閱讀 Kingfisher (一) KingfisherManager

之前在寫Objective-C的時候，都是通過使用SDWebImage來實現圖片異步下載以及緩存的。
現在的工作環境已經完全都是使用Swift了，為了工作中能夠儘量介紹OC與Swift的混合開發，就開始了解Kingfisher這個庫了。

然而在處理圖片的時候還是會思考到下面這幾個問題：

進入畫面，需要下載多個產品的圖片。
用戶上下滑動畫面，如何避免同時觸發同一個內容的下載。
對於下載過的內容，如果進行緩存，在下次觸發下載時，可以先從緩存中讀取。
對於緩存的內容是如何管理的？

於是就有了這次研究Kingfisher的動機。

Kingfisher.swift

#if os(macOS)
    import AppKit
    public typealias Image = NSImage
    public typealias Color = NSColor
    public typealias ImageView = NSImageView
    typealias Button = NSButton
#else
    import UIKit
    public typealias Image = UIImage
    public typealias Color = UIColor
    #if !os(watchOS)
    public typealias ImageView = UIImageView
    typealias Button = UIButton
    #endif
#endif

#if os(macOS)

import AppKit

public typealias Image = NSImage

public typealias Color = NSColor

public typealias ImageView = NSImageView

typealias Button = NSButton

#else

import UIKit

public typealias Image = UIImage

public typealias Color = UIColor

#if !os(watchOS)

public typealias ImageView = UIImageView

typealias Button = UIButton

#endif

Kingfisher同時支持macOS、watchOS、iOS以及tvOS平台

他通過#if os(macOS) 判斷系統類型
typealias 為一個已存在的類別取一個別名
根據不同的系統：
為不同系統中的變量類型建立一樣的類型名稱，比如將image對應到macOS中的NSImage以及iOS中的UIImage
引入不同的framework，比如macOS中的AppKit以及iOS中的UIKit

/**
 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

/**

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

之前使用SDWebImage或者使用Swift2.x版本的Kingfisher一個設定圖片的基本方法大概是這樣：

cell.cellImageView.kf_setImageWithURL(URL, placeholderImage: nil,.....

1	cell.cellImageView.kf_setImageWithURL(URL, placeholderImage: nil,.....

但Kingfisher在Swift3出來以後，就把調用方法給修改成類似這樣：

cell.cellImageView.kf.setImage(with: url,........

1	cell.cellImageView.kf.setImage(with: url,........

讓語法更Swift化。

kingfisher%e6%9e%b6%e6%a7%8b

從這張圖開始，接口的部分相當於對UIImageView或者

UIImageView+Kingfisher.swift

KingfisherManager

在這個類裡有多個工具函數，比如setImage(with URL)等等，不過最後都會調用KingfisherManager中下面這個方法：

    public func setImage(with resource: Resource?,
                         placeholder: Image? = nil,
                         options: KingfisherOptionsInfo? = nil,
                         progressBlock: DownloadProgressBlock? = nil,
                         completionHandler: CompletionHandler? = nil) -> RetrieveImageTask

public func setImage(with resource: Resource?,

placeholder: Image? = nil,

options: KingfisherOptionsInfo? = nil,

progressBlock: DownloadProgressBlock? = nil,

completionHandler: CompletionHandler? = nil) -> RetrieveImageTask

其中Resource包含兩個屬性cacheKey以及downloadURL，其中cacheKey就是downloadURL的字符串類型，之後會被利用作為cache存儲中的key。

而這個方法中會使用到單例KingfisherManager，這個類同時管理者downloader以及cache

let task = KingfisherManager.shared.retrieveImage

1	let task = KingfisherManager.shared.retrieveImage

然後通過單例會使用到這個方法：

    public func retrieveImage(with resource: Resource,
        options: KingfisherOptionsInfo?,
        progressBlock: DownloadProgressBlock?,
        completionHandler: CompletionHandler?) -> RetrieveImageTask

public func retrieveImage(with resource: Resource,

options: KingfisherOptionsInfo?,

progressBlock: DownloadProgressBlock?,

completionHandler: CompletionHandler?) -> RetrieveImageTask

其中options可以對下載、緩存做一些配置上的調整。

progressBlock對應的是一個closure，用來監控下載的進度：

public typealias DownloadProgressBlock = ((_ receivedSize: Int64, _ totalSize: Int64) -> ())

1	public typealias DownloadProgressBlock = ((_ receivedSize: Int64, _ totalSize: Int64) -> ())

completionHandler是一個用來在完成任務後做處理的closure

RetrieveImageTask

KingfisherManager所調用的方法中有返回一個RetrieveImageTask

public class RetrieveImageTask {
    
    static let empty = RetrieveImageTask()
    
    // If task is canceled before the download task started (which means the `downloadTask` is nil),
    // the download task should not begin.
    var cancelledBeforeDownloadStarting: Bool = false
    
    /// The disk retrieve task in this image task. Kingfisher will try to look up in cache first. This task represent the cache search task.
    public var diskRetrieveTask: RetrieveImageDiskTask?
    
    /// The network retrieve task in this image task.
    public var downloadTask: RetrieveImageDownloadTask?
    
    /**
    Cancel current task. If this task does not begin or already done, do nothing.
    */
    public func cancel() {
        // From Xcode 7 beta 6, the `dispatch_block_cancel` will crash at runtime.
        // It fixed in Xcode 7.1.
        // See https://github.com/onevcat/Kingfisher/issues/99 for more.
        if let diskRetrieveTask = diskRetrieveTask {
            diskRetrieveTask.cancel()
        }
        
        if let downloadTask = downloadTask {
            downloadTask.cancel()
        } else {
            cancelledBeforeDownloadStarting = true
        }
    }
}

public class RetrieveImageTask {

static let empty = RetrieveImageTask()

// If task is canceled before the download task started (which means the `downloadTask` is nil),

// the download task should not begin.

var cancelledBeforeDownloadStarting: Bool = false

/// The disk retrieve task in this image task. Kingfisher will try to look up in cache first. This task represent the cache search task.

public var diskRetrieveTask: RetrieveImageDiskTask?

/// The network retrieve task in this image task.

public var downloadTask: RetrieveImageDownloadTask?

/**

Cancel current task. If this task does not begin or already done, do nothing.

public func cancel() {

// From Xcode 7 beta 6, the `dispatch_block_cancel` will crash at runtime.

// It fixed in Xcode 7.1.

// See https://github.com/onevcat/Kingfisher/issues/99 for more.

if let diskRetrieveTask = diskRetrieveTask {

diskRetrieveTask.cancel()

}

if let downloadTask = downloadTask {

downloadTask.cancel()

} else {

cancelledBeforeDownloadStarting = true

}

RetrieveImageTask中包括了幾個屬性

cancelledBeforeDownloadStarting – 用來記錄是否在下載前就呼叫了cancel方法
diskRetrieveTask -從Disk獲取圖片的任務
downloadTask – 從網路獲取圖片的任務
func cancel() – diskRetrieveTask以及downloadTask的取消方法

retrieveImage方法

    public func retrieveImage(with resource: Resource,
        options: KingfisherOptionsInfo?,
        progressBlock: DownloadProgressBlock?,
        completionHandler: CompletionHandler?) -> RetrieveImageTask
    {
        let task = RetrieveImageTask()

        if let options = options, options.forceRefresh {
            _ = downloadAndCacheImage(
                with: resource.downloadURL,
                forKey: resource.cacheKey,
                retrieveImageTask: task,
                progressBlock: progressBlock,
                completionHandler: completionHandler,
                options: options)
        } else {
            tryToRetrieveImageFromCache(
                forKey: resource.cacheKey,
                with: resource.downloadURL,
                retrieveImageTask: task,
                progressBlock: progressBlock,
                completionHandler: completionHandler,
                options: options)
        }
        
        return task
    }

public func retrieveImage(with resource: Resource,

options: KingfisherOptionsInfo?,

progressBlock: DownloadProgressBlock?,

completionHandler: CompletionHandler?) -> RetrieveImageTask

{

let task = RetrieveImageTask()

if let options = options, options.forceRefresh {

_ = downloadAndCacheImage(

with: resource.downloadURL,

forKey: resource.cacheKey,

retrieveImageTask: task,

progressBlock: progressBlock,

completionHandler: completionHandler,

options: options)

} else {

tryToRetrieveImageFromCache(

forKey: resource.cacheKey,

with: resource.downloadURL,

retrieveImageTask: task,

progressBlock: progressBlock,

completionHandler: completionHandler,

options: options)

}

return task

}

通過判斷option是否為設定為強制刷新來調用downloadAndCacheImage或者tryToRetrieveImageFromCache。

這裡可以看到Kingfisher將下載的部分(Downloader)給獨立了出來，這樣在其他地方就可以復用了（比如從緩存中讀取，如果不存在還是要進行下載）。

閱讀 Kingfisher (一) KingfisherManager

Kingfisher.swift

UIImageView+Kingfisher.swift

KingfisherManager

RetrieveImageTask

retrieveImage方法

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Kingfisher.swift

UIImageView+Kingfisher.swift

KingfisherManager

RetrieveImageTask

retrieveImage方法

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