AMAsynchronousOperation

AMAsynchronousOperation+Private.h

#import "AMAsynchronousOperation.h"

@interface AMAsynchronousOperation ()

@property (nonatomic, assign, readwrite) AMAsynchronousOperationState state;

@end

#ifndef AMAsynchronousOperation_FinishIfCancelled
#define AMAsynchronousOperation_FinishIfCancelled() \
if (self.isCancelled) { \
    self.state = AMAsynchronousOperationStateFinished; \
    return; \
    }
#endif

AMAsynchronousOperation.h

#import <Foundation/Foundation.h>

typedef NS_ENUM(NSUInteger, AMAsynchronousOperationState) {
    AMAsynchronousOperationStateReady,
    AMAsynchronousOperationStateExecuting,
    AMAsynchronousOperationStateFinished,
};

@interface AMAsynchronousOperation : NSOperation

#pragma mark Subclasses

@property (nonatomic, assign, readonly) AMAsynchronousOperationState state;

@end

AMAsynchronousOperation.m

#import "AMAsynchronousOperation.h"
#import "AMAsynchronousOperation+Private.h"

@implementation AMAsynchronousOperation

- (instancetype)init
{
    self = [super init];
    if (self) {
        _state = AMAsynchronousOperationStateReady;
    }
    return self;
}

- (BOOL)isAsynchronous
{
    return YES;
}

- (BOOL)isReady
{
    return super.isReady && AMAsynchronousOperationStateReady == self.state;
}

- (BOOL)isExecuting
{
    return AMAsynchronousOperationStateExecuting == self.state;
}

- (BOOL)isFinished
{
    return AMAsynchronousOperationStateFinished == self.state;
}

- (void)start
{
    if (self.isCancelled) {
        self.state = AMAsynchronousOperationStateFinished;
    } else {
        self.state = AMAsynchronousOperationStateReady;
        [self main];
    }
}

- (NSString *)keyForState:(AMAsynchronousOperationState)state
{
    switch (state) {
        case AMAsynchronousOperationStateReady:
            return @"isReady";
        case AMAsynchronousOperationStateExecuting:
            return @"isExecuting";
        case AMAsynchronousOperationStateFinished:
            return @"isFinished";
    }
}

- (void)setState:(AMAsynchronousOperationState)newState
{
    if (_state == newState) {
        return;
    }
    
    AMAsynchronousOperationState oldState = _state;
    
    [self willChangeValueForKey:[self keyForState:oldState]];
    [self willChangeValueForKey:[self keyForState:newState]];
    [self willChangeValueForKey:@"state"];
    
    _state = newState;
    
    [self didChangeValueForKey:@"state"];
    [self didChangeValueForKey:[self keyForState:oldState]];
    [self didChangeValueForKey:[self keyForState:newState]];
    }

- (void)main
{
    AMAsynchronousOperation_FinishIfCancelled();
    
    self.state = AMAsynchronousOperationStateExecuting;
}

@end

@kielgillard thanks for pointing that out!