Question

I would like to make a data manager class that retrieves data from plist, and I wonder if I should make a class with all class methods which read plist every time when the method is called and returns requested value, or create a class initializer which initializes an array(instance variable) with the plist data and all methods are instance methods which get data from the array.

I would like to know which is more expensive:reading plist many times (like 50 times) or instantiating an object, or simply which is better.

Thank you for your help in advance.

Answer 1

This is one of the classical tradeoffs in programming – speed vs. memory use. The technique of reading something once and storing it on a faster medium (in this example, in memory) is called caching. It’s very popular, and for good reason. Mass storage devices are still magnitudes slower than RAM, and network access is magnitudes slower than local mass storage.

If you assume the data will be asked of the manager often, and if you assume the plist won’t change (or you can detect changes), then read the plist on the first access to the getter, store it in an iVar, and answer only the iVar as long as the plist hasn’t changed. This uses a little more memory, but is a lot faster for subsequent accesses.

NOTE: This approach would be harmful for very, very large files. If you are concerned about memory usage, implement the - (void)didReceiveMemoryWarning method in your viewControllers, and flush the cache (delete it) when you are low on memory.

A getter method could look like this:

- (NSArray *)data
{
    if (!cacheArray) {
        //what we do now is called "lazy initialization": we initialize our array only when we first need the data.
        //This is elegant, because it ensures the data will always be there when you ask for it,
        //but we don't need to initialize for data that might never be needed, and we automatically re-fill the array in case it has been deleted (for instance because of low memory)
        cacheArray = ... //read array from plist here; be sure to retain or copy it
    }
    return cacheArray;
}