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Custom NSValueTransformer in xcode 6 with swift

Posted by: admin November 30, 2017 Leave a comment


Did anyone successfully implement a custom NSValueTransformer in xcode 6 beta with swift?

I have the following swift class:

import Foundation

class myTransformer: NSValueTransformer {

  let amount = 100

  override class func transformedValueClass() -> AnyClass!
    return NSNumber.self

  override func transformedValue(value: AnyObject!) -> AnyObject! {
    return value.integerValue + amount

So all this transformer should do is, adding 100 to a given value in the gui.

As you can see, the transformer class appears now in the Value Transformer drop down in IB.

enter image description here

But if I choose this transformer the application crashes with:

2014-08-27 20:12:17.686 cdTest[44134:303] 
*** Terminating app due to uncaught exception 'NSInternalInconsistencyException', 
reason: 'Cannot find value transformer with name newTransformer'

Is it right to register this transformer in the AppDelegate with

override class func initialize() {
  let newTransformer = myTransformer()

Does anyone know how this whole stuff should work?

kind regards!


After you initialise newTransformer you should also include the line:

NSValueTransformer.setValueTransformer(newTransformer, forName: "myTransformer")

Then in your Interface Builder you should use myTransformer instead of newTransformer under the Value Transformer dropdown.


From Xcode release notes:

If you set a Swift subclass of NSValueTransformer as a binding’s value
transformer, the XIB or storyboard will contain an invalid reference
to the class, and the binding will not work properly at runtime. You
can either enter a mangled class name into the Value Transformer field
or add the @objc(…) attribute to the NSValueTransformer subclass to
solve this problem. (17495784)

From Swift guide:

To make your Swift class accessible and usable back in Objective-C,
make it a descendant of an Objective-C class or mark it with the @objc
attribute. To specify a particular name for the class to use in
Objective-C, mark it with @objc(<#name#>), where <#name#> is the name
that your Objective-C code will use to reference the Swift class. For
more information on @objc, see Swift Type Compatibility.


Declare your class as @objc(myTransformer) class myTransformer: NSValueTransformer and then you can use “myTransformer” as name…