源码阅读-Kingfisher
最后更新：2018-01-16
使用教程:
1. 开始使用
-
桥接 KingFisher， 利用 KingfisherCompatible协议来处理,
此处与 SnapKit的处理方式还是有点不同， SnapKit 是使用 ConstraintViewDSL 对象来设置, 对View 来设置方法, 当然这两种方式都可以； -
设置Image 利用
扩展extension来处理/** A type that has Kingfisher extensions. */ public protocol KingfisherCompatible { associatedtype CompatibleType var kf: CompatibleType { get } } public extension KingfisherCompatible { public var kf: Kingfisher<Self> { get { return Kingfisher(self) } } } extension Image: KingfisherCompatible { } #if !os(watchOS) extension ImageView: KingfisherCompatible { } extension Button: KingfisherCompatible { } #endif extension Kingfisher where Base: ImageView {} extension Kingfisher where Base: Image {} extension Kingfisher where Base: UIApplication {} extension Kingfisher where Base: UIButton {}
此处有一个极佳的使用场景，就是可以利用这种方式将 String 、NSString、NSAttributeString转换为 NSAttributeString
-
协议 Resource
协议Resource里面定义了缓存时使用的var cacheKey: String { get }以及 下载时候使用的var downloadURL: URL { get }; 对于cacheKey没什么好说的，但是对于downloadURL, 作者定义为URL类型, 查看 Alamofire 源码，我们可以看到一个 protocol URLConvertible ,直接将 String 转换为 URL, 不知道这样做对用户来说，是否更加方便? -
协议 Placeholder
在 SDWebImage - UIImageView+WebCache中,placeholderImage为一张UIImage对象. 此处作者进行了扩展, 协议Placeholder定义了add与remove方式, 任何对象只要遵循协议即可, 作者默认实现了Image, 如果你需要一个View来充当 PlaceHolder, 你只要让你的 View 遵循这个协议即可。 -
KingfisherOptionsInfoItem &
defaultOptions
作者在源码中, 一直传递着defaultOptions值,defaultOptions是用于存储 枚举KingfisherOptionsInfoItem值， 其里面可以定义一系列的操作，最基础的如 下载downloader(ImageDownloader)与 缓存.targetCache(cache), 刚看时候，非常懵逼, 深入进去, 截取核心部分代码:precedencegroup ItemComparisonPrecedence { associativity: none higherThan: LogicalConjunctionPrecedence } infix operator <== : ItemComparisonPrecedence // This operator returns true if two `KingfisherOptionsInfoItem` enum is the same, without considering the associated values. func <== (lhs: KingfisherOptionsInfoItem, rhs: KingfisherOptionsInfoItem) -> Bool { switch (lhs, rhs) { case (.targetCache(_), .targetCache(_)): return true case (.originalCache(_), .originalCache(_)): return true case (.downloader(_), .downloader(_)): return true case (.transition(_), .transition(_)): return true case (.downloadPriority(_), .downloadPriority(_)): return true case (.forceRefresh, .forceRefresh): return true case (.fromMemoryCacheOrRefresh, .fromMemoryCacheOrRefresh): return true case (.forceTransition, .forceTransition): return true case (.cacheMemoryOnly, .cacheMemoryOnly): return true case (.onlyFromCache, .onlyFromCache): return true case (.backgroundDecode, .backgroundDecode): return true case (.callbackDispatchQueue(_), .callbackDispatchQueue(_)): return true case (.scaleFactor(_), .scaleFactor(_)): return true case (.preloadAllAnimationData, .preloadAllAnimationData): return true case (.requestModifier(_), .requestModifier(_)): return true case (.processor(_), .processor(_)): return true case (.cacheSerializer(_), .cacheSerializer(_)): return true case (.imageModifier(_), .imageModifier(_)): return true case (.keepCurrentImageWhileLoading, .keepCurrentImageWhileLoading): return true case (.onlyLoadFirstFrame, .onlyLoadFirstFrame): return true case (.cacheOriginalImage, .cacheOriginalImage): return true default: return false } } extension Collection where Iterator.Element == KingfisherOptionsInfoItem { func lastMatchIgnoringAssociatedValue(_ target: Iterator.Element) -> Iterator.Element? { return reversed().first { $0 <== target } } func removeAllMatchesIgnoringAssociatedValue(_ target: Iterator.Element) -> [Iterator.Element] { return filter { !($0 <== target) } } } public extension Collection where Iterator.Element == KingfisherOptionsInfoItem { /// The `ImageDownloader` which is specified. public var downloader: ImageDownloader { if let item = lastMatchIgnoringAssociatedValue(.downloader(.default)), case .downloader(let downloader) = item { return downloader } return ImageDownloader.default } /// Or the placeholder will be used while downloading. public var keepCurrentImageWhileLoading: Bool { return contains { $0 <== .keepCurrentImageWhileLoading } } }此处作者重载了操作符, 更多内容可以参考SwiftTips-操作符、 Apple官方-operator-precedence、Operator Declarations; 取值的时候，先
reversed(), 然后在取·first(), 是不是觉得很妙?
哦， 顺便提一句, 代码中的if case 语法可以参考: 模式匹配第四弹：if case，guard case，for case考虑一下: 作者这个做法根 Optionset 实现很像，能否使用 Optionset 来处理 KingfisherOptionsInfoItem 呢？
2. 下载图片 DownloadImage
前面说了这么多, 还没提到真正下载的部分, 在extension Kingfisher where Base: ImageView{} 中，通过调用 KingfisherManager.shared.retrieveImage() 方法来下载, 下载的任务顺利转交到 KingfisherManager去了. KingfisherManager通过 options.forceRefresh 判断是直接去下载图片 (ImageDownloader)还是 去从缓存(ImageCache)中获取;
2.1 ImageDownloader 下载图片
ImageDownloader是KingFisher中的下载器。 简单查看一下文档, 定义了一个内部类: ImageFetchLoad, URLSession 以及相关的配置, 三个DispatchQueue。 值得一提的是, 喵神在设计NSURLSessionTaskDelegate的时候, 单独设计出一个ImageDownloaderSessionHandler, 原因可以查看issues-235。
现在, 让我们一起来看一下具体的实现吧, 里面的核心方法是:
open func downloadImage(with url: URL,
retrieveImageTask: RetrieveImageTask? = nil,
options: KingfisherOptionsInfo? = nil,
progressBlock: ImageDownloaderProgressBlock? = nil,
completionHandler: ImageDownloaderCompletionHandler? = nil) -> RetrieveImageDownloadTask? { }
前面几个是构建 URLRequest, 对 URLRequest进行modifier, 对 url进行判断等一系列操作, 在这个方法里面调用了-setup: 方法:
func setup(progressBlock: ImageDownloaderProgressBlock?, with completionHandler: ImageDownloaderCompletionHandler?, for url: URL, options: KingfisherOptionsInfo?, started: @escaping ((URLSession, ImageFetchLoad) -> Void)) {
func prepareFetchLoad() {
barrierQueue.sync(flags: .barrier) {
let loadObjectForURL = fetchLoads[url] ?? ImageFetchLoad()
let callbackPair = (progressBlock: progressBlock, completionHandler: completionHandler)
loadObjectForURL.contents.append((callbackPair, options ?? KingfisherEmptyOptionsInfo))
fetchLoads[url] = loadObjectForURL
if let session = session {
started(session, loadObjectForURL)
}
}
}
if let fetchLoad = fetchLoad(for: url), fetchLoad.downloadTaskCount == 0 {
if fetchLoad.cancelSemaphore == nil {
fetchLoad.cancelSemaphore = DispatchSemaphore(value: 0)
}
cancelQueue.async {
_ = fetchLoad.cancelSemaphore?.wait(timeout: .distantFuture)
fetchLoad.cancelSemaphore = nil
prepareFetchLoad()
}
} else {
prepareFetchLoad()
}
}
2.1.1 dispatch_barrier_sync
现在我们好好分析这部分的代码, 一开始, 调用if let fetchLoad = fetchLoad(for: url), 我们进入这个方法:
func fetchLoad(for url: URL) -> ImageFetchLoad? {
var fetchLoad: ImageFetchLoad?
barrierQueue.sync(flags: .barrier) { fetchLoad = fetchLoads[url] }
return fetchLoad
}
可以看到有一个 barrierQueue.sync(flags: .barrier) {}, 这是一个栅栏, 如果你曾经看过 SDWebImage的源码, 你可以在里面看到 dispatch_barrier_sync; 这个保证了线程安全, 可以查看:SDWebImage-关于dispatch_barrier_sync的issue 以及 Kingfisher-关于线程安全问题;
看到这里,你也就明白为什么用 dispatch_barrier_sync了吧。 在作者初始化的时候, 使用的是 barrierQueue = DispatchQueue(label: "com.onevcat.Kingfisher.ImageDownloader.Barrier.\(name)", attributes: .concurrent), 一个并发的队列, 我一开始没想明白为什么会这样。后来在查看官方文档中看到这么一段话:
The queue you specify should be a concurrent queue that you create yourself using the dispatch_queue_create function. If the queue you pass to this function is a serial queue or one of the global concurrent queues, this function behaves like the dispatch_sync function.
延伸阅读:通过GCD中的dispatch_barrier_(a)sync加强对sync中所谓等待的理解
2.1.2 DispatchSemaphore 信号量
接下来存在着一段这样的代码:
if fetchLoad.cancelSemaphore == nil {
fetchLoad.cancelSemaphore = DispatchSemaphore(value: 0)
}
cancelQueue.async {
_ = fetchLoad.cancelSemaphore?.wait(timeout: .distantFuture)
fetchLoad.cancelSemaphore = nil
prepareFetchLoad()
}
简答说一下信号量, 可以去看我的简易的测试文件
- 信号量 value 为0的时候,阻塞当前线程, value大于0的时候,当前线程执行;
- 当执行
semaphonre.wait()的时候, value值减一; - 当执行
semaphonre.signal()的时候, value值加一; - 初始化的时候, value值不能小于0,
wait()与signal()必须配对;
那我们来分析这段代码, 初始化DispatchSemaphore(value: 0)阻塞了, 那么在接下来 _ = fetchLoad.cancelSemaphore?.wait(timeout: .distantFuture) 一直阻塞这里。 当下载失败的时候, 调用leftSignal = fetchLoad.cancelSemaphore?.signal() ?? 0。 如果下载成功了, 会直接 self.cleanFetchLoad(for: url); 如果没有失败, 是不是感觉会一直阻塞着?
当然不会, 这是因为, 当你取消任务的时候func cancelDownloadingTask(_ task: RetrieveImageDownloadTask), task.internalTask.cancel() 会发通知给func urlSession(_ session: URLSession, task: URLSessionTask, didCompleteWithError error: Error?) , 但不是立即的，系统在后台做了一些不为人知的事情, 如果就在此时, 同样的url请求进来了,那么你就需要阻塞住, 等前面的取消任务完成再执行。
2.1.3 prepareFetchLoad()
func prepareFetchLoad() {
barrierQueue.sync(flags: .barrier) {
let loadObjectForURL = fetchLoads[url] ?? ImageFetchLoad()
let callbackPair = (progressBlock: progressBlock, completionHandler: completionHandler)
loadObjectForURL.contents.append((callbackPair, options ?? KingfisherEmptyOptionsInfo))
fetchLoads[url] = loadObjectForURL
if let session = session {
started(session, loadObjectForURL)
}
}
}
首先查看 fetchLoads里面是否有对应的 ImageFetchLoad 对象, 然后将回调数组保存: loadObjectForURL.contents.append((callbackPair, options ?? KingfisherEmptyOptionsInfo)); 这样就可以使得一个url, 下载一次, 但是可以多个回调的问题;
2.1.4 下载完成处理 processImage
代码太长, 就不贴了, 你可以去文件查看;
异步对下载好的图片数据进行处理, 同一个url 可能会有多个回调，因此遍历来处理, 处理完成之后回调回去;
至此, ImageDownloader的任务已经结束了;
2.1 ImageCache 缓存图片
如果你仔细看过 KingfisherOptionsInfoItem枚举, 你会发现存在两个Cache: targetCache(ImageCache) & originalCache(ImageCache);
targetCache(ImageCache)是用来缓存最终的图片的, 例如你下载好一张原图之后, 你利用 ImageProcessor 进行了处理, 例如缩小到原来的一半, 处理后的图片就是利用 targetCache(ImageCache) 来处理;
originalCache(ImageCache) 就是缓存原图;
虽然枚举值不一样, 但是都是用 ImageCache来处理;
ImageCache利用的是 NSCache 来缓存图片.
2.2.1 存储图片
首先将图片存储在缓存 NSCache中, 如果需要存储在磁盘上，利用串行队列异步的进行存储原图;
open func store(_ image: Image,
original: Data? = nil,
forKey key: String,
processorIdentifier identifier: String = "",
cacheSerializer serializer: CacheSerializer = DefaultCacheSerializer.default,
toDisk: Bool = true,
completionHandler: (() -> Void)? = nil)
{
let computedKey = key.computedKey(with: identifier)
memoryCache.setObject(image, forKey: computedKey as NSString, cost: image.kf.imageCost)
func callHandlerInMainQueue() {
if let handler = completionHandler {
DispatchQueue.main.async {
handler()
}
}
}
if toDisk {
ioQueue.async {
if let data = serializer.data(with: image, original: original) {
if !self.fileManager.fileExists(atPath: self.diskCachePath) {
do {
try self.fileManager.createDirectory(atPath: self.diskCachePath, withIntermediateDirectories: true, attributes: nil)
} catch _ {}
}
self.fileManager.createFile(atPath: self.cachePath(forComputedKey: computedKey), contents: data, attributes: nil)
}
callHandlerInMainQueue()
}
} else {
callHandlerInMainQueue()
}
}
延伸阅读: NSCache
2.2.2 获取图片
获取图片首先从内存中获取，如果没有，在根据条件判断是否从磁盘上获取,
@discardableResult
open func retrieveImage(forKey key: String,
options: KingfisherOptionsInfo?,
completionHandler: ((Image?, CacheType) -> Void)?) -> RetrieveImageDiskTask?
{
// No completion handler. Not start working and early return.
guard let completionHandler = completionHandler else {
return nil
}
var block: RetrieveImageDiskTask?
let options = options ?? KingfisherEmptyOptionsInfo
let imageModifier = options.imageModifier
if let image = self.retrieveImageInMemoryCache(forKey: key, options: options) {
options.callbackDispatchQueue.safeAsync {
completionHandler(imageModifier.modify(image), .memory)
}
} else if options.fromMemoryCacheOrRefresh { // Only allows to get images from memory cache.
options.callbackDispatchQueue.safeAsync {
completionHandler(nil, .none)
}
} else {
var sSelf: ImageCache! = self
block = DispatchWorkItem(block: {
// Begin to load image from disk
if let image = sSelf.retrieveImageInDiskCache(forKey: key, options: options) {
if options.backgroundDecode {
sSelf.processQueue.async {
let result = image.kf.decoded
sSelf.store(result,
forKey: key,
processorIdentifier: options.processor.identifier,
cacheSerializer: options.cacheSerializer,
toDisk: false,
completionHandler: nil)
options.callbackDispatchQueue.safeAsync {
completionHandler(imageModifier.modify(result), .memory)
sSelf = nil
}
}
} else {
sSelf.store(image,
forKey: key,
processorIdentifier: options.processor.identifier,
cacheSerializer: options.cacheSerializer,
toDisk: false,
completionHandler: nil
)
options.callbackDispatchQueue.safeAsync {
completionHandler(imageModifier.modify(image), .disk)
sSelf = nil
}
}
} else {
// No image found from either memory or disk
options.callbackDispatchQueue.safeAsync {
completionHandler(nil, .none)
sSelf = nil
}
}
})
sSelf.ioQueue.async(execute: block!)
}
return block
}
关于 DispatchWorkItem相关资料，你可以看这里
2.2.3 删除图片
作者给我们提供了如下方法来删除内存和缓存的图片
open func removeImage(forKey key: String,
processorIdentifier identifier: String = "",
fromMemory: Bool = true,
fromDisk: Bool = true,
completionHandler: (() -> Void)? = nil) {}
@objc public func clearMemoryCache() {}
open func clearDiskCache(completion handler: (()->())? = nil) {}
// 删除过期的缓存的文件
open func cleanExpiredDiskCache(completion handler: (()->())? = nil) {}
在清除过期缓存文件的时候,作者将过期的文件全部删除, 如果超过了磁盘文件的大小，也会按照使用的顺序来进行删除.
open func cleanExpiredDiskCache(completion handler: (()->())? = nil) {
// Do things in cocurrent io queue
ioQueue.async {
var (URLsToDelete, diskCacheSize, cachedFiles) = self.travelCachedFiles(onlyForCacheSize: false)
for fileURL in URLsToDelete {
do {
try self.fileManager.removeItem(at: fileURL)
} catch _ { }
}
if self.maxDiskCacheSize > 0 && diskCacheSize > self.maxDiskCacheSize {
let targetSize = self.maxDiskCacheSize / 2
// Sort files by last modify date. We want to clean from the oldest files.
let sortedFiles = cachedFiles.keysSortedByValue {
resourceValue1, resourceValue2 -> Bool in
if let date1 = resourceValue1.contentAccessDate,
let date2 = resourceValue2.contentAccessDate
{
return date1.compare(date2) == .orderedAscending
}
// Not valid date information. This should not happen. Just in case.
return true
}
for fileURL in sortedFiles {
do {
try self.fileManager.removeItem(at: fileURL)
} catch { }
URLsToDelete.append(fileURL)
if let fileSize = cachedFiles[fileURL]?.totalFileAllocatedSize {
diskCacheSize -= UInt(fileSize)
}
if diskCacheSize < targetSize {
break
}
}
}
DispatchQueue.main.async {
if URLsToDelete.count != 0 {
let cleanedHashes = URLsToDelete.map { $0.lastPathComponent }
NotificationCenter.default.post(name: .KingfisherDidCleanDiskCache, object: self, userInfo: [KingfisherDiskCacheCleanedHashKey: cleanedHashes])
}
handler?()
}
}
}
fileprivate func travelCachedFiles(onlyForCacheSize: Bool) -> (urlsToDelete: [URL], diskCacheSize: UInt, cachedFiles: [URL: URLResourceValues]) {
let diskCacheURL = URL(fileURLWithPath: diskCachePath)
let resourceKeys: Set<URLResourceKey> = [.isDirectoryKey, .contentAccessDateKey, .totalFileAllocatedSizeKey]
let expiredDate: Date? = (maxCachePeriodInSecond < 0) ? nil : Date(timeIntervalSinceNow: -maxCachePeriodInSecond)
var cachedFiles = [URL: URLResourceValues]()
var urlsToDelete = [URL]()
var diskCacheSize: UInt = 0
for fileUrl in (try? fileManager.contentsOfDirectory(at: diskCacheURL, includingPropertiesForKeys: Array(resourceKeys), options: .skipsHiddenFiles)) ?? [] {
do {
let resourceValues = try fileUrl.resourceValues(forKeys: resourceKeys)
// If it is a Directory. Continue to next file URL.
if resourceValues.isDirectory == true {
continue
}
// If this file is expired, add it to URLsToDelete
if !onlyForCacheSize,
let expiredDate = expiredDate,
let lastAccessData = resourceValues.contentAccessDate,
(lastAccessData as NSDate).laterDate(expiredDate) == expiredDate
{
urlsToDelete.append(fileUrl)
continue
}
if let fileSize = resourceValues.totalFileAllocatedSize {
diskCacheSize += UInt(fileSize)
if !onlyForCacheSize {
cachedFiles[fileUrl] = resourceValues
}
}
} catch _ { }
}
return (urlsToDelete, diskCacheSize, cachedFiles)
}
