OS X: Bindings, KVC, and KVO - Core Data - page 143

that we provide in Interface Builder. In our recipe application, we have one
NSArrayController that is bound to the Recipe entity in Core Data. Because we also
bound that NSArrayController to our NSManagedObjectContext , it is able to retrieve
those Recipe entities automatically and make them available to the rest of the
user interface. When our interface is loaded, those NSArrayController objects go
out to that NSManagedObjectContext and ask for the entities that currently exist.
Once they are loaded into the NSArrayController objects, any view element asso-
ciated with them will be notified, via KVO, that the data is available for display.
All of this happens behind the scenes; we're not required to write code for
any of it.
NSFormatter
Users expect fields in the interface to accept their input and format it appro-
priately. This is where NSFormatter objects come into play. When dealing with
any type of number, it is best to add an NSNumberFormatter to the text field or
table column and define its display. Likewise, when working with dates, use
an NSDateFormatter on the field or column to ensure the data is formatted and
validated correctly before it is stored in the Core Data repository. When
working with Core Data, it is sometimes necessary to manipulate the display
of the data so the user's input can be validated and also so it can be displayed
in a usable form. For instance, we're not creating a very good user experience
if we display currency as 3.99 rather than $3.99 or display a date in raw
seconds.
In our application, we used an NSNumberFormatter to display the quantity in the
Ingredients column of our second NSTableView . If we were to add a shopping
list to our application, we would also use NSNumberFormatter objects to display
currency and NSDateFormatter objects to show date and time information.
To add an NSFormatter to a field (either a column or a text field), select it in the
Library palette and drag it onto the interface element. Once it is in place, we
can configure its details in the Attributes inspector, as shown in Figure 29,
NSNumberFormatter Attributes inspector , on page 147 . The Attributes inspector
allows us to configure exactly how the data is presented to the user.
In addition to properly displaying number and date data, the NSFormatter
classes accept input from the user and send that input back to the model in
the correct format. For example, by applying NSNumberFormatter to the Quantity
column of the ingredients table, we are guaranteed to receive an NSNumber
back from the user interface.