Question

I’ve a model object that extends from NSObject called column. Column has two fields, properties and data. Properties is an NSDictionary that stores arbitrary key/value pairs while data is an array.

Columns come in arrays and I need to sort them by a value I know will always be in properties, a value with the key ordinal_position.

I’m using the following code to sort but it always fails because there is no “ordinal_position” in the model object, only in the properties of the model object:

NSArray *unsortedArray = [resultStore allValues];
NSSortDescriptor *sortDescriptor = [[[NSSortDescriptor alloc] initWithKey:@"ordinal_position" ascending:YES] autorelease];

NSArray *sortedArray = [unsortedArray sortedArrayUsingDescriptors:[NSArray arrayWithObject:sortDescriptor]];

How can I tell NSSortDescriptor to look for the property in a dictionary in the model object?

Answer 1

You should define your own selector that returns an NSComparisonResult and implement it:

- (NSComparisonResult)compareByProperty:(Column *)aColumn {
    id thisValue = [self valueForKey:@"ordinal_position"];
    id otherValue = [self valueForKey:@"ordinal_position"];

    return [thisValue compare:otherValue]; // Replace to suit your logic
}

Then use the NSArray method sortedArrayUsingSelector::

NSArray *sortedArray = [unsortedArray sortedArrayUsingSelector:@selector(compareByProperty:)];

Note that your comparison method must exist on your model object, not on the class you’re calling this from – the idiom is that you’re comparing some given model object (self) to another model object (aColumn) from within the first object.