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Re: How to implement readonly property
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Re: How to implement readonly property


  • Subject: Re: How to implement readonly property
  • From: Steve Sisak <email@hidden>
  • Date: Fri, 07 Dec 2012 21:01:38 -0500

At 7:56 PM +0700 11/12/12, Gerriet M. Denkmann wrote:
- (NSDictionary *)someDictionary;
{
	static NSDictionary *someDictionary;
	static dispatch_once_t justOnce;
	dispatch_once( &justOnce, ^
		{
			// create a temp dictionary (might take some time)
			someDictionary = temp;
		}
	);

	return someDictionary;
}

Here's what I usually do:

assume that _someDictionary is an instance variable initialized to nil and never changed once initialized to non-nil

- (NSDictionary *)someDictionary;
{
  if (!_someDictionary)
  {
    @synchronized (self)
    {
      if (!_someDictionary)
      {
        // create a temp dictionary (might take some time)
         _someDictionary = temp;
      }
    }
  }

  return _someDictionary;
}

the outer if avoids the overhead of @synchronized if _someDictionary is already created -- this is just an optimization

the inner if is necessary to resolve the race condition if multiple threads make it past the outer one

HTH,

-Steve
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