NSValueTransformer

Scripting Forums AppleScriptObjC and Xcode
#1

Hello Everybody,

I just wanted to know: there is a “Value Transformer” field in IB bindings. I’ve read this value transformer is a class which will be invoked to return a result.

Say I want an image to be set on a “Green” picture if a value “test” in the arrangedObjects of a controller is equal to a value “target” in the same array. To do it with bindings, do I have to subclass NSValueTransformer?

So to set the “hidden” flag of a control: it’s easy to do if the condition is boolean (control hidden if this property <> 0, using NSNegateBoolean and so on). But what if I want the control to be hidden if the property is, say, “3” or “less than 2”?

I know how to do it by code, it’s easy, it does not require more than an IF test. But with bindings?

Maybe I’m a bit compulsive with bindings but I love the concept. :stuck_out_tongue:

Regards,

#2

Hi,

actually the proper way is keyPathsForValuesAffectingValueForKey: of NSObject which uses key value observing.
Short ObjC explanation, but it should work also in ASOC.

There is a generic +keyPathsForValuesAffecting method. represents the key to be entered in the bindings key path field. The method must return a set of (literal string) keypaths.

The respective variables must be key-value-observing compliant (defined as properties)

For example you want to hide something depending on the number of items in an array,
the +keyPathsForValuesAffecting method is


+ (NSSet *)keyPathsForValuesAffectingHideSomething
{
	return [NSSet setWithObject:@"myArray"];
}

myArray is the instance variable property of the array

The corresponding key method is

- (BOOL)hideSomething
{
	return [self.myArray count] > 3;
}

Consider that the method starts with a lowercase letter,
but the letter after keyPathsForValuesAffecting in the other method is a capital letter!

now enter hideSomething in the key path of the hidden binding of your UI element.

This works also for example with an image


+ (NSSet *)keyPathsForValuesAffectingMainImage
{
	return [NSSet setWithObject:@"imageStatus"];
}


- (NSImage *)mainImage
{
	if (self.imageStatus == 0)
		return [NSImage imageNamed:@"blueImage"]; 
    else if (self.imageStatus == 1)
        return [NSImage imageNamed:@"greenImage"];
    else
        return nil;
}

imageStatus is an integer instance variable property
the key mainImage will be bound to the key path of the value property of an NSImageView object

#3

Hi Stefan,

Thank you for this detailed answer! How much time does it require to gain your knowledge of cocoa? I’m a bit afraid to know the answer – are you a pro developer or just an enthusiastic amateur?

The problem is that good programers don’t spend too much time with ASOC and turn themselves to Objective-C. But I’m not a good programer and stick to ASOC :slight_smile:

I’ll see if I can transfer this code into something working.

Thank you very much again!

Regards,

#4

I regard myself as an enthusiastic amateur, I learned Cocoa/Objective-C as a normal foreign language.

You’re right. I’m using ASOC only for Apple Events to communicate between applications.
Although ASOC is a pretty clever Cocoa bridge, there are too many restrictions in comparison to native C/Objective-C