Monthly Archives: March 2013

Flexible method calls with NSInvocation

I was recently working on a means to perform concurrent saves (for performance reasons) of some records to Parse .     As I worked through this I realized that there may be some benefit in creating a generic dispatcher that wasn’t tied to a particular source of those records.   My goal was to give this generic dispatcher the means to retrieve a total record count and set of records in a range, without having to know the details of where the count and records came from.

The dispatcher would also be given parameters that told it how many records to save at once (Parse has a saveAll feature for persisting objects to the cloud), along with how many saves to perform concurrently.


At first I explored using one or more selectors that I passed from my custom record save method to the generic dispatcher.     I quickly ran into the warning “performSelector may cause a leak because its selector is unknown”.  I found an excellent post on Stack Overflow that discusses this issue.  One proposed explanation for this warning was that since the selector is unknown to the compiler, ARC cannot enforce proper memory management.

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Singletons and synchronized code

I’ve been making more use of threads and recently needed to allow two threads (neither of them the main thread) to coordinate with each other.    In this particular case I was logging into Parse in one thread and moving some data from the Address Book to Core Data in the other thread.   Once both threads completed, I wanted to call a routine that would allow me to communicate with Parse and update some records in the cloud as needed.


As an aside, I wanted to put in a quick plug for Parse.  It’s a great service that allows me to persist objects in the cloud, perform queries, etc.   The documentation is great and the team has been very responsive on their forums.  I hope to write about my experience with Parse more extensively in the future.


Conceptually, it isn’t difficult to think about how to synchronize the completion of the threads before kicking off my update method.   I just need to keep a flag somewhere that both threads can check.

@property (nonatomic) BOOL readyForParseUpdate;

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