Watching Core Data objects for updates or deletions

CDZObjectWatcher.h

//
//  CDZObjectWatcher.h
//
//  Created by Chris Dzombak.
//  Copyright (c) 2015 Chris Dzombak. All rights reserved.
//

@import Foundation;
@import CoreData;

typedef NS_ENUM(NSUInteger, CDZObjectWatchType) {
    CDZObjectWatchTypeDeleted = 0,
    CDZObjectWatchTypeUpdated,
};

typedef void(^CDZWatchedObjectsBlock)(NSSet *objects);

@interface CDZObjectWatcher : NSObject

/**
 Initialize a watcher that watches the given Core Data store coordinator for
 changes of the given type and executes the given block when a change occurs.

 Your block will be called on the main queue.
 */
- (instancetype)initWithPersistentStoreCoordinator:(NSPersistentStoreCoordinator *)psc watchType:(CDZObjectWatchType)watchType block:(CDZWatchedObjectsBlock)block;

- (void)watchObject:(NSManagedObject *)object;
- (void)watchObjectsInArray:(NSArray *)objects;
- (void)watchObjectsInSet:(NSSet *)objects;

- (void)stopWatchingObject:(NSManagedObject *)object;

@end

CDZObjectWatcher.m

//
//  CDZObjectWatcher.m
//
//  Created by Chris Dzombak.
//  Copyright (c) 2015 Chris Dzombak. All rights reserved.
//
//  This class's design and API are heavily based on MCDeleteWatcher,
//      found at http://www.cimgf.com/2014/02/25/deleting-objects-in-core-data/
//
//  Created by Marcus S. Zarra on 2/24/14.
//  Copyright (c) 2014 MartianCraft. All rights reserved.
//  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 "CDZObjectWatcher.h"
#import <dispatch/dispatch.h>

@interface CDZObjectWatcher ()

@property (nonatomic, readonly, weak) NSPersistentStoreCoordinator *persistentStoreCoordinator;
@property (nonatomic, readonly) NSMutableSet *objectsToWatch;
@property (nonatomic, readonly) CDZWatchedObjectsBlock watchedObjectsBlock;
@property (nonatomic, readonly) CDZObjectWatchType watchType;

@end

@implementation CDZObjectWatcher

#pragma mark - Object Lifecycle

- (id)initWithPersistentStoreCoordinator:(NSPersistentStoreCoordinator *)psc watchType:(CDZObjectWatchType)watchType block:(CDZWatchedObjectsBlock)objectsBlock {
    NSParameterAssert(psc);
    NSParameterAssert(objectsBlock);

    self = [super init];
    if (self) {
        _persistentStoreCoordinator = psc;
        _watchedObjectsBlock = [objectsBlock copy];
        _watchType = watchType;
        _objectsToWatch = [NSMutableSet set];

        [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(contextUpdated:) name:NSManagedObjectContextObjectsDidChangeNotification object:nil];
    }
    return self;
}

- (void)dealloc {
    [[NSNotificationCenter defaultCenter] removeObserver:self];
}

+ (dispatch_queue_t)backgroundQueue {
    static dispatch_queue_t q = NULL;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        q = dispatch_queue_create("com.cdz.objectwatcher", DISPATCH_QUEUE_SERIAL);
    });
    return q;
}

#pragma mark - Public API

- (void)watchObject:(NSManagedObject *)object {
    NSParameterAssert(object);

    dispatch_async([[self class] backgroundQueue], ^{
        [[self objectsToWatch] addObject:object];
    });
}

- (void)watchObjectsInArray:(NSArray *)objects {
    NSParameterAssert(objects);

    dispatch_async([[self class] backgroundQueue], ^{
        [[self objectsToWatch] addObjectsFromArray:objects];
    });
}

- (void)watchObjectsInSet:(NSSet *)objects {
    NSParameterAssert(objects);

    dispatch_async([[self class] backgroundQueue], ^{
        [[self objectsToWatch] unionSet:objects];
    });
}

- (void)stopWatchingObject:(NSManagedObject *)object {
    NSParameterAssert(object);

    dispatch_async([[self class] backgroundQueue], ^{
        [[self objectsToWatch] removeObject:object];
    });
}

#pragma mark - Notification Handling

- (void)contextUpdated:(NSNotification *)notification {
    if (!self.watchedObjectsBlock) {
        return;
    }

    if ([[notification object] persistentStoreCoordinator] != self.persistentStoreCoordinator) {
        return;
    }

    NSString *objectsKey = self.watchType == CDZObjectWatchTypeDeleted ? NSDeletedObjectsKey : NSUpdatedObjectsKey;
    NSSet *notificationObjects = [[notification userInfo] objectForKey:objectsKey];

    dispatch_async([[self class] backgroundQueue], ^{
        NSMutableSet *objects = [NSMutableSet set];

        for (NSManagedObject *object in notificationObjects) {
            if ([self.objectsToWatch containsObject:object]) {
                [objects addObject:object];
            }
        }

        if ([objects count]) {
            dispatch_async(dispatch_get_main_queue(), ^{
                [self watchedObjectsBlock](objects);
            });
        }
    });
}

@end