Coffee & Cocoa hot bits and bytes

Some time ago I wrote about the restrictions of bindings and the fact that custom cells often need more than one data element - see Data for a custom cell in a NSTableView. There were some very good comments on the article and also the request for a sample code.

So I took some code from Sofa Control and put together a sample project for download. The project has a class called ImageTextCell that is a subclass of NSCell and draws two text lines and an icon.

It works like this:

Step 1: Set custom cell on NSTableColumn

As a first step you have to configure your NSTableView to use the custom cell instead of the standard one.

To do that instantiate the custom cell and set that instance as the data cell of a column in the NSTableView. Best place to do that is the awakeFromNib in a controller class (see MainController.m)

// appList is a NSTableView object
NSTableColumn* column = [[appList tableColumns] objectAtIndex:0];
ImageTextCell* cell = [[[ImageTextCell alloc] init] autorelease];
[column setDataCell: cell];

As the custom cell needs more vertical space than the standard cell you have to set the row height of the NSTableView to 34 pixel in Interface Builder.

Step 2: Getting the data to the cell

The tricky thing is how to get the necessary data to the custom cell. In the example we are using a complex object called AppInfo. The AppInfo object has an image and two text elements. These data shall be displayed by the custom cell.

We have a NSArray that holds a number of these AppInfo objects. The column of the NSTableView is bound to display these objects. The binding is configured in Interface Builder.

With this the cell has access to the AppInfo objects. Each time the drawWithFrame method is called the cell can get access to the AppInfo object with [self objectValue]. As we do want to reuse the cell we don’t want to to hardcode the access to AppInfo objects.

Therefore we use an indirection by introducing delegate methods that are responsible to retrieve the data for the cell.

- (NSImage*) iconForCell: (ImageTextCell*) cell data: (NSObject*) data;
- (NSString*) primaryTextForCell: (ImageTextCell*) cell data: (NSObject*) data;
- (NSString*) secondaryTextForCell: (ImageTextCell*) cell data: (NSObject*) data;

The methods are implemented in the controller and you have to set the controller as the delegate on the cell instance (see awakeFromNib in MainController.m).

NSCell copies objects

Each time the method setObjectValue on NSCell is called it creates a copy of the given object. Therefore the AppInfo object implements the NSCopying method.

There are cases where you do not want to create copies of your objects (e.g. when you are using CoreData objects). For these cases I don’t bind the data object itself to the NSTableColumn but instead I bind object identifiers. Some kind of primary key that uniquely identifies an object.

In this case the cell’s objectValue is the object identifier and we have to resolve the correct data object. The ImageTextCell therefore supports an optional delegate method to map an object identifier to the correct data object. A typical implementation looks like this.

- (NSObject*) dataElementForCell: (ImageTextCell*) cell {
    NSObject* uniqueId = [cell objectValue];
    ... build up a core data fetch request
    NSObject* dataObject = ...executeFetch...
    return dataObject;
}

KeyPath instead of delegate methods

In the previous article Markus suggested to use keypath.

Instead of implementing delegate methods one could set keypaths on the ImageTextCell object that are used to retrieve the values from the data object. This works perfectly for simple cases where the getter methods of the data object map perfectly to the display values of the cell.

Instead of setting a delegate and implementing the delegate methods you would configure the cell as following:

[cell setPrimaryTextKeyPath: @"displayName"];
[cell setSecondaryTextKeyPath: @"details"];
[cell setIconKeyPath: @"icon"];

Interface Builder Integration

It would be great if there is an easy way to define binding information for custom cells in Interface Builder. Leopard comes with a totally new Interface Builder and perhaps a way to extend the binding inspector for custom cells as well. We’ll see.

This entry was posted on Friday, May 4th, 2007 at 12:06 pm and is filed under Cocoa. You can follow any responses to this entry through the RSS 2.0 feed. Responses are currently closed, but you can trackback from your own site.