多线程：下载管理器（模拟 SDWebImage）

#import "ViewController.h"
#import "DownLoadImageManager.h"
#import "AppInfo.h"
@interface ViewController ()
@property (weak, nonatomic) IBOutlet UIImageView *imageView;

@property (nonatomic, copy) NSString *currentURL; // 记录当前下载的URL

@end

@implementation ViewController

- (void)viewDidLoad {
[super viewDidLoad];
// Do any additional setup after loading the view, typically from a nib.
NSLog(@"%@",NSHomeDirectory());
}

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {

NSArray *data = [AppInfo appInfoList];

// 连续点击的时候，只显示最后一张
AppInfo *app = data[arc4random_uniform(data.count)];

// 判断要下载的URL跟上一个下载的URL的关系
if (![app.icon isEqualToString:self.currentURL] && self.currentURL) {
// 取消上一个下载操作
[[DownLoadImageManager shareManager]cancelDownload:self.currentURL];
}

self.currentURL = app.icon;

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

}

- (IBAction)cancel:(id)sender {

}

@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];
//
//}
@end

#import <UIKit/UIKit.h>

@interface DownLoadImageManager : NSObject
+ (instancetype)shareManager;
- (void)downloadOperationWithURLString:(NSString *)URLString finish:(void(^)(UIImage *image))finish;
- (void)cancelDownload:(NSString *)URLString;
@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;
}

@end