Cocoa Bindings in OS X Storyboards

Q:  Given that there is no File’s Owner in an OS X storyboard, how can I bind the UI elements in my view controller scenes to my application's model objects?

A: In an OS X storyboard, every scene has a controller. We use that controller to manage the content of the scene and interact with the other parts of the application, thus you don’t need a File’s Owner object anymore. (For more information on File’s Owner, see the About File's Owner section of Resource Programming Guide.)

In your view controller scene, the controller is an object of the NSViewController subclass. You can use its representedObject property, or make any number of other properties that are more type-safe, to represent the model your view controller works with, and then bind through them.

For Cocoa bindings to work correctly, be sure that the model properties are ready for use when Cocoa bindings need them. This is especially important when you need to load the model data immediately after the application launch. To achieve that, there are two cases to consider:

• For a document based application, you can set up the model properties in the -makeWindowControllers method of your document class, as shown in Listing 1.

  
  
  

  Listing 1  Document based application

  - (void)makeWindowControllers {

  /* Your code to set up the window controller(s) for your document*/

  // Set up the view controller’s representedObject property here,

  // assume the contentViewController of the current window controller

  // is the view controller you working with.

  yourViewController = (YourViewController *)windowController.contentViewController;

  yourViewController.representedObject = <your_model_object>;

  }

• For a non-document based application, you might set up the model properties in the -awakeFromNib method of your view controller class, as shown in Listing 2.

  
  
  

  Listing 2  Non-document based application

  - (void)awakeFromNib {

  [super awakeFromNib];

  if (! self.representedObject) {

  self.representedObject = <your_model_object>;

  }

  }

Once the model properties have been set up, you can safely use key paths starting with "self.representedObject" (or other properties you defined). Figure 1 shows you how to bind an NSArrayController object within a view controller scene to the managedObjectContext property of the view controller's representedObject (which represents a document object owning that property).

Be aware that the object of a model property may be replaced when it is written back through Cocoa bindings. As an example, when you bind an NSArrayController object to an NSMutableArray property, NSArrayController uses the mutableArrayValueForKey: method defined in NSKeyValueCoding.h to write back changes. Your NSMutableArray object will be replaced with a new array if you don't implement the array mutation methods for the property. The details are covered in NSKeyValueCoding.h.

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