On 12 Nov 2012, at 13:39, Marco Tabini <[email protected]> wrote:
>> This is completely the wrong way to implement a property. The static
>> variable will be shared between all instances. Here's how you should be
>> doing a lazy loaded var:
>>
>> @implementation MyClass
>> {
>> NSDictionary *_someDictionary
>> }
>>
>> - (NSDictionary *)someDictionary
>> {
>> static dispatch_once_t justOnce;
>> dispatch_once(&justOnce, ^
>> {
>> someDictionary = [[NSDictionary alloc] initWithObjectsAndKeys: ……
>> nil];
>> });
>> return someDictionary;
>> }
>
> I don't think this does what you think it does; my understanding is that
> dispatch_once will execute only once for the lifetime of an app, so the code
> you posted will only run once for the first object that requires is, and then
> never run again, resulting in _dictionary being Nil in all other instances.
Very good point! My bad.
> I understood the OP's request as wanting to implement a singleton, which,
> based on your reading, may not be the case. dispatch_once will be fine for a
> singleton, but if you need a thread-safe, lazily-instantiated read-only
> property, maybe something like this will do the trick:
I'm pretty sure he doesn't want a singleton, as he was talking about a
property. And yeh, sorry about the misinformation there!
>
> @implementation MyClass {
> NSDictionary *_someDictionary
> }
>
> - (NSDictionary *) someDictionary {
> @synchronized(self) {
> if (!_someDictionary) {
> _someDictionary = [[NSDictionary alloc] initWith… ]
> }
> }
>
> return _someDictionary;
> }
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