IOS-多线程（NSOperation）

一、基础用法

 //
 //  ViewController.m
 //  IOS_0120_NSOperation
 //
 //  Created by ma c on 16/1/20.
 //  Copyright © 2016年 博文科技. All rights reserved.
 //

 #import "ViewController.h"

 @interface ViewController ()<UITableViewDelegate>

 @property (nonatomic, strong) UIImageView *imageView;

 @end

 @implementation ViewController
 /*
  一、简介
  1.NSOperation的作用
  配合使用NSOperation和NSOperationQueue也能实现多线程编程

  2.NSOperation和NSOperationQueue实现多线程的具体步骤
  1>先将需要执行的操作封装到一个NSOperation对象中
  2>然后将NSOperation对象添加到NSOperationQueue中
  3>系统会自动将NSOperationQueue中的NSOperation取出来
  4>将取出的NSOperation封装的操作放到一条新线程中执行

  二、NSOperation的子类
  1.NSOperation是个抽象类，并不具备封装操作的能力，必须使用它的子类

  2.使用NSOperation子类的方式有3种
  1>NSInvocationOperation
  2>NSBlockOperation
  3>自定义子类继承NSOperation，实现内部相应的方法

  三、具体使用
  1.NSInvocationOperation
  1>创建NSInvocationOperation对象
  - (id)initWithTarget:(id)target selector:(SEL)sel object:(id)arg;

  2>调用start方法开始执行操作
  - (void)start;
  一旦执行操作，就会调用target的sel方法

  3>注意
  默认情况下，调用了start方法后并不会开一条新线程去执行操作，而是在当前线程同步执行操作
  只有将NSOperation放到一个NSOperationQueue中，才会异步执行操作

  2.NSBlockOperation
  1>创建NSBlockOperation对象
  + (id)blockOperationWithBlock:(void (^)(void))block;

  2>通过addExecutionBlock:方法添加更多的操作
  - (void)addExecutionBlock:(void (^)(void))block;

  3>注意：只要NSBlockOperation封装的操作数 > 1，就会异步执行操作

 三、NSOperationQueue
  1.NSOperationQueue的作用
  NSOperation可以调用start方法来执行任务，但默认是同步执行的
  如果将NSOperation添加到NSOperationQueue（操作队列）中，系统会自动异步执行NSOperation中的操作

  2.添加操作到NSOperationQueue中
  - (void)addOperation:(NSOperation *)op;
  - (void)addOperationWithBlock:(void (^)(void))block;

  四、最大并发数
  1.什么是并发数
  同时执行的任务数
  比如，同时开3个线程执行3个任务，并发数就是3

  2.最大并发数的相关方法
  - (NSInteger)maxConcurrentOperationCount;
  - (void)setMaxConcurrentOperationCount:(NSInteger)cnt;

  五、队列的取消、暂停、恢复
  取消队列的所有操作
  - (void)cancelAllOperations;
  提示：也可以调用NSOperation的- (void)cancel方法取消单个操作

  暂停和恢复队列
  - (void)setSuspended:(BOOL)b; // YES代表暂停队列，NO代表恢复队列
  - (BOOL)isSuspended;

  六、操作优先级
  1.设置NSOperation在queue中的优先级，可以改变操作的执行优先级
  - (NSOperationQueuePriority)queuePriority;
  - (void)setQueuePriority:(NSOperationQueuePriority)p;

  2.优先级的取值
  NSOperationQueuePriorityVeryLow = -8L,
  NSOperationQueuePriorityLow = -4L,
  NSOperationQueuePriorityNormal = 0,
  NSOperationQueuePriorityHigh = 4,
  NSOperationQueuePriorityVeryHigh = 8

  七、操作的监听
  1.可以监听一个操作的执行完毕
  - (void (^)(void))completionBlock;
  - (void)setCompletionBlock:(void (^)(void))block;

  八、操作的依赖
  NSOperation之间可以设置依赖来保证执行顺序
  比如一定要让操作A执行完后，才能执行操作B，可以这么写
  [operationB addDependency:operationA]; // 操作B依赖于操作A
  可以在不同queue的NSOperation之间创建依赖关系

  注意：不能相互依赖 比如A依赖B，B依赖A

  九、第三方框架的使用建议
  1.用第三方框架的目的
  1> 开发效率：快速开发，人家封装好的一行代码顶自己写的N行
  2> 为了使用这个功能最牛逼的实现

  2.第三方框架过多，很多坏处(忽略不计)
  1> 管理、升级、更新
  2> 第三方框架有BUG，等待作者解决
  3> 第三方框架的作者不幸去世、停止更新（潜在的BUG无人解决）
  4> 感觉：自己好水

  3.比如
  流媒体：播放在线视频、音频（边下载边播放）
  非常了解音频、视频文件的格式
  每一种视频都有自己的解码方式（C\C++）

  4.总结
  1> 站在巨人的肩膀上编程
  2> 没有关系，使劲用那么比较稳定的第三方框架

 十、SDWebImage
  1.什么是SDWebImage
  iOS中著名的牛逼的网络图片处理框架
  包含的功能：图片下载、图片缓存、下载进度监听、gif处理等等
  用法极其简单，功能十分强大，大大提高了网络图片的处理效率
  国内超过90%的iOS项目都有它的影子
  2.项目地址
  https://github.com/rs/SDWebImage

  1> 常用方法
  - (void)sd_setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder;
  - (void)sd_setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder options:(SDWebImageOptions)options;
  - (void)sd_setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder completed:(SDWebImageCompletionBlock)completedBlock;
  - (void)sd_setImageWithURL:(NSURL *)url placeholderImage:(UIImage *)placeholder options:(SDWebImageOptions)options progress:(SDWebImageDownloaderProgressBlock)progressBlock completed:(SDWebImageCompletionBlock)completedBlock;

  2> 内存处理：当app接收到内存警告时
  当app接收到内存警告
  - (void)applicationDidReceiveMemoryWarning:(UIApplication *)application
  {
  SDWebImageManager *mgr = [SDWebImageManager sharedManager];

  // 1.取消正在下载的操作
  [mgr cancelAll];

  // 2.清除内存缓存
  [mgr.imageCache clearMemory];
  }
  3> SDWebImageOptions
  * SDWebImageRetryFailed : 下载失败后，会自动重新下载
  * SDWebImageLowPriority : 当正在进行UI交互时，自动暂停内部的一些下载操作
  * SDWebImageRetryFailed | SDWebImageLowPriority : 拥有上面2个功能

  十一、自定义NSOperation
  自定义NSOperation的步骤很简单
  重写- (void)main方法，在里面实现想执行的任务

  重写- (void)main方法的注意点
  自己创建自动释放池（因为如果是异步操作，无法访问主线程的自动释放池）
  经常通过- (BOOL)isCancelled方法检测操作是否被取消，对取消做出响应

  */

 - (void)viewDidLoad {
     [super viewDidLoad];

 //    [self useNSInvocationOperation];
 //    [self useBaseNSBlockOperation];
 //    [self useNSBlockOperation];
 //    [self useNSOperationQueue];
 //    [self useAddDependency];

     self.imageView = [[UIImageView alloc] initWithFrame:CGRectMake(, , , )];
     [self.view addSubview:self.imageView];
     [self communicate];

 }

 #pragma mark - 通讯
 - (void)communicate
 {
     NSOperationQueue *queue = [[NSOperationQueue alloc] init];
     //取消队列所有操作
 //    [queue cancelAllOperations];
 //    NSLog(@"didCancelAllOperations");
     //暂停队列
     [queue setSuspended:YES];
     //恢复队列
     [queue setSuspended:NO];
     //异步下载图片
     [queue addOperationWithBlock:^{
         NSURL *url = [NSURL URLWithString:@"http://images.haiwainet.cn/2016/0113/20160113015030150.jpg"];
         NSData *data = [[NSData alloc] initWithContentsOfURL:url];
         UIImage *image = [UIImage imageWithData:data];
         //回到主线程，显示图片
         [[NSOperationQueue mainQueue] addOperationWithBlock:^{
             self.imageView.image = image;
         }];
     }];
 }

 - (void)scrollViewWillBeginDragging:(UIScrollView *)scrollView
 {
 //    [queue setSuspended:YES]; //暂停队列中所有任务
 }

 - (void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate
 {
 //    [queue setSuspended:NO]; //恢复队列中所有任务
 }

 #pragma mark - 依赖
 - (void)useAddDependency
 {
     /*三个异步执行操作：操作C依赖于操作B，操作B依赖于操作A*/

     //创建一个队列
     NSOperationQueue *queue = [[NSOperationQueue alloc] init];

     NSBlockOperation *operationA = [NSBlockOperation blockOperationWithBlock:^{
         NSLog(@"A1--------%@",[NSThread currentThread]);
     }];
     [operationA addExecutionBlock:^{
         NSLog(@"A2--------%@",[NSThread currentThread]);
     }];
     [operationA setCompletionBlock:^{
         NSLog(@"依赖于A--------%@",[NSThread currentThread]);

     }];
     NSBlockOperation *operationB = [NSBlockOperation blockOperationWithBlock:^{
         NSLog(@"B--------%@",[NSThread currentThread]);
     }];
     NSBlockOperation *operationC = [NSBlockOperation blockOperationWithBlock:^{
         NSLog(@"C--------%@",[NSThread currentThread]);
     }];
     //设置依赖
     [operationB addDependency:operationA];
     [operationC addDependency:operationB];

     [queue addOperation:operationA];
     [queue addOperation:operationB];
     [queue addOperation:operationC];
 }

 #pragma mark - 队列中直接添加任务
 - (void)useNSOperationQueue
 {
     //1.创建operation的时候，添加一个任务
     NSBlockOperation *operation1 = [NSBlockOperation blockOperationWithBlock:^{
         NSLog(@"--------下载图片1--------%@",[NSThread currentThread]);
     }];
     NSBlockOperation *operation2 = [NSBlockOperation blockOperationWithBlock:^{
         NSLog(@"--------下载图片2--------%@",[NSThread currentThread]);
     }];

     //2.创建队列（非主队列）会自动异步执行任务，并发
     NSOperationQueue *queue = [[NSOperationQueue alloc] init];

     //3.设置最大并发数
     queue.maxConcurrentOperationCount = ;

     //4.添加操作到队列中
     [queue addOperation:operation1];
     [queue addOperation:operation2];

     [queue addOperationWithBlock:^{
         NSLog(@"--------下载图片3--------%@",[NSThread currentThread]);
     }];
 }

 #pragma mark - 多操作添加到队列
 - (void)useNSBlockOperation
 {
     //创建operation的时候，添加一个任务
     NSBlockOperation *operation1 = [NSBlockOperation blockOperationWithBlock:^{
         NSLog(@"--------下载图片1--------%@",[NSThread currentThread]);
     }];
     NSBlockOperation *operation2 = [NSBlockOperation blockOperationWithBlock:^{
         NSLog(@"--------下载图片2--------%@",[NSThread currentThread]);
     }];
     //创建队列
     NSOperationQueue *queue = [[NSOperationQueue alloc] init];
     //主队列
 //    NSOperationQueue *queue = [NSOperationQueue mainQueue];
     //添加操作到队列中
     [queue addOperation:operation1];
     [queue addOperation:operation2];

     //[operation start];
 }

 #pragma mark - NSBlockOperation
 - (void)useBaseNSBlockOperation
 {
     //创建operation的时候，添加一个任务
 //    NSBlockOperation *operation = [NSBlockOperation blockOperationWithBlock:^{
 //        NSLog(@"--------下载图片1--------%@",[NSThread currentThread]);
 //    }];

     NSBlockOperation *operation = [[NSBlockOperation alloc] init];
     //操作中添加任务
     [operation addExecutionBlock:^{
         NSLog(@"--------下载图片2--------%@",[NSThread currentThread]);
     }];
     [operation addExecutionBlock:^{
         NSLog(@"--------下载图片3--------%@",[NSThread currentThread]);
     }];

     //任务大于1，才会异步执行
     [operation start];
 }

 #pragma mark - NSInvocationOperation
 - (void)useNSInvocationOperation
 {
     //创建队列
     NSOperationQueue *queue = [[NSOperationQueue alloc] init];
     //创建操作
     NSInvocationOperation *operation = [[NSInvocationOperation alloc] initWithTarget:self selector:@selector(download) object:nil];

     //opreation直接调用start，是同步执行（在当前线程执行操作）
     //[operation start];

     //添加操作到队列中，会自动异步执行
     [queue addOperation:operation];
 }

 - (void)download
 {
     NSLog(@"download-------%@",[NSThread currentThread]);
 }

 - (void)didReceiveMemoryWarning {
     [super didReceiveMemoryWarning];
     // Dispose of any resources that can be recreated.
 }

 @end

二、防止重复下载，与沙盒缓存

 //
 //  BWApp.h
 //  IOS_0120_NSOperation防止重复下载
 //
 //  Created by ma c on 16/1/20.
 //  Copyright © 2016年 博文科技. All rights reserved.
 //

 #import <Foundation/Foundation.h>

 @interface BWApp : NSObject

 @property (nonatomic, copy) NSString *name;
 @property (nonatomic, copy) NSString *download;
 @property (nonatomic, copy) NSString *icon;

 + (instancetype)appWithDict:(NSDictionary *)dict;

 @end

 //
 //  BWApp.m
 //  IOS_0120_NSOperation防止重复下载
 //
 //  Created by ma c on 16/1/20.
 //  Copyright © 2016年 博文科技. All rights reserved.
 //

 #import "BWApp.h"

 @implementation BWApp

 + (instancetype)appWithDict:(NSDictionary *)dict
 {
     BWApp *app = [[self alloc] init];
     [app setValuesForKeysWithDictionary:dict];
     return app;
 }

 @end

 //
 //  BWTableViewController.m
 //  IOS_0120_NSOperation防止重复下载
 //
 //  Created by ma c on 16/1/20.
 //  Copyright © 2016年 博文科技. All rights reserved.
 //

 #import "BWTableViewController.h"
 #import "BWApp.h"

 #define appImageCachesPath(url) [[NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES) lastObject] stringByAppendingPathComponent:[url lastPathComponent]]

 @interface BWTableViewController ()

 //所有应用数据
 @property (nonatomic, strong) NSMutableArray *appsArray;
 //存放所有下载操作的队列
 @property (nonatomic, strong) NSOperationQueue *queue;
 //存放所有下载操作(url是key，operation对象是value)
 @property (nonatomic, strong) NSMutableDictionary *operationDict;
 //缓存图片
 @property (nonatomic, strong) NSMutableDictionary *imageDict;

 @end

 @implementation BWTableViewController

 #pragma mark - 懒加载
 - (NSMutableArray *)appsArray
 {
     if (!_appsArray) {
         NSMutableArray *appArr = [[NSMutableArray alloc] init];

         NSString *path = [[NSBundle mainBundle] pathForResource:@"apps" ofType:@"plist"];

         NSArray *dictArr = [NSArray arrayWithContentsOfFile:path];

         for (NSDictionary *dict in dictArr) {

             BWApp *app = [BWApp appWithDict:dict];
             [appArr addObject:app];
         }
         _appsArray = appArr;
     }
     return _appsArray;
 }

 - (NSOperationQueue *)queue
 {
     if (!_queue) {
         _queue = [[NSOperationQueue alloc] init];
     }
     return _queue;
 }

 - (NSMutableDictionary *)operationDict
 {
     if (!_operationDict) {
         _operationDict = [[NSMutableDictionary alloc] init];
     }
     return _operationDict;
 }

 - (NSMutableDictionary *)imageDict
 {
     if (!_imageDict) {
         _imageDict = [[NSMutableDictionary alloc] init];
     }
     return _imageDict;
 }

 #pragma mark - 初始化
 - (void)viewDidLoad {
     [super viewDidLoad];

 }

 - (void)didReceiveMemoryWarning
 {
     [super didReceiveMemoryWarning];

     //移除所有下载操作
     [self.queue cancelAllOperations];
     [self.operationDict removeAllObjects];
     //移除所有图片缓存
     [self.imageDict removeAllObjects];
 }

 #pragma mark - Table view data source

 - (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView {

     return ;
 }

 - (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section {

     return self.appsArray.count;
 }

 - (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
     static NSString *cellID = @"cellID";

     UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:cellID];

     if (!cell) {
         cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleSubtitle reuseIdentifier:cellID];
     }
     //取出模型
     BWApp *app = [self.appsArray objectAtIndex:indexPath.row];

     cell.textLabel.text = app.name;
     cell.detailTextLabel.text = app.download;

     //先从imageDict缓存中取出图片url对应的image
     UIImage *image = self.imageDict[app.icon];

     if (image) { //说明图片已经下载成功过
         cell.imageView.image = image;
     }
     else //说明图片未下载（没有缓存）
     {
         //获取caches路径,拼接文件路径
         NSString *filePath = appImageCachesPath(app.icon);

         NSData *data = [NSData dataWithContentsOfFile:filePath];

         if (data) { //沙盒中存在这个图片
             cell.imageView.image = [UIImage imageWithData:data];
         }
         else{
             //显示占位图片
             cell.imageView.image = [UIImage imageNamed:@"placeholder"];
         }

         [self download:app.icon andIndexPath:indexPath];
     }
     return cell;
 }

 - (void)download:(NSString *)imageUrl andIndexPath:(NSIndexPath *)indexPath
 {
     //取出当前图片对应的下载操作（operation对象）
     NSBlockOperation *operation = self.operationDict[imageUrl];

     if (operation)  return;

 //    __weak BWTableViewController *vc = self;
     __weak typeof(self) appVC = self;

     //创建操作，下载图片
     operation = [NSBlockOperation blockOperationWithBlock:^{
         NSURL *url = [NSURL URLWithString:imageUrl];
         NSData *data = [NSData dataWithContentsOfURL:url];
         UIImage *image = [UIImage imageWithData:data];

         //回到主线程
         [[NSOperationQueue mainQueue] addOperationWithBlock:^{

             //判断图片是否存在
             if (image) {
                 //存放图片到字典中
                 appVC.imageDict[imageUrl] = image;
 #warning 沙盒缓存
                 //将图片存入沙盒之中 UIImage --> NSData --> File(文件)
                 NSData *data = UIImagePNGRepresentation(image);

 //                //获取caches路径
 //                NSString *cachesPath = [NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES) lastObject];
 //
 //                //拼接文件路径
 //                NSString *fileName = [imageUrl lastPathComponent];
 //                NSString *filePath = [cachesPath stringByAppendingPathComponent:fileName];

                 //写入缓存
                 [data writeToFile:appImageCachesPath(imageUrl) atomically:YES];

                 //UIImageJPEGRepresentation(UIImage * _Nonnull image, CGFloat compressionQuality)
             }
             //从字典中移除下载操作
             [appVC.operationDict removeObjectForKey:imageUrl];
             //刷新表格
             //[self.tableView reloadData];
             [appVC.tableView reloadRowsAtIndexPaths:@[indexPath] withRowAnimation:UITableViewRowAnimationNone];
         }];
     }];
     //添加操作到队列中
     [appVC.queue addOperation:operation];
     //添加操作到字典中（为了解决重复下载）
     appVC.operationDict[imageUrl] = operation;
 }
 //保证图片只下载过一次
 //当用户开始拖拽表格时
 - (void)scrollViewWillBeginDragging:(UIScrollView *)scrollView
 {
     //暂停下载
     [self.queue setSuspended:YES];
 }
 //用户停止拖拽表格时
 - (void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate
 {
     //恢复下载
     [self.queue setSuspended:NO];
 }

 @end

三、自定义NSOperation

 //
 //  BWApp.h
 //  IOS_0120_NSOperation防止重复下载
 //
 //  Created by ma c on 16/1/20.
 //  Copyright © 2016年 博文科技. All rights reserved.
 //

 #import <Foundation/Foundation.h>

 @interface BWApp : NSObject

 @property (nonatomic, copy) NSString *name;
 @property (nonatomic, copy) NSString *download;
 @property (nonatomic, copy) NSString *icon;

 + (instancetype)appWithDict:(NSDictionary *)dict;

 @end

 //
 //  BWApp.m
 //  IOS_0120_NSOperation防止重复下载
 //
 //  Created by ma c on 16/1/20.
 //  Copyright © 2016年 博文科技. All rights reserved.
 //

 #import "BWApp.h"

 @implementation BWApp

 + (instancetype)appWithDict:(NSDictionary *)dict
 {
     BWApp *app = [[self alloc] init];
     [app setValuesForKeysWithDictionary:dict];
     return app;
 }

 @end

 //
 //  BWDownloadOperation.h
 //  IOS_0120_NSOperation防止重复下载
 //
 //  Created by ma c on 16/1/21.
 //  Copyright © 2016年 博文科技. All rights reserved.
 //

 #import <Foundation/Foundation.h>
 #import <UIKit/UIKit.h>

 @class BWDownloadOperation;

 @protocol BWDownloadOperationDelegate <NSObject>

 @optional
 - (void)downloadOperation:(BWDownloadOperation *)operation didFinishDownload:(UIImage *)image;

 @end

 @interface BWDownloadOperation : NSOperation

 @property (nonatomic, copy) NSString *imageUrl;
 @property (nonatomic, strong) NSIndexPath *indexPath;

 @property (nonatomic, weak) id<BWDownloadOperationDelegate> delegate;

 @end

 //
 //  BWDownloadOperation.m
 //  IOS_0120_NSOperation防止重复下载
 //
 //  Created by ma c on 16/1/21.
 //  Copyright © 2016年 博文科技. All rights reserved.
 //

 #import "BWDownloadOperation.h"

 @implementation BWDownloadOperation

 - (void)main
 {
     @autoreleasepool {

         if (self.isCancelled)  return;

         //创建操作，下载图片
         NSURL *url = [NSURL URLWithString:self.imageUrl];
         NSData *data = [NSData dataWithContentsOfURL:url];
         UIImage *image = [UIImage imageWithData:data];

         if (self.isCancelled)  return;

         //回到主线程
         [[NSOperationQueue mainQueue] addOperationWithBlock:^{
             if ([self.delegate respondsToSelector:@selector(downloadOperation:didFinishDownload:)]) {
                 [self.delegate downloadOperation:self didFinishDownload:image];
             }
         }];
     }
 }
 @end

 //
 //  BWTableViewController.m
 //  IOS_0120_NSOperation防止重复下载
 //
 //  Created by ma c on 16/1/20.
 //  Copyright © 2016年 博文科技. All rights reserved.
 //

 #import "BWTableViewController.h"
 #import "BWApp.h"
 #import "BWDownloadOperation.h"

 #define appImageCachesPath(url) [[NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES) lastObject] stringByAppendingPathComponent:[url lastPathComponent]]

 @interface BWTableViewController ()<BWDownloadOperationDelegate>

 //所有应用数据
 @property (nonatomic, strong) NSMutableArray *appsArray;
 //存放所有下载操作的队列
 @property (nonatomic, strong) NSOperationQueue *queue;
 //存放所有下载操作(url是key，operation对象是value)
 @property (nonatomic, strong) NSMutableDictionary *operationDict;
 //缓存图片
 @property (nonatomic, strong) NSMutableDictionary *imageDict;

 @end

 @implementation BWTableViewController

 #pragma mark - 懒加载
 - (NSMutableArray *)appsArray
 {
     if (!_appsArray) {
         NSMutableArray *appArr = [[NSMutableArray alloc] init];

         NSString *path = [[NSBundle mainBundle] pathForResource:@"apps" ofType:@"plist"];

         NSArray *dictArr = [NSArray arrayWithContentsOfFile:path];

         for (NSDictionary *dict in dictArr) {

             BWApp *app = [BWApp appWithDict:dict];
             [appArr addObject:app];
         }
         _appsArray = appArr;
     }
     return _appsArray;
 }

 - (NSOperationQueue *)queue
 {
     if (!_queue) {
         _queue = [[NSOperationQueue alloc] init];
     }
     return _queue;
 }

 - (NSMutableDictionary *)operationDict
 {
     if (!_operationDict) {
         _operationDict = [[NSMutableDictionary alloc] init];
     }
     return _operationDict;
 }

 - (NSMutableDictionary *)imageDict
 {
     if (!_imageDict) {
         _imageDict = [[NSMutableDictionary alloc] init];
     }
     return _imageDict;
 }

 #pragma mark - 初始化
 - (void)viewDidLoad {
     [super viewDidLoad];

 }

 - (void)didReceiveMemoryWarning
 {
     [super didReceiveMemoryWarning];

     //移除所有下载操作
     [self.queue cancelAllOperations];
     [self.operationDict removeAllObjects];
     //移除所有图片缓存
     [self.imageDict removeAllObjects];
 }

 #pragma mark - Table view data source

 - (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView {

     return ;
 }

 - (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section {

     return self.appsArray.count;
 }

 - (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
     static NSString *cellID = @"cellID";

     UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:cellID];

     if (!cell) {
         cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleSubtitle reuseIdentifier:cellID];
     }
     //取出模型
     BWApp *app = [self.appsArray objectAtIndex:indexPath.row];

     cell.textLabel.text = app.name;
     cell.detailTextLabel.text = app.download;

     //先从imageDict缓存中取出图片url对应的image
     UIImage *image = self.imageDict[app.icon];

     if (image) { //说明图片已经下载成功过
         cell.imageView.image = image;
     }
     else //说明图片未下载（没有缓存）
     {
         //获取caches路径,拼接文件路径
         NSString *filePath = appImageCachesPath(app.icon);

         NSData *data = [NSData dataWithContentsOfFile:filePath];

         if (data) { //沙盒中存在这个图片
             cell.imageView.image = [UIImage imageWithData:data];
         }
         else{
             //显示占位图片
             cell.imageView.image = [UIImage imageNamed:@"placeholder"];
         }

         [self download:app.icon andIndexPath:indexPath];
     }
     return cell;
 }

 - (void)download:(NSString *)imageUrl andIndexPath:(NSIndexPath *)indexPath
 {
     //取出当前图片对应的下载操作（operation对象）
     BWDownloadOperation *operation = self.operationDict[imageUrl];

     if (operation)  return;

     //创建操作下载图片
     operation = [[BWDownloadOperation alloc] init];
     operation.imageUrl = imageUrl;
     operation.indexPath = indexPath;

     //设置代理
     operation.delegate = self;

     //添加操作到队列中
     [self.queue addOperation:operation];

     //添加操作到字典中（为了解决重复下载）
     self.operationDict[imageUrl] = operation;

 }

 #pragma mark - 下载代理方法

 - (void)downloadOperation:(BWDownloadOperation *)operation didFinishDownload:(UIImage *)image
 {

     //UIImageJPEGRepresentation(<#UIImage * _Nonnull image#>, <#CGFloat compressionQuality#>)

     //判断图片是否存在
     if (image) {
         //存放图片到字典中
         self.imageDict[operation.imageUrl] = image;

 #warning 沙盒缓存
         //将图片存入沙盒之中 UIImage --> NSData --> File(文件)
         NSData *data = UIImagePNGRepresentation(image);

         //写入缓存
         [data writeToFile:appImageCachesPath(operation.imageUrl) atomically:YES];

     }
     //从字典中移除下载操作(防止operationDict的下载操作越来越大，保证下载失败后能重新下载)
     [self.operationDict removeObjectForKey:operation.imageUrl];
     //刷新表格
     //[self.tableView reloadData];
     [self.tableView reloadRowsAtIndexPaths:@[operation.indexPath] withRowAnimation:UITableViewRowAnimationNone];
 }

 //保证图片只下载过一次
 //当用户开始拖拽表格时
 - (void)scrollViewWillBeginDragging:(UIScrollView *)scrollView
 {
     //暂停下载
     [self.queue setSuspended:YES];
 }
 //用户停止拖拽表格时
 - (void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate
 {
     //恢复下载
     [self.queue setSuspended:NO];
 }

 @end

四、SDWebImage

 //
 //  BWApp.h
 //  IOS_0120_NSOperation防止重复下载
 //
 //  Created by ma c on 16/1/20.
 //  Copyright © 2016年 博文科技. All rights reserved.
 //

 #import <Foundation/Foundation.h>

 @interface BWApp : NSObject

 @property (nonatomic, copy) NSString *name;
 @property (nonatomic, copy) NSString *download;
 @property (nonatomic, copy) NSString *icon;

 + (instancetype)appWithDict:(NSDictionary *)dict;

 @end

 //
 //  BWApp.m
 //  IOS_0120_NSOperation防止重复下载
 //
 //  Created by ma c on 16/1/20.
 //  Copyright © 2016年 博文科技. All rights reserved.
 //

 #import "BWApp.h"

 @implementation BWApp

 + (instancetype)appWithDict:(NSDictionary *)dict
 {
     BWApp *app = [[self alloc] init];
     [app setValuesForKeysWithDictionary:dict];
     return app;
 }

 @end

 //
 //  BWTableViewController.m
 //  IOS_0120_NSOperation防止重复下载
 //
 //  Created by ma c on 16/1/20.
 //  Copyright © 2016年 博文科技. All rights reserved.
 //

 #import "BWTableViewController.h"
 #import "BWApp.h"
 #import "UIImageView+WebCache.h"

 @interface BWTableViewController ()

 //所有应用数据
 @property (nonatomic, strong) NSMutableArray *appsArray;

 @end

 @implementation BWTableViewController

 #pragma mark - 懒加载
 - (NSMutableArray *)appsArray
 {
     if (!_appsArray) {
         NSMutableArray *appArr = [[NSMutableArray alloc] init];

         NSString *path = [[NSBundle mainBundle] pathForResource:@"apps" ofType:@"plist"];

         NSArray *dictArr = [NSArray arrayWithContentsOfFile:path];

         for (NSDictionary *dict in dictArr) {

             BWApp *app = [BWApp appWithDict:dict];
             [appArr addObject:app];
         }
         _appsArray = appArr;
     }
     return _appsArray;
 }

 #pragma mark - 初始化
 - (void)viewDidLoad {
     [super viewDidLoad];

 }

 - (void)didReceiveMemoryWarning
 {
     [super didReceiveMemoryWarning];
 }

 #pragma mark - Table view data source

 //- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView {
 //
 //    return 1;
 //}

 - (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section {

 //    return self.appsArray.count;
     return ;
 }

 - (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {

     static NSString *cellID = @"cellID";

     UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:cellID];

     if (!cell) {
         cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleSubtitle reuseIdentifier:cellID];
     }
     //取出模型
     BWApp *app = [self.appsArray objectAtIndex:indexPath.row];
     //设置基本信息
     cell.textLabel.text = app.name;
     cell.detailTextLabel.text = app.download;

     //下载图片
 //    NSURL *url = [NSURL URLWithString:@"http://img.cnblogs.com/ad/not-to-stop-questioning.jpg"];

     NSURL *url = [NSURL URLWithString:app.icon];
     UIImage *image = [UIImage imageNamed:@"placeholder"];
 //    [cell.imageView sd_setImageWithURL:url placeholderImage:image];

     SDWebImageOptions options = SDWebImageRetryFailed | SDWebImageLowPriority;

     [cell.imageView sd_setImageWithURL:url placeholderImage:image options:options progress:^(NSInteger receivedSize, NSInteger expectedSize) {

         NSLog(@"下载进度：%f",(double)receivedSize / expectedSize);

     } completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, NSURL *imageURL) {
         NSLog(@"-----图片加载完毕-----%@",image);
     }];

     return cell;
 }

 @end

 - (void)applicationDidReceiveMemoryWarning:(UIApplication *)application
 {
     SDWebImageManager *maneger = [SDWebImageManager sharedManager];
     //1.取消正在下载的操作
     [maneger cancelAll];
     //2.清除内存缓存
     [maneger.imageCache clearMemory];
     maneger.imageCache.maxCacheAge = ***;
 }