One way binding to NSSlider

I was trying to bind an NSSlider's value to another object. I figured it would be nice to set the slider's value in IB and have it act as default for the bound object. While binding the slider to the object ([myObject bind:@"opacity" toObject:opacitySlider withKeyPath:@"value" options:nil];), my code kept crashing with
*** Terminating app due to uncaught exception 'NSUnknownKeyException', 
reason: '[<NSSlider 0x125040> valueForUndefinedKey:]: this class is not key value coding-compliant for the key value.'

What ? NSSlider does not have a value key ? That's new ! It's there in IB, it shows up in its exposedBindings … Turns out only one way is supported : binding the view (slider) to the object.

BAD [myObject bind:@"opacity" toObject:opacitySlider withKeyPath:@"value" options:nil];
GOOD [opacitySlider bind:@"value" toObject: myObject withKeyPath:@"opacity" options:nil];

I already had that kind of crash with NSTextField. [slider valueForKey:@"value"] will crash, too. I suppose all the controls act alike, that means there's no KVC way to interact with them.

2008 06 06

Yes there is, you use the accessors defined in NSControl (objectValue, doubleValue, floatValue, intValue, stringValue).

Binding to something requires a KVO-compliant key. Binding from something can evidently be faked (or at least renamed).

Patrick Geiller
2008 06 06

Any idea why it's 'faked' and just one way ? It seems like more work than just making a 'value' property KVC compliant.

James Bedford
2010 06 10

This page and it's comments helped me out - thanks! :)

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