[swift-users] EnumerateUsingObjects in Swift

[Doug Hill]

Thanks for the help. I'm still trying to figure out how Swift works, particularly what the error messages mean. This has been driving me a little nuts trying to figure out what is wrong via sometimes cryptic errors. Also, it seems like getting generic programming working in Swift is more difficult than I'm used to (even than C++!) so this answer helps figure out how the compiler works.

Doug Hill


> On Jan 23, 2017, at 7:04 PM, Zhao Xin <owenzx at gmail.com> wrote:
> 
> It seems to me that you didn't initialize your `myArray` before you casted it. That caused the problem.
> 
> Zhaoxin
> 
> On Tue, Jan 24, 2017 at 9:34 AM, Jon Shier via swift-users <swift-users at swift.org <mailto:swift-users at swift.org>> wrote:
> enumerateObjects(options:using:) exists on NSArray in Swift. And I was able to create your generic class just fine:
> 
> class Test<T> {
>     var array: [T] = []
>     
>     init() {
>         var temp = array as NSArray
>     }
> }
> 
> I’m not sure what the canonical parallel array enumeration would be, but you can do it using concurrentPerform:
> 
> let array = [“one”, “two”]
> DispatchQueue.concurrentPerform(iterations: array.count) { index in
>     print(array[index])
> }
> 
> 
>> On Jan 23, 2017, at 8:20 PM, Doug Hill via swift-users <swift-users at swift.org <mailto:swift-users at swift.org>> wrote:
>> 
>> I'm trying to accomplish the equivalent functionality of -[NSArray enumerateUsingObjects:…] in Swift. Doing a Googles search, I see that one would need to call the equivalent method on the bridged NSArray version of your Swift array:
>> 
>> var myNSArray : NSArray = mySwiftArray as NSArray
>> 
>> Here's the problem I'm running into; I have the following class:
>> 
>> class Tester<typeA>
>> {
>> 	var myArray : [typeA]
>> 
>> 	init()
>> 	{
>> 		var temp = self. myArray as NSArray
>> 	}
>> }
>> 
>> Which produces a compiler error:
>> 
>> 'cannot convert value of type '[typeA]' to type 'NSArray' in coercion'
>> 
>> Ok, this makes some sense since I'm guessing NSArray requires each element to to be an NSObject but this array type Array<typeA> could be a non-NSObject.
>> 
>> However, this makes my code harder to write since I now have to make sure any array has element type NSObject to use enumerateUsingObjects. Not something I can either guarantee or even desire.
>> 
>> The reason I like enumerateUsingObjects is that it supports a functional style of programming and is better at creating work items for each object by dispatching each array item on multiple cores/processors/threads for me. Writing this method myself would require figuring out to pass an object to a dispatch invocation. But looking through the swift API's, I don't see any GCD method for passing an object to dispatch_sync/async. I see versions of these methods that takes a context parameter but then takes a C function instead of a block, so not very Swift-like and potentially unsafe.
>> 
>> Does this mean enumerateUsingObjects is generally not all that useful in Swift? Are there better alternatives? Any ideas on how best to handle this situation would be appreciated.
>> 
>> Doug Hill

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