Kingfisher源码解读，代码片段

ReadKingfisherSource.swift

///ImageView+Kingfisher.swift
/// 设置图片内部方法
func setImage(
        with source: Source?,
        placeholder: Placeholder? = nil,
        parsedOptions: KingfisherParsedOptionsInfo,
        progressBlock: DownloadProgressBlock? = nil,
        completionHandler: ((Result<RetrieveImageResult, KingfisherError>) -> Void)? = nil) -> DownloadTask?
    {
        var mutatingSelf = self
        guard let source = source else {
            mutatingSelf.placeholder = placeholder
            mutatingSelf.taskIdentifier = nil
            completionHandler?(.failure(KingfisherError.imageSettingError(reason: .emptySource)))
            return nil
        }

        var options = parsedOptions
        // 判断是否设置站位图
        let isEmptyImage = base.image == nil && self.placeholder == nil
        if !options.keepCurrentImageWhileLoading || isEmptyImage {
            // Always set placeholder while there is no image/placeholder yet.
            mutatingSelf.placeholder = placeholder
        }
        // 判断是否有加载UI
        let maybeIndicator = indicator
        maybeIndicator?.startAnimatingView()
        // 生成标志符号
        let issuedIdentifier = Source.Identifier.next()
        // 绑定标识符
        mutatingSelf.taskIdentifier = issuedIdentifier
        // 判断是否显示预加载动画
        if base.shouldPreloadAllAnimation() {
            options.preloadAllAnimationData = true
        }
        // 设置进度回调，如果有的话
        if let block = progressBlock {
            options.onDataReceived = (options.onDataReceived ?? []) + [ImageLoadingProgressSideEffect(block)]
        }
        // 判断是否设置进度显示的UI
        if let provider = ImageProgressiveProvider(options, refresh: { image in
            self.base.image = image
        }) {
            options.onDataReceived = (options.onDataReceived ?? []) + [provider]
        }

        options.onDataReceived?.forEach {
            $0.onShouldApply = { issuedIdentifier == self.taskIdentifier }
        }
        // 获取图片，监听回调
        let task = KingfisherManager.shared.retrieveImage(
            with: source,
            options: options,
            downloadTaskUpdated: {
                mutatingSelf.imageTask = $0
            },
            completionHandler: { result in
                CallbackQueue.mainCurrentOrAsync.execute {
                    // 停止加载动画，如果有的话
                    maybeIndicator?.stopAnimatingView()
                    // 判断当前任务id与回调的任务Id是否是同一个, 如果不是就是不当前控件的图片
                    guard issuedIdentifier == self.taskIdentifier else {
                        let reason: KingfisherError.ImageSettingErrorReason
                        do {
                            let value = try result.get()
                            reason = .notCurrentSourceTask(result: value, error: nil, source: source)
                        } catch {
                            reason = .notCurrentSourceTask(result: nil, error: error, source: source)
                        }
                        let error = KingfisherError.imageSettingError(reason: reason)
                        completionHandler?(.failure(error))
                        return
                    }
                    // 清除当前任务
                    mutatingSelf.imageTask = nil
                    // 清除当前任务Id
                    mutatingSelf.taskIdentifier = nil

                    switch result {
                    case .success(let value):
                        // 是否有设置转场动画, 如果没有直接设置图片
                        guard self.needsTransition(options: options, cacheType: value.cacheType) else {
                            mutatingSelf.placeholder = nil
                            self.base.image = value.image
                            completionHandler?(result)
                            return
                        }
                        // 开始转场动画，并设置图片
                        self.makeTransition(image: value.image, transition: options.transition) {
                            completionHandler?(result)
                        }

                    case .failure:
                        // 加载失败，判断是否有设置失败的图片，如果有设置，则显示失败站位图
                        if let image = options.onFailureImage {
                            self.base.image = image
                        }
                        completionHandler?(result)
                    }
                }
            }
        )
        // 为当前控件绑定下载任务
        mutatingSelf.imageTask = task
        return task
    }

/// KingfisherManager.swift

 private func retrieveImage(
        with source: Source,
        context: RetrievingContext,
        completionHandler: ((Result<RetrieveImageResult, KingfisherError>) -> Void)?) -> DownloadTask? {
        let options = context.options
        /// 判断是否需要强制刷新
        if options.forceRefresh {
            /// 加载并缓存图片
            return loadAndCacheImage(
                source: source,
                context: context,
                completionHandler: completionHandler)?.value
        } else {
            /// 从缓存中获取图片
            let loadedFromCache = retrieveImageFromCache(
                source: source,
                context: context,
                completionHandler: completionHandler)
            /// 在缓存中获取到了数据，直接返回
            if loadedFromCache {
                return nil
            }
            if options.onlyFromCache {
                let error = KingfisherError.cacheError(reason: .imageNotExisting(key: source.cacheKey))
                completionHandler?(.failure(error))
                return nil
            }
            /// 缓存中没有数据，则开始加载数据并缓存
            return loadAndCacheImage(
                source: source,
                context: context,
                completionHandler: completionHandler)?.value
        }
    }
// ImageDownloader.swift
    /// 开始下载任务
    private func startDownloadTask(
        context: DownloadingContext,
        callback: SessionDataTask.TaskCallback
    ) -> DownloadTask
    {
        /// 添加下载任务
        let downloadTask = addDownloadTask(context: context, callback: callback)
        let sessionTask = downloadTask.sessionTask
        // 如果任务已经开始，则返回
        guard !sessionTask.started else {
            return downloadTask
        }
        sessionTask.onTaskDone.delegate(on: self) { (self, done) in
            let (result, callbacks) = done
            // 通知外部已经下载了图片数据
            self.reportDidDownloadImageData(result: result, url: context.url)
            switch result {
            // 下载完成，开始将二进制数据转换为image
            case .success(let (data, response)):
                // 生成图片数据处理器
                let processor = ImageDataProcessor(
                    data: data, callbacks: callbacks, processingQueue: context.options.processingQueue
                )
                // 监听处理器处理结果回调
                processor.onImageProcessed.delegate(on: self) { (self, done) in
                    let (result, callback) = done
                    self.reportDidProcessImage(result: result, url: context.url, response: response)

                    let imageResult = result.map { ImageLoadingResult(image: $0, url: context.url, originalData: data) }
                    let queue = callback.options.callbackQueue
                    queue.execute { callback.onCompleted?.call(imageResult) }
                }
                // 开始将二进制数据转换为image
                processor.process()

            case .failure(let error):
                callbacks.forEach { callback in
                    let queue = callback.options.callbackQueue
                    queue.execute { callback.onCompleted?.call(.failure(error)) }
                }
            }
        }
        /// 通知外部即将下载图片
        reportWillDownloadImage(url: context.url, request: context.request)
        // 开始sessionTask
        sessionTask.resume()
        return downloadTask
    }
    
/// kf前缀(为什么是通过ImageView.kf.setImage的方式调用)
/// 定一个Wrapper结构体
public struct KingfisherWrapper<Base> {
    public let base: Base
    public init(_ base: Base) {
        self.base = base
    }
}

/// 定义一个协议，什么都不写
public protocol KingfisherCompatible: AnyObject { }

public protocol KingfisherCompatibleValue {}

/// 为协议扩展一个计算属性（kf的类型为KingfisherWrapper，同时指定KingfisherWrapper的泛型为当前调用者Self）
extension KingfisherCompatible {
    public var kf: KingfisherWrapper<Self> {
        get { return KingfisherWrapper(self) }
        set { }
    }
}

/// 对应的类型（UIImageView）：遵守协议
extension UIImageView: KingfisherCompatible {}

/// 扩展定义的结构，指定Base为UIImageView。 这样就相当于UIImageView中有一个计算属性kf，kf可以调用结构体Wrapper里面的方法与属性
extension KingfisherWrapper where Base == UIImageView {
    
}