When to use enumerateObjectsUsingBlock vs. for

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Ultimately, use whichever pattern you want to use and comes more naturally in the context.

While for(... in ...) is quite convenient and syntactically brief, enumerateObjectsUsingBlock: has a number of features that may or may not prove interesting:

  • enumerateObjectsUsingBlock: will be as fast or faster than fast enumeration (for(... in ...) uses the NSFastEnumeration support to implement enumeration). Fast enumeration requires translation from an internal representation to the representation for fast enumeration. There is overhead therein. Block-based enumeration allows the collection class to enumerate contents as quickly as the fastest traversal of the native storage format. Likely irrelevant for arrays, but it can be a huge difference for dictionaries.

  • “Don’t use enumerateObjectsUsingBlock when you need to modify local variables” – not true; you can declare your locals as __block and they’ll be writable in the block.

  • enumerateObjectsWithOptions:usingBlock: supports either concurrent or reverse enumeration.

  • with dictionaries, block based enumeration is the only way to retrieve the key and value simultaneously.

Personally, I use enumerateObjectsUsingBlock: more often than for (... in ...), but – again – personal choice.

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