一 简单的使用
如果只是简单地使用NSOperation，只需要重载main这个方法，在这个方法里面添加需要执行的操作
取消事件
默认情况下，一个
NSOperation开始执行之后，会一直执行任务到结束，就比如下面的DownloadOperation，默认会执行完main方法中的所有代码。NSOperation提供了一个cancel方法，可以取消当前的操作。如果是自定义
NSOperation的话，需要手动处理这个取消事件。比如，一旦调用了cancel方法，应该马上终止main方法的执行，并及时回收一些资源。处理取消事件的具体做法是：在
main方法中定期地调用isCancelled属性检测操作是否已经被取消，也就是说是否调用了cancel方法，如果返回YES，表示已取消，则立即让main方法返回。
DownloadOperation.h
@protocol DownloadOperationDelegate;
@interface DownloadOperation : NSOperation
@property(nonatomic, copy) NSString *imageUrl;
@property(nonatomic, assign) id<DownloadOperationDelegate> delegate;
- (instancetype)initWithURL:(NSString *)url
delegate:(id<DownloadOperationDelegate>)delegate;
@end
@protocol DownloadOperationDelegate <NSObject>
//这个方法不规范,这么写只是为了方便
- (void)downloadFinishedWithImage:(UIImage *)image;
@end
DownloadOperation.m
@implementation DownloadOperation
- (instancetype)initWithURL:(NSString *)url delegate:(id<DownloadOperationDelegate>)delegate
{
if (self = [super init]) {
_imageUrl = url;
_delegate = delegate;
}
return self;
}
- (void)main
{
@autoreleasepool {
NSLog(@"Thread : %@", [NSThread currentThread]);
//经常检测isCancelled属性，NSOperation可能会被cancel掉
if (self.isCancelled) return;
NSURL *url = [NSURL URLWithString:self.imageUrl];
NSData *data = [NSData dataWithContentsOfURL:url];
if (self.isCancelled) return;
UIImage *image = [UIImage imageWithData:data];
if (self.isCancelled) return;
if ([self.delegate respondsToSelector:@selector(downloadFinishedWithImage:)]) {
dispatch_async(dispatch_get_main_queue(), ^{
[self.delegate downloadFinishedWithImage:image];
});
}
}
}
@end
使用
- (void)OPERATION06
{
DownloadOperation *op1 = [[DownloadOperation alloc] initWithURL:@"http://i1.hoopchina.com.cn/u/1705/08/144/1144/414d5be1" delegate:self];
DownloadOperation *op2 = [[DownloadOperation alloc] initWithURL:@"http://i1.hoopchina.com.cn/u/1705/08/144/1144/2241e6c1" delegate:self];
NSOperationQueue *queue = [NSOperationQueue new];
[queue addOperations:@[op1, op2] waitUntilFinished:NO];
// [op2 cancel];
}
- (void)downloadFinishedWithImage:(UIImage *)image
{
NSLog(@"image.size %@", NSStringFromCGSize(image.size));
}
