SDWebImagePrefetcher option to only store the loaded images in the disk cache (not memory)

[edenman]

New Issue Checklist

• [yup] I have read and understood the CONTRIBUTING guide
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Issue Info

Info | Value |
-------------------------|-------------------------------------|
Platform Name | ios
Platform Version | 11.4.1
SDWebImage Version | 4.3.3
Integration Method | cocoapods
Xcode Version | Xcode 9

Issue Description and Steps

I love SDWebImagePrefetcher: I have some very large images that I know I'll need to download if the user goes to a particular screen in my app. I'd like to ensure those images are downloaded, but I _don't_ want to keep them in memory unless the user actually goes to that screen. I see .cacheMemoryOnly as an option....any chance a .cacheDiskOnly option could be added? Happy to submit a PR if you're open to it.

[dreampiggy]

See 5.x's SDWebImageContextStoreCacheType feauture. You can pass .disk and let it store disk cache only.

imageView.sd_setImage(withURL: url, placeholderImage: nil, context: [.storeCacheType: SDImageCacheType.disk])

In 4.x, however we have no plan to add this.

[dreampiggy]

For SDWebImagePrefetcher, use the context property with the context option SDWebImageContextStoreCacheType.

So, close this issue due the the solution above.