- Launch Xcode.
- From the File menu, select New Project and scroll to "Core Data Application". screenshot
- Click "Next".
- Name the project appropriately.
- Click "Finish".
Xcode will prepare a blank Core Data Application project for you.
AbstractTree is a Cocoa sample application that demonstrates how to use Core Data and Bindings with NSTreeController. The data model is intentionally simple and abstract - you can extend it to any hierarchical model suitable for your problem domain. Likewise, the user interface is minimal - just an outline view and buttons for creating and removing nodes in the tree. In addition, the project shows how to implement drag and drop for managing parent-child relationships of the Core Data content.
This tutorial will cover each step necessary for creating the project. By default, many detailed sections are not displayed, but can be viewed by clicking links found in the document.
Using Xcode, create a new Core Data Application project.
AbstractTree's data model consists of a single entity, "Node", representing a generic tree node. Most importantly, nodes have a to-one "parent" relationship and a to-many "children" relationship. (Nodes also have a name, so that there's something to display in the UI of the application.) In a general sense, trees have several types of nodes - root nodes, container nodes, and leaf nodes. A root node is a node without a parent; containers have children; leaves have no children. A data model could represent a tree in many different ways - we're just keeping things simple in this sample. So, in AbstractTree, a root node is implicitly any node with a nil parent, and a leaf node is implicitly any node that has no children.
The children relationship is the key to NSTreeController functionality. AbstractTree has a single entity in its hierarchy, so this is not an issue, but if there are multiple entities in a hierarchy they must all have a common key for accessing children. Without a common key, NSTreeController will be unable to manage its content. The parent relationship is present for two reasons: (1) modeling inverse relationships is a strongly encouraged practice when using Core Data, and (2) we test the parent relationship to determine the root(s) of our tree in the UI.
The application delegate object provides the necessary Core Data support by loading the managed object model and creating the persistent store coordinator and managed object context objects. All of this is in boiler-plate code provided by the Xcode template - you don't have to do anything here, unless you wish to customize these actions for your application. For this project, we need to add some IBOutlets and implement some additional methods to manage drag and drop.
We need an outlet to the NSOutlineView so we can register it for receiving drag operations, and to the NSTreeController to use it as a mediator in the drag operation.
For drag and drop, we make the application delegate the data source of the NSOutlineView. This does not obligate us to implement the "standard" data source methods, because that functionality will be provided by binding to the NSTreeController. Instead, we simply implement the data source methods specific to drag operations. In the drag, we use a URL representation of the Core Data objects, a simple but clean way of uniquely specifying them ideal for pasteboard operations.
If you are following along with your own project, add the outlets to your application delegate and copy the source implementations found below the line "#pragma mark Code Added for AbstractTree Drag and Drop".
AbstractTree’s UI is minimal: it consists of a single window containing a NSOutlineView and two NSButtons. The outline view shows the nodes in the tree, hierarchically arranged, and the buttons allow you to create new nodes and remove existing nodes. Node names will be editable by double-clicking them directly in the tree. We will use a NSTreeController to mediate between the outline view, the buttons, and the data model for the tree.
In MainMenu.nib, the NSTreeController needs to be configured. Set it to "Entity" Mode, with an Entity Name of "Node". In addition, the "Prepares Content" should be checked and the Fetch Predicate should be "parent == nil". This predicate will constrain the controller to objects which are root nodes because those nodes will not have any value for their parent relationship. It is also necessary to configure the Key Path for Children, which will be simply "children" - matching the name of the appropriate relationship in the data model. The Count and Leaf key paths can be left blank.
We need to connect a number of outlets and actions in Interface Builder in order for messages to pass between the objects in the application. Some of these connections are already in place when the project is created, so we'll just focus on the ones you have to add. Specifically, the connections needed are:
Bindings are the final step in making the application come together. Two objects will be bound - the tree controller and the outline view's table column. The tree controller will bind its managedObjectContext to that of the application delegate. The table column will bind its value to the tree controller. The bindings are:
You should be able to build and run the project from Xcode now. Try the Add and Remove buttons, they should create and destroy nodes displayed in the outline. Double-clicking a node in the outline should allow you to edit its name. Dragging nodes in the outline should allow you to reorganize the node hierarchy.
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