Objective-C-NSoperationカスタム実装

@class UIImage;
@class DownLoadOperation;

@protocol DownLoadOperationDelegate <NSObject>
@required
- (void) downloadOperation:(DownLoadOperation*) opertaion finishDownloadImg:(UIImage *)img;
@end

@interface DownLoadOperation : NSOperation
/**  operation */
@property (nonatomic, strong) NSString *urlStr;
/**  KO*/
@property (nonatomic, weak) id<DownLoadOperationDelegate> delegate;
@end

#import <UIKit/UIKit.h>

@implementation DownLoadOperation

- (void) main
{
    @autoreleasepool {
        if (self.isCancelled) {
            return;
        }
        NSURL *url = [NSURL URLWithString:self.urlStr];
        NSData *data = [NSData dataWithContentsOfURL:url];
        UIImage *img = [UIImage imageWithData:data];
        
        if (self.isCancelled) {
            return;
        }
        //update UI
        [[NSOperationQueue mainQueue] addOperationWithBlock:^{
            if ([self.delegate respondsToSelector:@selector(downloadOperation:finishDownloadImg:)]) {
                [_delegate downloadOperation:self finishDownloadImg:img];
            }
        }];
    }
}
@end

#import "DownLoadOperation.h"

@interface ViewController ()<DownLoadOperationDelegate>

@property (weak, nonatomic) IBOutlet UIImageView *imgView;

@end

@implementation ViewController

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

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
{
    if (_imgView.image) {
        return;
    }
    DownLoadOperation *operation = [[DownLoadOperation alloc] init];
    operation.urlStr = @"http://img4.duitang.com/uploads/item/201406/01/20140601231235_WRKGW.jpeg";
    operation.delegate = self;
    
    NSOperationQueue *queue = [[NSOperationQueue alloc] init];
    [queue addOperation:operation];
}

- (void)downloadOperation:(DownLoadOperation *)opertaion finishDownloadImg:(UIImage *)img
{
    _imgView.image = img;
}
@end