NSOperation subclass implementing +asyncOperationWithBlock:

JPHAsyncOperation.h

#import <Foundation/Foundation.h>

@interface AsyncOperation : NSOperation

@property (nonatomic, assign) BOOL isExecuting; // yes, explicity not '.. getter=isExecuting) BOOL executing'
@property (nonatomic, assign) BOOL isFinished; // the KVC keys must be isExecuting & isFinished, with the 'is'

typedef void (^FinishBlock)(void);

+ (instancetype)asyncOperationWithBlock:(void (^)(FinishBlock finishBlock))block;

+ (instancetype)asyncOperationOnMainThreadWithBlock:(void (^)(FinishBlock finishBlock))block;

@end

JPHAsyncOperation.m

#import "AsyncOperation.h"

@interface AsyncOperation ()
@property (nonatomic, strong) void (^operationBlock)(FinishBlock);
@property (nonatomic, assign) BOOL onMainThread;
@end

@implementation AsyncOperation

+ (instancetype)asyncOperationWithBlock:(void (^)(FinishBlock finishBlock))block
{
    return [[self alloc] initWithBlock:block toRunOnMainThread:NO];
}

+ (instancetype)asyncOperationOnMainThreadWithBlock:(void (^)(FinishBlock finishBlock))block
{
	AsyncOperation *obj = [[self alloc] initWithBlock:block toRunOnMainThread:YES];
	return obj;
}

- (instancetype)initWithBlock:(void (^)(FinishBlock finishBlock))block toRunOnMainThread:(BOOL)mainThread
{
	self = [super init];
	if (self)
	{
		self.operationBlock = block;
		self.onMainThread = mainThread;
	}
	return self;
}


- (void)start
{
	if (self.isCancelled || self.operationBlock == nil)
	{
		self.isFinished = YES;
	}
	else
	{
		self.isExecuting = YES;
		
		FinishBlock finishBlock = ^{
			self.isExecuting = NO;
			self.isFinished = YES;
		};
		
		if (self.onMainThread && ![NSThread isMainThread]) // necessary??
			dispatch_async(dispatch_get_main_queue(), ^{ self.operationBlock(finishBlock); });
		else
			self.operationBlock(finishBlock);
	}
}


- (BOOL)isConcurrent
{
    return YES;
}

// this makes it so will/didChangeValueForKey isFinished automatically get called, like normal properties
+ (BOOL)automaticallyNotifiesObserversForKey:(NSString *)key
{
	if ([key isEqualToString:@"isExecuting"] || [key isEqualToString:@"isFinished"])
		return YES;
	return [super automaticallyNotifiesObserversForKey:key];
}

@end