text: //
  //  UBSignalObserver.h
  //  UberSignals
  //
  //  Copyright (c) 2015 Uber Technologies, Inc.
  //
  //  Permission is hereby granted, free of charge, to any person obtaining a copy
  //  of this software and associated documentation files (the "Software"), to deal
  //  in the Software without restriction, including without limitation the rights
  //  to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
  //  copies of the Software, and to permit persons to whom the Software is
  //  furnished to do so, subject to the following conditions:
  //
  //  The above copyright notice and this permission notice shall be included in
  //  all copies or substantial portions of the Software.
  //
  //  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  //  IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  //  FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
  //  AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  //  LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
  //  OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
  //  THE SOFTWARE.
  
  #import <Foundation/Foundation.h>
  
  NS_ASSUME_NONNULL_BEGIN
  
  /**
   A SignalObserver is returned whenever an observer is added to a UBSignal. Use it to cancel this specific observation or change the OperationQueue on which to dispatch the callback on.
   */
  @interface UBSignalObserver : NSObject
  
  /*
   An opertion OperationQueue to dispath the callback on. If nil (default), the callback will be dispatched on the same queue as the signal fires on.
   */
  @property (nonatomic) NSOperationQueue *operationQueue;
  
  /*
   If YES, cancels the observer once the signal has fired the next time.
   */
  @property (nonatomic) BOOL cancelsAfterNextFire;
  
  /**
   Calls the observers callback block with the last data that has been fired by the signal. If no data has been fired by the Signal or the Signal doesn't fire any data, the observers block is not called and NO is returned.
   
   @return YES if the observer block was called with previous data or NO if the Signal didn't have any previous data.
   */
  - (BOOL)firePreviousData;
  
  /**
   Cancels the observation for this specific observer and callback block.
   */
  - (void)cancel;
  
  @end
  
  NS_ASSUME_NONNULL_END
  