Is it necessary to override bind:toObject:withKeyPath:options: in an NSView subclass to implement binding?

Question

I have an NSView subclass which has property which I want to be bindable. I've implemented the following in the subclass:

myView.h:

@property (readwrite, retain) NSArray *representedObjects;

myView.m:

@synthesize representedObjects;

+(void)initialize
{
    [self exposeBinding: @"representedObjects"];
}


-(void)bind:(NSString *)binding toObject:(id)observableController withKeyPath:(NSString *)keyPath options:(NSDictionary *)options
{
    if ([binding isEqualToString:@"representedObjects"]) {
        [observableController addObserver: self forKeyPath:@"arrangedObjects" options:NSKeyValueChangeNewKey context:nil];
    } else {
        [super bind: binding toObject:observableController withKeyPath:keyPath options: options];
    }
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
    if ([keyPath isEqualToString:@"arrangedObjects"]) {
        [self setRepresentedObjects: [object arrangedObjects]];
    }
}

I then create the binding to an arrayController in -[AppController awakeFromNib]:

[myView bind:@"representedObjects" toObject:arrayController withKeyPath:@"arrangedObjects" options: nil];

Is this the correct way of implementing binding? It involves a lot of boiler plate code which makes me think that I'm doing something wrong.

I thought that NSObject would automagically implement what I have done manually in -bind:toObject:withKeyPath:options: but this doesn't seem to be the case. If I comment out my -bind:toObject:withKeyPath:options: the setRepresentedObjects method is never called.

Additional info: I've done some more investigating and have reached the conclusion that my original approach is correct and you do have to over ride -bind:toObject:withKeyPath:options:. Here's a quote from Cocoa Bindings Programming Topics: How Do Bindings Work?:

In its bind:toObject:withKeyPath:options: method an object must as a minimum do the following:

• Determine which binding is being set
• Record what object it is being bound to using what keypath and with what options
• Register as an observer of the keypath of the object to which it is bound so that it receives notification of changes

The code sample in Listing 2 shows a partial implementation of Joystick’s bind:toObject:withKeyPath:options: method dealing with just the angle binding.

Listing 2 Partial implementation of the bind:toObject:withKeyPath:options method for the Joystick class:

static void *AngleBindingContext = (void *)@"JoystickAngle";

- (void)bind:(NSString *)binding
 toObject:(id)observableObject
 withKeyPath:(NSString *)keyPath
 options:(NSDictionary *)options
{
 // Observe the observableObject for changes -- note, pass binding identifier
 // as the context, so you get that back in observeValueForKeyPath:...
 // This way you can easily determine what needs to be updated.

if ([binding isEqualToString:@"angle"])
 {
    [observableObject addObserver:self
                   forKeyPath:keyPath
                  options:0
                  context:AngleBindingContext];

    // Register what object and what keypath are
    // associated with this binding
    observedObjectForAngle = [observableObject retain];
    observedKeyPathForAngle = [keyPath copy];

    // Record the value transformer, if there is one
    angleValueTransformer = nil;
    NSString *vtName = [options objectForKey:@"NSValueTransformerName"];
    if (vtName != nil)
    {
        angleValueTransformer = [NSValueTransformer
            valueTransformerForName:vtName];
    }
 }
 // Implementation continues...

This clearly shows that the Joystick class (which is an NSView subclass) needs to override -bind:toObject:withKeyPath:options:.

I find this surprising. I was skeptical of this conclusion as I have found no other code samples that do this. However, as the offical Apple documentation says I should over ride -bind:toObject:withKeyPath:options: I conclude that it is the correct approach.

I would be very happy if someone could prove me wrong!

Answer 1

No, you shouldn’t need that glue code.

What do you mean by “doesn’t seem to be the case”? What happens if you omit it?

Answer 2

I didn't think I needed that code! That leads to the next question: what am I doing wrong?

Answer 3

You definitely DO need to implement -bind:toObject:withKeyPath:options: in a custom view if you want to implement bindings in that view. Your implementation in myView.m is pretty much spot on.