工具类：给 ImageView 类增加下载图片的功能（模仿 SDWebImage/runtime 用法1：动态添加属性）

#import <UIKit/UIKit.h>

@interface DownLoadImageManager : NSObject
+ (instancetype)shareManager;
- (void)downloadOperationWithURLString:(NSString *)URLString finish:(void(^)(UIImage *image))finish;
- (void)cancelDownload:(NSString *)URLString;
// 内存警告的时候用来清除内存
- (void)clearMemory;
@end


#import "DownLoadImageManager.h"
#import "DownLoadOperation.h"
#import "NSString+Path.h"
@interface DownLoadImageManager ()
@property (nonatomic, strong) NSOperationQueue *queue; // 队列
@property (nonatomic, strong) NSMutableDictionary *operationCache; // 操作缓存池
@property (nonatomic, strong) NSMutableDictionary *imageCache; // 图片缓存池
@end

@implementation DownLoadImageManager
+ (instancetype)shareManager {
    static dispatch_once_t onceToken;
    static id instance;
    dispatch_once(&onceToken, ^{
        instance = [[self alloc]init];
    });
    return instance;
}

- (void)downloadOperationWithURLString:(NSString *)URLString finish:(void (^)(UIImage *))finish {

    NSAssert(finish != nil, @"完成回调不能为空");

    // 判断有没有缓存
    if ([self checkCache:URLString]) { // 有缓存
        // 不需要管缓存是以什么形式过来的，只需要从内存中读取图片就可以
        UIImage *image = [self.imageCache objectForKey:URLString];
        finish(image);
        return;
    }

    // 创建操作
    DownLoadOperation * op = [DownLoadOperation downloadOperationWithURLString:URLString finish:^(UIImage *image) {
        // 移除操作
        [self.operationCache removeObjectForKey:URLString];
        // 把图片缓存起来
        if (image) {
            [self.imageCache setObject:image forKey:URLString];
        }

        finish(image);
    }];

    // 添加到队列
    [self.queue addOperation:op];

    // 把操作缓存起来
    [self.operationCache setObject:op forKey:URLString];
}


- (BOOL)checkCache:(NSString *)URLString {

    // 判断是否有缓存
    // 先判断内存缓存，再判断沙盒缓存
    if ([self.imageCache objectForKey:URLString]) {
        NSLog(@"内存缓存");
        // 有缓存
        return YES;
    }

    // 判断沙盒
    // 取出沙盒缓存
    NSString *path = [URLString appendCache];
    NSData *data = [NSData dataWithContentsOfFile:path];
    UIImage *image = [UIImage imageWithData:data];

    if (image) {
        NSLog(@"沙盒缓存");
        // 缓存到内存
        [self.imageCache setObject:image forKey:URLString];
        return YES;
    }

    return NO;
}


// 取消操作
- (void)cancelDownload:(NSString *)URLString {
    // 取出对应的操作
    DownLoadOperation *op = [self.operationCache objectForKey:URLString];

    [op cancel];
}

- (NSMutableDictionary *)operationCache {
    if (_operationCache == nil) {
        _operationCache = [NSMutableDictionary dictionary];
    }
    return _operationCache;
}

- (NSMutableDictionary *)imageCache {
    if (_imageCache == nil) {
        _imageCache = [NSMutableDictionary dictionary];
    }
    return _imageCache;
}

- (NSOperationQueue *)queue {
    if (_queue == nil) {
        _queue = [[NSOperationQueue alloc]init];
        // SDWebImage 也是 6
        _queue.maxConcurrentOperationCount = 6;
    }
    return _queue;
}


- (void)clearMemory {
    [self.imageCache removeAllObjects];
    [self.queue cancelAllOperations];
}

@end

#import <UIKit/UIKit.h>

@interface DownLoadOperation : NSOperation

// 创建一个下载操作
+ (instancetype)downloadOperationWithURLString:(NSString *)URLString finish:(void(^)(UIImage *image))finish;


@end



#import "DownLoadOperation.h"
#import "NSString+Path.h"
@interface DownLoadOperation ()
// 图片下载的URLString
@property (nonatomic, copy) NSString *URLString;
// 下载完成之后的回调
@property (nonatomic, copy) void(^finish)(UIImage *image);
@end

@implementation DownLoadOperation
// 线程入口
- (void)main {
    // 需要添加自动释放池
    @autoreleasepool {
        // 断言 （只在debug模式下有作用）在上架之后不起作用
        NSAssert(self.finish != nil, @"完成回调不能为空");
        // 下载操作
        // 首先要有网址
        NSURL *url = [NSURL URLWithString:self.URLString];



        // 接收二进制数据
       NSData *data = [NSData dataWithContentsOfURL:url];

        // 如果有数据就保存图片
        if (data) {
            NSLog(@"%@",NSHomeDirectory());
            // 2. 沙盒缓存
            [data writeToFile:[self.URLString appendCache] atomically:YES];
        }

        // 关键节点判断 (耗时操作)
        if (self.isCancelled) {
            NSLog(@"操作已经被取消");
            // 操作已经被取消
            return;
        }


        UIImage *image = [UIImage imageWithData:data];
        // 完成之后的回调
        // 下载完成之后，回到主线程调用
        [[NSOperationQueue mainQueue]addOperationWithBlock:^{
            self.finish(image);
        }];


    }
}


+ (instancetype)downloadOperationWithURLString:(NSString *)URLString finish:(void (^)(UIImage *))finish {
    DownLoadOperation *op = [[self alloc]init];
    op.URLString = URLString;
    op.finish = finish;
    return op;
}

// SDWebImage 在start 方法中取消正在执行的操作 (已经提交了几千份代码)
// 不管操作是否已经被取消，都会调用 start 方法
// 如果重写start 方法来取消操作，很非常麻烦
// 有兴趣可以去看SDWebImage 的 start 方法
//- (void)start {
//    NSLog(@"cancel = %d",self.isCancelled);
//    if (self.isCancelled)return;
//
////    NSLog(@"%s",__FUNCTION__);
//    [super start];
//
//}

#import <Foundation/Foundation.h>

@interface NSString (Path)
- (NSString *)appendDocument;
- (NSString *)appendCache;
- (NSString *)appendTmp;

@end



#import "NSString+Path.h"

@implementation NSString (Path)
- (NSString *)appendDocument {
    NSString *root = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES)lastObject];

    return [root stringByAppendingPathComponent:[self lastPathComponent]];
}

- (NSString *)appendCache {
    NSString *root = [NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES)lastObject];

    return [root stringByAppendingPathComponent:[self lastPathComponent]];
}

- (NSString *)appendTmp {
    NSString *root = NSTemporaryDirectory();

    return [root stringByAppendingPathComponent:[self lastPathComponent]];
}
@end

#import <UIKit/UIKit.h>

@interface UIImageView (WebImage)
- (void)setImageWithURLString:(NSString *)URLString;
@end



#import "UIImageView+WebImage.h"
#import <objc/runtime.h>
#import "DownLoadImageManager.h"

@interface UIImageView ()
@property (nonatomic, copy) NSString *currentURL;
@end

@implementation UIImageView (WebImage)
- (void)setImageWithURLString:(NSString *)URLString {

    // 分类不能直接添加属性 -> 如果能添加属性，用分类下载图片就完成了
    /*
     运行时(Runtime)来动态添加属性

     会没有用过(了解过)运行时机制 -> 第三方
     什么是运行时？ OC是运行时做的

     运行时通常我们会用到三个： 
     1. 动态添加属性 -> 分类 (BlockKit)
     2. 交换方法地址 -> 安全上(怎么让数组越界而程序不崩,字典)
     3. 获取属性列表 -> 字典转模型 

     */
    // 下载图片
    self.image = nil;
    // 判断要下载的URL跟上一个下载的URL的关系
    if (![URLString isEqualToString:self.currentURL] && self.currentURL) {
        // 取消上一个下载操作
        [[DownLoadImageManager shareManager]cancelDownload:self.currentURL];
    }

    self.currentURL = URLString;

    [[DownLoadImageManager shareManager]downloadOperationWithURLString:self.currentURL finish:^(UIImage *image) {
        self.image = image;
    }];
}

#define CZCurrentURL "currentURL"
- (NSString *)currentURL {

//    @selector(currentURL) void * 相当于 oc 的 ID
    return objc_getAssociatedObject(self, CZCurrentURL);
}

- (void)setCurrentURL:(NSString *)currentURL {
    /*
     1. 绑定的对象
     2. 相当于属性名字 selector
     3. 属性的值
     4. 修饰方式

     OBJC_ASSOCIATION_ASSIGN = 0, assign
    OBJC_ASSOCIATION_RETAIN_NONATOMIC = 1, retain(strong) nonatomic

    OBJC_ASSOCIATION_COPY_NONATOMIC = 3, copy nonatomic

     OBJC_ASSOCIATION_RETAIN = 01401, retain(strong) atomic
    OBJC_ASSOCIATION_COPY = 01403     copy atomic

     */
    // 动态绑定值到对象上
    objc_setAssociatedObject(self, CZCurrentURL, currentURL, OBJC_ASSOCIATION_COPY_NONATOMIC);
}

@end