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Sorting Table View Rows

Table views can be configured so that users can click on a column’s header to toggle scrolling from none to ascending or descending order. You use sort descriptors to enable this behavior.

Sorting a Table View Programmatically

Your app can specify a default sort descriptor to use when displaying the table view data. It does this using the NSTableViewsetSortDescriptors: method. The sort descriptors are an array of NSSortDescriptors instances. For example, the following code sorts the content based on the last name and first name:

    [theTableView setSortDescriptors:[NSArray arrayWithObjects:

                                      [NSSortDescriptor sortDescriptorWithKey:@"lastName" ascending:YES selector:@selector(compare:)],

                                      [NSSortDescriptor sortDescriptorWithKey:@"firstName" ascending:YES selector:@selector(compare:)],

                                      nil]];

This code snippet sorts the content indirectly through the delegate callback (see Responding to a Sort Request) but won’t allow a user to click in a table header to change the sort order for that column. To give users this capability, you need to configure sorting on a per-column basis. In a table configured for per-column sorting, the user’s click on a table header modifies the table view’s sort descriptor to reflect the current sort state of the table view (you can use the sortDescriptors method to access the sort descriptor).

Configuring Sorting for Individual Table Columns

You can sort individual table columns programmatically by specifying an NSSortDescriptor prototype instance for the appropriate NSTableColumn instance, or by using Interface Builder.

Configuring Per-Column Sorting in Code

You can set a sort method for a column programmatically by creating a sort descriptor prototype. You create the prototype by using the method sortDescriptorWithKey:ascending:selector: to create an NSSortDescriptor instance. The resulting sort descriptor instance is then set as the table column’s sorting prototype using the method setSortDescriptorPrototype:. When a user clicks the header, the table view’s sort descriptors are modified and the table column header reflects the sort direction.

The following code creates a sort descriptor prototype for a lastName column using localized sorting:

NSTableColumn *tableColumn = [theTableView tableColumnWithIdentifier:@"lastName"];

NSSortDescriptor *lastNameSortDescriptor = [NSSortDescriptor sortDescriptorWithKey:@"lastName"

                                                                         ascending:YES

                                                                          selector:@selector(compare:)];

[tableColumn setSortDescriptorPrototype:lastNameSortDescriptor];

Configuring Per-Column Sorting in Interface Builder

To use Interface Builder to configure per-column sorting, select a table column and open the Table Column Attributes inspector. The inspector contains three controls that help you configure sorting behavior:

The Sort Key field specifies the key in the table view data model that the column sorts on.
The Selector field contains the selector that the sort uses. Tabbing to the field causes the field to fill with the compare: selector.
This comparison selector expects a single parameter—the value that needs to be sorted—and returns an NSComparisonResult. You can substitute any method selector that adheres to that pattern, including custom methods of your own.
The compare: method works with NSString, NSDate, and NSNumber objects. If your table column contains only strings, you may want to consider using the caseInsensitiveCompare: method if case sensitivity is unimportant. However, consider replacing these method signatures with the localizedCompare: or localizedCaseInsensitiveCompare: methods to take into the account the user’s language requirements.
The Order pop-up menu allows you to specify the initial order of the sort, as ascending (default) or descending.

Leaving the Sort Key and Selector fields blank disables sorting of the column using the settings within Interface Builder. However, it does not disable sorting of the column using other techniques.

Responding to a Sort Request

When a table view is sorted through user action or by setting the table view’s sort descriptors, the data source receives a tableView:sortDescriptorsDidChange: message. The data source must implement this method to support sorting.

The tableView:sortDescriptorsDidChange: method applies the table view’s current sort descriptors to the model collection, and then reloads the table view data. A typical implementation of this delegate method is shown in Listing 7-1.

Listing 7-1 Sample implementation of the delegate method tableView:sortDescriptorsDidChange:

- (void)tableView:(NSTableView *)tableView sortDescriptorsDidChange:(NSArray *)oldDescriptors

    [peopleArray sortUsingDescriptors: [tableView sortDescriptors]];

    [tableView reloadData];

The implementation shown in Listing 7-1 sorts the current model content using the table view sort descriptors, but it also loses the initial order of the data. If the initial order is important to your app, you’ll want to take steps to cache the original order so that it can be restored.

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