Refactored for Efficiency

Refactoring your code is for more then keeping it DRY. It also gives you a chance to make it more efficient.

Take a look at the following Objective-C. Its goal is to return the full path to a sessions folder which is a subdirectory in the users documents directory.

-(NSString *)filePath {
    NSString *baseDirectory = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) objectAtIndex:0];
    NSString *sessionPath = [baseDirectory stringByAppendingPathComponent:self.sessionName];
    return [sessionPath stringByAppendingPathComponent:self.fileName];   
}

It accomplishes its task but is relatively expensive. There are two string concatenations, each of which involve another allocation of memory.

We can speed this up a bit by rewritting it like this:

-(NSString *)filePath {
    return [NSString stringWithFormat:@"%@/%@/%@",
         [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) objectAtIndex:0], 
         self.sessionName, 
         self.fileName
    ]; 
}

We can also cache the result as it doesn’t change during its session. We simply add a NSString *_filePath to the containing class and refactor further.

-(NSString *)filePath {
    if(_filePath == null) {   
        _filePath = [NSString stringWithFormat:@"%@/%@/%@",
             [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) objectAtIndex:0], 
             self.sessionName, 
             self.fileName
        ]; 
    }
    return _filePath;
}

The end result is a method that is twice is fast the first time it is called and even more so each additional call due to memoization.