调用方式：

cell.imgView.sin_setImageWithUrl(url: "图片链接", placeholderImage: #imageLiteral(resourceName: "user_icon12"))

实现代码:

//
//  DownLoadManager.swift
//  QueueLoadImg
//
//  Created by 郑东喜 on 2018/5/19.
//  Copyright © 2018年 郑东喜. All rights reserved.
//

import UIKit
extension UIImageView {
    /// 下载图片链接
    ///
    /// - Parameter url: 图片链接
    func sin_setImageWithUrl(url:String) {
        
        sin_setImageWithUrl(url: url, placeholderImage: UIImage())
    }
    
    /// 下载图片链接
    /// - Parameter placeholderImage: 占位图
    /// - Parameter url: 图片链接
    func sin_setImageWithUrl(url:String,placeholderImage:UIImage) {
        
        self.image = placeholderImage
        SINImageDownloaderManager.shared.downloadImage(url: url) {[weak self] (image) in
            self?.image = image
        }
    }
}

extension String {
    func cacheDir() -> String {
        
        let cachePath = NSSearchPathForDirectoriesInDomains(FileManager.SearchPathDirectory.cachesDirectory, FileManager.SearchPathDomainMask.userDomainMask, true).last!
        
        let filePath = (cachePath as NSString).appendingPathComponent((self as NSString).lastPathComponent)
        
        return filePath
    }
}

class SINImageDownloaderManager {
    // 单粒
    static let shared = SINImageDownloaderManager()
    // 内存缓存图片，根据图片操作
    var cacheImages:Dictionary<String, Any> = [:]
    // 保存回调闭包
    var completionDict = [String:(image:UIImage) -> Void]()
    // 发现了swift的bug，当定义闭包时， var completion = (image:UIImage) -> Void()，这种写法到赋值后极易引发xcode代码颜色变黑，久不变回，望大神指出这种写法的错误。
    // 队列
    var queue:OperationQueue = {
        let queue = OperationQueue.init()
        queue.maxConcurrentOperationCount = 3
        return queue
    }()
    
    func downloadImage(url:String,completion:@escaping (_ image:UIImage)->Void) {
        
        completionDict[url] = completion
        
        // 从内存缓存中取图片
        let cacheImage = cacheImages[url] as! UIImage?
        if cacheImage != nil {
            if completionDict[url] != nil {
                completionDict[url]!(cacheImage!)
                return
            }
        }
        
        // 从沙盒cache中获取图片
        let cachePath = url.cacheDir()
        print(cachePath)
        let sanboxData = NSData.init(contentsOfFile: cachePath)
        
        if (sanboxData != nil) {
            let sandboxImage = UIImage.init(data: sanboxData! as Data)
            
            // 保存到内存缓存
            cacheImages[url] = sandboxImage
            // 回调
            if completionDict[url] != nil {
                completionDict[url]!(sandboxImage!)
                return
            }
        }
        
        // 内存和沙盒中均没有图片，开始下载
        queue.addOperation {
            
            if let _url = URL.init(string: url) {
                
                let downloadData = NSData.init(contentsOf: _url)
                
                if downloadData != nil {
                    let downloadImage = UIImage.init(data: downloadData! as Data)
                    // 回主线程
                    OperationQueue.main.addOperation {
                        // 回调
                        if self.completionDict[url] != nil && downloadImage != nil {
                            self.completionDict[url]!(downloadImage!)
                        }
                        // 保存到内存
                        self.cacheImages[url] = downloadImage
                        // 保存到沙盒
                        downloadData?.write(to: URL.init(fileURLWithPath: cachePath), atomically: true)
                    }
                }
            }
        }
    }
}