What is the role of selector in UILocalizedIndexedCollation's sectionForObject:(id)object collationStringSelector:(SEL)selector method

Question

I am a beginner of iOS development and while going through this document (iOS Developer Guide about configuring a TableView with Indexed List) I came across this:

// Listing 4.7
for (State *theState in statesTemp) {
        NSInteger sect = [theCollation sectionForObject:theState collationStringSelector:@selector(name)];
        theState.sectionNumber = sect;
    }

I could not figure out the selector (@selector(name)) and its purpose, nor could I find the method with the name passed in the selector i.e. name. I googled for examples to find a better explanation, and came across this example.

In the code listing, there is a statement which is a method call:     

self.tableData = [self partitionObjects:objects collationStringSelector:@selector(title)];

now the selector is called title. I have not been able to find a better explanation, and my question is what is the purpose of this selector and the method referred by this selector, and what should it do and return.

Answer 1

In general

With the @selector(title:) you define which method will be called.

in my example it will call

- (void) title:(id)someObject {}

Be carefull with the semicolon at the end! If you have a semicolon at the end you method will have parameters like mine above.

Your code states just @selector(title) and will call a method title without a parameter like this:

- (void)title {}

Specific to UILocalizedIndexCollation

The docs state:

selector
A selector that identifies a method returning an identifying string for object that is used in collation. The method should take no arguments and return an NSString object. For example, this could be a name property on the object.

So i would suggest you implement it like this

self.tableData = [self partitionObjects:objects collationStringSelector:@selector(title)];
 ...
- (NSString *)title {
     NSString *title;
     // some code to fill title with an identifier for your object
     return title;
}

Answer 2

Try replace the title with self:

self.tableData = [self partitionObjects:objects collationStringSelector:@selector(self)];

worked for me