Kingfisher是swift中加载网络图片的框架，类似于SDWebImage，加载缓存网络图片，兼容macOS， IOS
一、基本使用方法：
ima1.kf.setImage(with: url)，它通过协议定义了kf，UIImageVIew实现了这个协议实际上就是本身
二、关于缓存图片
基本策略：1内存中，2磁盘中，下载最新的缓存到内存和磁盘
存在磁盘中的是image转成的data存储
在内存中存的的是NSCache，使用它的原因：
1.NSCache类结合了各种自动删除策略，以确保不会占用过多的系统内存
如果其它应用需要内存时，系统自动执行这些策略
当调用这些策略时，会从缓存中删除一些对象，以最大限度减少内存的占用。
2.NSCache是线程安全的，我们可以在不同的线程中添加、删除和查询缓存中的对象，而不需要锁定缓存区域。
不像NSMutableDictionary对象，一个缓存对象不会拷贝key对象。
NSCache [key:value(StorageObject对象里边有属性key，属性value)]
如何存在内存里的
通过一个单利对象存储，单利中有属性memoryStorage，里边有个NSCache
KingfisherManager.shared
1.增加了一个清楚内存缓存的策略
通过一个定时器定期清除过期的缓存，默认为2分钟（可以设定）清理一次
删除缓存时间大于5分钟（可以设定）的对象，保证了一个对象在内存中存在时间是5到7分钟
2.默认的缓存对象最大个数是无限大
3.内存大小的celue
// Bitmap memory cost with bytes. 计算每个图片的大小B
var cost: Int {
let pixel = Int(size.width * size.height * scale * scale)
guard let cgImage = cgImage else {
return pixel * 4
}
return pixel * cgImage.bitsPerPixel / 8
}
最大占用内存数totalCostLimit 是 物理内存的1/4
let totalMemory = ProcessInfo.processInfo.physicalMemory
let costLimit = totalMemory / 4
三、 disk磁盘中缓存策略
每次进入后台时，和将要推出应用时 清理7天（可以设置）之前的缓存，如果缓存大于设定值的最大值则删除最早缓存的data
磁盘中存取的是data
有关其中的缓存的key 是url 如果url中不包含具体图片的名称，但后台更改图片但是url地址不变时不会加载中新图片
四、可配置的设置项
通过KingfisherOptionsInfo 可以配置一些设置
eg：只从内存中取，内存中没有就直接下载，或者设置为直接下载最新的，设置请求类型等
所有内容如下：
/// Represents the available option items could be used in `KingfisherOptionsInfo`.
public enum KingfisherOptionsInfoItems {
/// Kingfisher will use the associated `ImageCache` object when handling related operations,
/// including trying to retrieve the cached images and store the downloaded image to it.
//目标图片，修饰完处理器处理完之后的图片最终图片缓存，不指定会默认创建，可以设定最大缓存数等，过期时间默认为5分钟也就是说取图片时发现时5分钟之前缓存的会重新下载图片
case targetCache(ImageCache)
/// The `ImageCache` for storing and retrieving original images. If `originalCache` is
/// contained in the options, it will be preferred for storing and retrieving original images.
/// If there is no `.originalCache` in the options, `.targetCache` will be used to store original images.
///
/// When using KingfisherManager to download and store an image, if `cacheOriginalImage` is
/// applied in the option, the original image will be stored to this `originalCache`. At the
/// same time, if a requested final image (with processor applied) cannot be found in `targetCache`,
/// Kingfisher will try to search the original image to check whether it is already there. If found,
/// it will be used and applied with the given processor. It is an optimization for not downloading
/// the same image for multiple times.
case originalCache(ImageCache)
/// Kingfisher will use the associated `ImageDownloader` object to download the requested images.
case downloader(ImageDownloader)
/// Member for animation transition when using `UIImageView`. Kingfisher will use the `ImageTransition` of
/// this enum to animate the image in if it is downloaded from web. The transition will not happen when the
/// image is retrieved from either memory or disk cache by default. If you need to do the transition even when
/// the image being retrieved from cache, set `.forceRefresh` as well.
case transition(ImageTransition)
/// Associated `Float` value will be set as the priority of image download task. The value for it should be
/// between 0.0~1.0. If this option not set, the default value (`URLSessionTask.defaultPriority`) will be used.
case downloadPriority(Float)
/// If set, Kingfisher will ignore the cache and try to fire a download task for the resource.
case forceRefresh
/// If set, Kingfisher will try to retrieve the image from memory cache first. If the image is not in memory
/// cache, then it will ignore the disk cache but download the image again from network. This is useful when
/// you want to display a changeable image behind the same url at the same app session, while avoiding download
/// it for multiple times.
case fromMemoryCacheOrRefresh
/// If set, setting the image to an image view will happen with transition even when retrieved from cache.
/// See `.transition` option for more.
case forceTransition
/// If set, Kingfisher will only cache the value in memory but not in disk.
case cacheMemoryOnly
/// If set, Kingfisher will wait for caching operation to be completed before calling the completion block.
case waitForCache
/// If set, Kingfisher will only try to retrieve the image from cache, but not from network. If the image is
/// not in cache, the image retrieving will fail with an error.
case onlyFromCache
/// Decode the image in background thread before using. It will decode the downloaded image data and do a off-screen
/// rendering to extract pixel information in background. This can speed up display, but will cost more time to
/// prepare the image for using.
case backgroundDecode
/// The associated value of this member will be used as the target queue of dispatch callbacks when
/// retrieving images from cache. If not set, Kingfisher will use main queue for callbacks.
@available(*, deprecated, message: "Use `.callbackQueue(CallbackQueue)` instead.")
case callbackDispatchQueue(DispatchQueue?)
/// The associated value will be used as the target queue of dispatch callbacks when retrieving images from
/// cache. If not set, Kingfisher will use `.mainCurrentOrAsync` for callbacks.
///
/// - Note:
/// This option does not affect the callbacks for UI related extension methods. You will always get the
/// callbacks called from main queue.
case callbackQueue(CallbackQueue)
/// The associated value will be used as the scale factor when converting retrieved data to an image.
/// Specify the image scale, instead of your screen scale. You may need to set the correct scale when you dealing
/// with 2x or 3x retina images. Otherwise, Kingfisher will convert the data to image object at `scale` 1.0.
case scaleFactor(CGFloat)
/// Whether all the animated image data should be preloaded. Default is `false`, which means only following frames
/// will be loaded on need. If `true`, all the animated image data will be loaded and decoded into memory.
///
/// This option is mainly used for back compatibility internally. You should not set it directly. Instead,
/// you should choose the image view class to control the GIF data loading. There are two classes in Kingfisher
/// support to display a GIF image. `AnimatedImageView` does not preload all data, it takes much less memory, but
/// uses more CPU when display. While a normal image view (`UIImageView` or `NSImageView`) loads all data at once,
/// which uses more memory but only decode image frames once.
case preloadAllAnimationData
/// The `ImageDownloadRequestModifier` contained will be used to change the request before it being sent.
/// This is the last chance you can modify the image download request. You can modify the request for some
/// customizing purpose, such as adding auth token to the header, do basic HTTP auth or something like url mapping.
/// The original request will be sent without any modification by default.
case requestModifier(ImageDownloadRequestModifier)
/// The `ImageDownloadRedirectHandler` contained will be used to change the request before redirection.
/// This is the posibility you can modify the image download request during redirect. You can modify the request for
/// some customizing purpose, such as adding auth token to the header, do basic HTTP auth or something like url
/// mapping.
/// The original redirection request will be sent without any modification by default.
case redirectHandler(ImageDownloadRedirectHandler)
/// Processor for processing when the downloading finishes, a processor will convert the downloaded data to an image
/// and/or apply some filter on it. If a cache is connected to the downloader (it happens when you are using
/// KingfisherManager or any of the view extension methods), the converted image will also be sent to cache as well.
/// If not set, the `DefaultImageProcessor.default` will be used.
case processor(ImageProcessor)
/// Supplies a `CacheSerializer` to convert some data to an image object for
/// retrieving from disk cache or vice versa for storing to disk cache.
/// If not set, the `DefaultCacheSerializer.default` will be used.
case cacheSerializer(CacheSerializer)
/// An `ImageModifier` is for modifying an image as needed right before it is used. If the image was fetched
/// directly from the downloader, the modifier will run directly after the `ImageProcessor`. If the image is being
/// fetched from a cache, the modifier will run after the `CacheSerializer`.
///
/// Use `ImageModifier` when you need to set properties that do not persist when caching the image on a concrete
/// type of `Image`, such as the `renderingMode` or the `alignmentInsets` of `UIImage`.
case imageModifier(ImageModifier)
/// Keep the existing image of image view while setting another image to it.
/// By setting this option, the placeholder image parameter of image view extension method
/// will be ignored and the current image will be kept while loading or downloading the new image.
case keepCurrentImageWhileLoading
/// If set, Kingfisher will only load the first frame from an animated image file as a single image.
/// Loading an animated images may take too much memory. It will be useful when you want to display a
/// static preview of the first frame from a animated image.
///
/// This option will be ignored if the target image is not animated image data.
case onlyLoadFirstFrame
/// If set and an `ImageProcessor` is used, Kingfisher will try to cache both the final result and original
/// image. Kingfisher will have a chance to use the original image when another processor is applied to the same
/// resource, instead of downloading it again. You can use `.originalCache` to specify a cache or the original
/// images if necessary.
///
/// The original image will be only cached to disk storage.
case cacheOriginalImage
/// If set and a downloading error occurred Kingfisher will set provided image (or empty)
/// in place of requested one. It's useful when you don't want to show placeholder
/// during loading time but wants to use some default image when requests will be failed.
case onFailureImage(Image?)
/// If set and used in `ImagePrefetcher`, the prefetching operation will load the images into memory storage
/// aggressively. By default this is not contained in the options, that means if the requested image is already
/// in disk cache, Kingfisher will not try to load it to memory.
case alsoPrefetchToMemory
/// If set, the disk storage loading will happen in the same calling queue. By default, disk storage file loading
/// happens in its own queue with an asynchronous dispatch behavior. Although it provides better non-blocking disk
/// loading performance, it also causes a flickering when you reload an image from disk, if the image view already
/// has an image set.
///
/// Set this options will stop that flickering by keeping all loading in the same queue (typically the UI queue
/// if you are using Kingfisher's extension methods to set an image), with a tradeoff of loading performance.
case loadDiskFileSynchronously
/// The expiration setting for memory cache. By default, the underlying `MemoryStorage.Backend` uses the
/// expiration in its config for all items. If set, the `MemoryStorage.Backend` will use this associated
/// value to overwrite the config setting for this caching item.
case memoryCacheExpiration(StorageExpiration)
/// The expiration extending setting for memory cache. The item expiration time will be incremented by this value after access.
/// By default, the underlying `MemoryStorage.Backend` uses the initial cache expiration as extending value: .cacheTime.
/// To disable extending option at all add memoryCacheAccessExtendingExpiration(.none) to options.
case memoryCacheAccessExtendingExpiration(ExpirationExtending)
/// The expiration setting for memory cache. By default, the underlying `DiskStorage.Backend` uses the
/// expiration in its config for all items. If set, the `DiskStorage.Backend` will use this associated
/// value to overwrite the config setting for this caching item.
case diskCacheExpiration(StorageExpiration)
/// Decides on which queue the image processing should happen. By default, Kingfisher uses a pre-defined serial
/// queue to process images. Use this option to change this behavior. For example, specify a `.mainCurrentOrAsync`
/// to let the image be processed in main queue to prevent a possible flickering (but with a possibility of
/// blocking the UI, especially if the processor needs a lot of time to run).
case processingQueue(CallbackQueue)
/// Enable progressive image loading, Kingfisher will use the `ImageProgressive` of
case progressiveJPEG(ImageProgressive)
}