Which of the following will be efficient or faster in iOS? - objective-c

Question

Which of the following will be efficient or faster in iOS?
/* Public Static Variable */
static UICollectionView *mDocsCollection;
#interface ClassA ()
{
}
#end
#implementation ClassA
- (UICollectionView *)documentsCollection
{
if (!mDocsCollection) {
mDocsCollection = (UICollectionView *)[self.view viewWithTag:VIEW_TAG_DOCS_COLLECTION];
}
return mDocsCollection;
}
#end
/* Interface Variable */
#interface ClassA ()
{
UICollectionView *mDocsCollection;
}
#end
#implementation ClassA
- (UICollectionView *)documentsCollection
{
if (!mDocsCollection) {
mDocsCollection = (UICollectionView *)[self.view viewWithTag:VIEW_TAG_DOCS_COLLECTION];
}
return mDocsCollection;
}
#end
/* Private Static Variable */
- (UICollectionView *)documentsCollection
{
static UICollectionView *docsCollection;
if (!docsCollection) {
docsCollection = (UICollectionView *)[self.view viewWithTag:VIEW_TAG_DOCS_COLLECTION];
}
return docsCollection;
}
/* Typecasting is done every time (No Variables used) */
- (UICollectionView *)documentsCollection
{
return (UICollectionView *)[self.view viewWithTag:VIEW_TAG_DOCS_COLLECTION];
}

Answer 1

With the speed of todays devices I doubt there would be any measurable difference. However there are some other observations:
As per other comments, it's not a good idea to use static variables in these situations. In fact, I recommend only using statics where absolutely necessary.
Again unless necessary, I recommend to people to avoid using viewWithTag:. It may seem like an easy solution to finding controls at first. But once you project becomes larger or other programmers join, it's a likely cause of on going bugs. I recommend using IBOutlets and/or UIView extends so that you have direct access to correctly typed references to the controls that always point to the control you want.
And by using an IBOutlet or property, you simply would not need any of these methods :)

Answer 2

The last is slowest. The others are equally fast, probably only a few machine instructions apart.
The file-scope static and the method-scope static do the same thing. The ivar does something very different. This matters if you have multiple instances of ClassA or multiple threads, in which case the statics are probably simply wrong.

Answer 3

I doubt the difference will be observable in this case, but you could benchmark it and see. A simple-minded benchmark would do something like:
CFAbsoluteTime startTime = CFAbsoluteTimeGetCurrent();
// put the code to benchmark here
NSLog(#"elapsed = %f seconds", CFAbsoluteTimeGetCurrent() - startTime);
You might want to repeat your tests many times so you establish a confidence level on the numbers you generate. Also note that the order in which you test them can affect the results, so you might want to delay the benchmarking code until the app has really finished loading and quieted down.
Theoretically, if you call this documentsCollection method a lot, storing the value in a variable might offer some performance improvement, but you'd only bother with this sort of implementation when using some very expensive method or if you're calling it a lot.
FYI, the static implementations can result in very different behavior if its possible that you might reinstantiate ClassA more than once (i.e. dismiss the view controller and re-present it later). The static variables are not unique to just this instance of ClassA, but are shared across instances. Given that you're dealing with views, I doubt this behavior is desirable.