macOS NSValueTransformer Converting BOOL to NSString

[stadidas]

I wanted to create a value transformer that would convert a BOOL value to either a Yes or No string, for displaying data using bindings. However, the method that must be overriden in a NSValueTransformer subclass, - (id)transformedValueid)value, expects an object to be passed in. As I am passing in a boolean, the only way I have found that works is this:

- (id)transformedValue:(id)value
{
	if ((int)value == 16866128)
	{
		return @"Yes";
	}
	return @"No";
}

However, this is obviously really hacky, and also makes the NSTextField subviews that are bound to the value with the transformer applied redraw in a strange way.

I'm sure there must be a better way of converting booleans to text, does anyone have any pointers?

 

[toddburch]

I'm still learning the language, but figured I'd add to the discussion...

My understanding is that BOOL isn't a class, but a special type handled by the preprocessor. So, isn't passing it under the (id) umbrella in variable "value" incorrect?

 

[kainjow]

id is going to be an NSObject subclass, and most likely for your situation it'll be a BOOL inside an NSNumber. So try something along the lines of:

- (id)transformedValue:(id)value {
    if ([value boolValue])
    {
        return @"Yes";
    }
    return @"No";
}

Edit: what you're doing in your code is comparing the pointer address, and the reason I'm guessing that it works is because when you create an NSNumber via numberWithBool:, NSNumber internally will cache that object for YES and for NO and always use the same one.

 

[stadidas]

I'm still learning the language, but figured I'd add to the discussion...

My understanding is that BOOL isn't a class, but a special type handled by the preprocessor. So, isn't passing it under the (id) umbrella in variable "value" incorrect?

I am aware of this; this is why I wanted to find a correct way of achieving what I wanted.

id is going to be an NSObject subclass, and most likely for your situation it'll be a BOOL inside an NSNumber. So try something along the lines of:

- (id)transformedValue:(id)value {
    if ([value boolValue])
    {
        return @"Yes";
    }
    return @"No";
}

Edit: what you're doing in your code is comparing the pointer address, and the reason I'm guessing that it works is because when you create an NSNumber via numberWithBool:, NSNumber internally will cache that object for YES and for NO and always use the same one.

Ah, interesting, thanks Kainjow. Your method also worked. I should have thought of this myself, but it was late and I'd been coding for hours. Honest

 

[toddburch]

I was going in the right direction, as shown by kainjow.

I was going to suggest something like this, but then realized BOOL wasn't a class...

-(id) transformedValue: (id) value { 
   if (value isKindOf: [BOOL class]) { 
      if (value==YES) return @"Yes" ; 
      return @"No" ; 
   }
   return super(value) ; 
}

But then I realized that [BOOL class] would have failed.