Simple implementation of a thread safe mutable dictionary. In most cases, you want NSCache instead, but it can be useful in situations where you want to manually control what is evicted from the cache in low memory situations.**Warning:** I only use this for setting/getting keys. Enumeration is not thread safe here and will still throw exception…

PSPDFThreadSafeMutableDictionary.m

//
// PSPDFThreadSafeMutableDictionary.m
//
// Copyright (c) 2013 Peter Steinberger, PSPDFKit GmbH. 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 <Foundation/Foundation.h>

// Dictionary-Subclasss whose primitive operations are thread safe.
@interface PSPDFThreadSafeMutableDictionary : NSMutableDictionary
@end

// ----------------------------------------------------------------

//
//  PSPDFThreadSafeMutableDictionary.m
//  PSPDFKit
//
//  Copyright (c) 2013 PSPDFKit GmbH. All rights reserved.
//

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

#define LOCKED(...) OSSpinLockLock(&_lock); \
__VA_ARGS__; \
OSSpinLockUnlock(&_lock);

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

///////////////////////////////////////////////////////////////////////////////////////////
#pragma mark - NSObject

- (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;
}

///////////////////////////////////////////////////////////////////////////////////////////
#pragma mark - NSMutableDictionary

- (void)setObject:(id)anObject forKey:(id<NSCopying>)aKey {
    LOCKED(_dictionary[aKey] = anObject)
}

- (void)addEntriesFromDictionary:(NSDictionary *)otherDictionary {
    LOCKED([_dictionary addEntriesFromDictionary:otherDictionary]);
}

- (void)setDictionary:(NSDictionary *)otherDictionary {
    LOCKED([_dictionary setDictionary:otherDictionary]);
}

- (void)removeObjectForKey:(id)aKey {
    LOCKED([_dictionary removeObjectForKey:aKey])
}

- (void)removeAllObjects {
    LOCKED([_dictionary removeAllObjects]);
}

- (NSUInteger)count {
    LOCKED(NSUInteger count = _dictionary.count)
    return count;
}

- (NSArray *)allKeys {
    LOCKED(NSArray *allKeys = _dictionary.allKeys)
    return allKeys;
}

- (NSArray *)allValues {
    LOCKED(NSArray *allValues = _dictionary.allValues)
    return allValues;
}

- (id)objectForKey:(id)aKey {
    LOCKED(id obj = _dictionary[aKey])
    return obj;
}

- (NSEnumerator *)keyEnumerator {
    LOCKED(NSEnumerator *keyEnumerator = [_dictionary keyEnumerator])
    return keyEnumerator;
}

- (id)copyWithZone:(NSZone *)zone {
    return [self mutableCopyWithZone:zone];
}

- (id)mutableCopyWithZone:(NSZone *)zone {
    LOCKED(id copiedDictionary = [[self.class allocWithZone:zone] initWithDictionary:_dictionary])
    return copiedDictionary;
}

- (NSUInteger)countByEnumeratingWithState:(NSFastEnumerationState *)state
                                  objects:(id __unsafe_unretained [])stackbuf
                                    count:(NSUInteger)len {
    LOCKED(NSUInteger count = [[_dictionary copy] countByEnumeratingWithState:state objects:stackbuf count:len]);
    return count;
}

- (void)performLockedWithDictionary:(void (^)(NSDictionary *dictionary))block {
    if (block) LOCKED(block(_dictionary));
}

- (BOOL)isEqual:(id)object {
    if (object == self) return YES;

    if ([object isKindOfClass:PSPDFThreadSafeMutableDictionary.class]) {
        PSPDFThreadSafeMutableDictionary *other = object;
        __block BOOL isEqual = NO;
        [other performLockedWithDictionary:^(NSDictionary *dictionary) {
            [self performLockedWithDictionary:^(NSDictionary *otherDictionary) {
                isEqual = [dictionary isEqual:otherDictionary];
            }];
        }];
        return isEqual;
    }
    return NO;
}

@end

Analyze your code in Xcode and you see warning in method: performLockedWithDictionary, if block is 0, then LOCKED macro would be executed particularly. Please use braces in if statement.

@buh: you are right. LOCKED expands to three statements and the if guards only the first. I would prefer defining the LOCKED as one statement with

#define LOCKED(...) do { \
OSSpinLockLock(&_lock); \
__VA_ARGS__; \
OSSpinLockUnlock(&_lock); \
} while (NO)

Without arguing, if you should use braces in the if, it will work as expected in both cases :-)

- (NSUInteger)countByEnumeratingWithState:(NSFastEnumerationState *)state
                                  objects:(id __unsafe_unretained [])stackbuf
                                    count:(NSUInteger)len {
    LOCKED(NSUInteger count = [[_dictionary copy] countByEnumeratingWithState:state objects:stackbuf count:len]);
    return count;
}

Unfortunately (I'm having a similar problem), I believe the above will crash. The current count will be returned, but enumeration will then proceed to happen on the copy of _dictionary, to which there are no strong references. So enumeration will only get a few elements in (an initial buffers worth.. not completely sure on the internal implementation of fast enumeration, but elements are loaded in parallel) before the new dictionary is released, at which point it will crash.

Since fast enumeration happens after this method is called, I'm not sure what the solution would be to keep the _dictionary copy around for the appropriate amount of time.

Found a solution, given that countByEnumeratingWithState: is called once before enumeration starts and once after it ends (godsend.. we have our "appropriate amount of time" indicator):

- (NSUInteger)countByEnumeratingWithState:(NSFastEnumerationState *)state
                                  objects:(id __unsafe_unretained [])stackbuf
                                    count:(NSUInteger)len {
    OSSpinLockLock(&_lock);

    NSDictionary *enumerationDict = objc_getAssociatedObject(self, &stackbuf);
    if (!enumerationDict) {
        objc_setAssociatedObject(self, &stackbuf, (enumerationDict = _dictionary.copy), OBJC_ASSOCIATION_RETAIN);
    }

    NSUInteger count = [enumerationDict countByEnumeratingWithState:state objects:stackbuf count:len];

    if (!count) {
        objc_setAssociatedObject(self, &stackbuf, nil, OBJC_ASSOCIATION_RETAIN);
    }

    OSSpinLockUnlock(&_lock);

    return count;
}

AFAIK, if you implement isEqual: then you must also implement the -(int)hash method (and probably use your LOCK() macro to return the [super hash] value).

@steipete the license at the top seems very permissive, but then there's the other license below in all caps. What's the deal with this file? And whatever it is, could we make the two licenses more consistent?

Hi,
I am new to IOS development, can this be used same as concurrent dictionary in JAVA.

I got a crash report with this as the backtrace:

0  libobjc.A.dylib                0x183064428 objc_msgSend + 8
1  CoreFoundation                 0x183b7ad0c mdict_rehashd + 180
2  CoreFoundation                 0x183b791fc -[__NSDictionaryM setObject:forKeyedSubscript:] + 440
3  SomeApp                        0x1018ae708 -[PSPDFThreadSafeMutableDictionary setObject:forKey:] + 800008

Curious if anyone else has gotten it, or knows what the root cause might be? There weren't any other threads in the crash report related to this class or dictionaries.