Kingfisher-swift缓存图片框架

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

⚠️

当有新的图片请求添加到ImageView上时，SDWebImage会取消之前的url请求，Kingfisher不会取消之前的的图片请求

这一点在tabelview中要格外注意，cell中的iamgeView，如果使用Kingfisher最好自己实现取消策略

SD，自带取消前面的url sd_cancelImageLoadOperationWithKey

KingFisher可以自己实现 iamgeView.kf.cancelDownloadTask()

sd 不能直接设置动图

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)
 }