Simple CancelToken for PromiseKit

CancelToken.h

#import <Foundation/Foundation.h>

typedef void (^MHCancelHandler)(void);


@interface MHCancelToken : NSObject

- (void)onCancel:(MHCancelHandler)cancelHandler;

- (void)cancel;

@end

CancelToken.m

#import "MHCancelToken.h"

typedef NS_ENUM(NSInteger, MHCancelTokenState)
{
    MHCancelTokenStateReady,
    MHCancelTokenStateCancelled
};


@interface MHCancelToken ()

@property (strong, nonatomic) NSMutableArray* cancelHandlers;
@property (nonatomic) MHCancelTokenState state;

@end


@implementation MHCancelToken

- (instancetype)init
{
    if (self = [super init]) {
        _cancelHandlers = [NSMutableArray array];
        _state = MHCancelTokenStateReady;
    }
    return self;
}

- (void)onCancel:(MHCancelHandler)cancelHandler
{
    @synchronized(self) {
        if (self.state == MHCancelTokenStateReady) {
            [self.cancelHandlers addObject:cancelHandler];
            return;
        }
    }
    
    cancelHandler();
}

- (void)cancel
{
    NSArray* cancelHandlersSnapshot;
    @synchronized(self) {
        if (self.state != MHCancelTokenStateReady) {
            return;
        }
        
        self.state = MHCancelTokenStateCancelled;
        
        cancelHandlersSnapshot = self.cancelHandlers;
        self.cancelHandlers = nil;
    }
    
    for (MHCancelHandler handler in cancelHandlersSnapshot) {
        handler();
    }
}

@end

I'm trying to get my head around this as I think I need it to make use of PromiseKit in my application. Why does onCancel call cancelHandler() in the last line? You can call cancel on the cancellation token from where the client/call site needs it which seems right, but what if I only want to cancel it from the call site and not when setting up the handler?

Would I be right to think that this is just to ensure that if this is to ensure cancellation occurs in an edge case connected to when the enclosing block is initially constructed?

eg: Your code example on the thread on cancelling in PromiseKit. I think its useful to have this example client along with your valuable gist on this matter.

- (PMKPromise*)GET:(NSString*)URLString
    parameters:(NSDictionary*)parameters
    cancelToken:(MHCancelToken*)cancelToken
{
    return [PMKPromise new:^(PMKPromiseFulfiller fulfiller, PMKPromiseRejecter rejecter) {
          NSURLSessionDataTask* task = [self.sessionManager GET:URLString
                                                     parameters:parameters
                                                        success:^(NSURLSessionDataTask* resultTask, id responseObject) {
                                                            fulfiller(responseObject);
                                                        }
                                                        failure:^(NSURLSessionDataTask* resultTask, NSError* error) {
                                                            rejecter(error);
                                                        }];

          [cancelToken onCancel:^{
              if (task.state != NSURLSessionTaskStateCompleted) {
                  [task cancel];
              }
          }];
    }];
}