Dictionary subclass whose primitive operations are thread safe. Most of the time you don't want locking at the collection level, but there are valid use cases for this, and it's interesting how to properly subclass the NSDictionary class cluster. We're using SpinLocks here, since it's highly unlikely that any of the operation locks for more than…

PSPDFThreadSafeMutableDictionary.m

@interface PSPDFThreadSafeMutableDictionary : NSMutableDictionary
@end


#import "PSPDFThreadSafeMutableDictionary.h"
#import <libkern/OSAtomic.h>

@implementation PSPDFThreadSafeMutableDictionary {
    OSSpinLock _lock;
    NSMutableDictionary *_dictionary; // Class Cluster!
}

- (id)init {
    return [self initWithCapacity:0];
}

- (id)initWithObjects:(NSArray *)objects forKeys:(NSArray *)keys {
    if ((self = [self initWithCapacity:objects.count])) {
        [objects enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
            _dictionary[keys[idx]] = obj;
        }];
    }
    return self;
}

- (id)initWithCapacity:(NSUInteger)capacity {
    if ((self = [super init])) {
        _dictionary = [[NSMutableDictionary alloc] initWithCapacity:capacity];
        _lock = OS_SPINLOCK_INIT;
    }
    return self;
}

- (void)setObject:(id)anObject forKey:(id<NSCopying>)aKey {
    OSSpinLockLock(&_lock);
    _dictionary[aKey] = anObject;
    OSSpinLockUnlock(&_lock);
}

- (void)removeObjectForKey:(id)aKey {
    OSSpinLockLock(&_lock);
    [_dictionary removeObjectForKey:aKey];
    OSSpinLockUnlock(&_lock);
}

- (NSUInteger)count {
    OSSpinLockLock(&_lock);
    NSUInteger count = _dictionary.count;
    OSSpinLockUnlock(&_lock);
    return count;
}

- (id)objectForKey:(id)aKey {
    OSSpinLockLock(&_lock);
    id obj = _dictionary[aKey];
    OSSpinLockUnlock(&_lock);
    return obj;
}

- (NSEnumerator *)keyEnumerator {
    OSSpinLockLock(&_lock);
    NSEnumerator *keyEnumerator = [_dictionary keyEnumerator];
    OSSpinLockUnlock(&_lock);
    return keyEnumerator;
}

@end

Thanks for this.

In Apple's subclassing notes, they mention that initWithObjects:forKeys:count: needs to be subclassed as it's a primitive method upon which other methods are based.

http://developer.apple.com/documentation/Cocoa/Reference/Foundation/Classes/NSDictionary_Class/

Also, the keys should be copied shouldn't they? So that they can no longer mutate

Hey... I gave this a test and it crashed because the dictionary was mutated while being enumerated... here's the code

// Define the dictionary
static int SIZE = 1000000;
PSPDFThreadSafeMutableDictionary *_dictionary = [[PSPDFThreadSafeMutableDictionary alloc] initWithCapacity:SIZE];

// Fill it up on the main thread
for (int _i = 0; _i < SIZE; _i++)
{
    [_dictionary setObject:[NSNumber numberWithInt:_i] forKey:[NSNumber numberWithInt:_i]];
}

// Remove objects asynchronously on a separate thread
dispatch_queue_t _q = dispatch_queue_create("second_thread", DISPATCH_QUEUE_CONCURRENT);
dispatch_async(_q, ^{
    for (int _i = 0; _i < SIZE; _i++)
    {
        [_dictionary removeObjectForKey:[NSNumber numberWithInt:_i]];
    }
});

// Print the values on the main thread
NSLog(@"%@", [_dictionary allValues]);