KVC vs fast enumeration

Question

Which of the following is faster and why?

CGFloat sum = 0;
for (UIView *v in self.subviews)
    sum += v.frame.size.height;

or

CGFloat sum = [[self.subviews valueForKeyPath:@"@sum.frame.size.height"] floatValue];

Answer 1

Really, a lot of how elegant (or clever) a language is comes down to how well it avoids loops. for, while; even fast enumeration expressions are a drag. No matter how you sugar-coat them, loops will be a block of code that does something that is much simpler to describe in natural language.

"get me the average salary of all of the employees in this array",

double totalSalary = 0.0;
for (Employee *employee in employees) {
  totalSalary += [employee.salary doubleValue];
}
double averageSalary = totalSalary / [employees count];

versus...

Fortunately, Key-Value Coding gives us a much more concise--almost Ruby-like--way to do this:

[employees valueForKeyPath:@"@avg.salary"];

KVC Collection Operators allows actions to be performed on a collection using key path notation in valueForKeyPath:.

Any time you see @ in a key path, it denotes a particular aggregate function whose result can be returned or chained, just like any other key path.

Fast Enumeration is Faster then KVC.

Hope it helps you.

Answer 2

First, your KVC expression won't work; KVC can't be used to retrieve struct members (save for very limited cases where a class has a special implementation of valueForKey: that does special processing).

Secondly, this smells like premature optimization.

Have you actually quantified a performance problem related to this code?

If not, you are wasting your time "optimizing" this code path.

That aside, using KVC for such operations adds considerable fragility to your code. The compiler cannot verify the expression passed to valueForKey: and, thus, what would be a compile time error or warning is now an error at runtime that'll only be found if you execute that codepath.