and want to have a discussion.

1. The KingfisherManager type is now used as a "glue" between ImageDownloader and ImageCache. Framework users/image extension could use KingfisherManager to download and cache images without knowing the detail of downloader and cache. However, I don't think it is the situation for ImagePrefetcher. The prefetcher property is even not used in KingfisherManager. It would be better to
2. Now ImagePrefetcher is relying KingfisherManager and caching the downloaded images to the manager's cache. It is probably not a good choice. There is no chance for a user who wants to cache the prefetched images to another cache besides the manager's one.

How do you think about these?

[krider2010]

@onevcat can you confirm what it would be better to do in point 1. ? It got cut off!

As for point 2. I did mention above "It is a simple implementation, using default managers etc. but could be expanded upon pretty easily to use different managers, downloaders, and queues etc." but I had no time to do this for our needs. I think it is a perfectly acceptable initial/default case. The solution here is to expand it more which I can look at doing.

[onevcat]

Oops, sorry for the cut off. I was considering whether we should contain a prefetcher in the manager as a member. In fact at least we are not using it at all in the manager.

But yes you are right. It is already a good start point for it now. I will consider how to improve/refactor this to a better way later before it could be released. Now please just fix that obvious things and I will merge it when everything looks good!

Thanks in advance!

[krider2010]

@onevcat let me know what you think of the latest changes :)

[onevcat]

Great. Thanks for this.