# isPartialStringValid(_:proposedSelectedRange:originalString:originalSelectedRange:errorDescription:) This method should be implemented in subclasses that want to validate user changes to a string in a field, where the user changes are not necessarily at the end of the string, and preserve the selection (or set a different one, such as selecting the erroneous part of the string the user has typed). ``` func isPartialStringValid(_ partialStringPtr: AutoreleasingUnsafeMutablePointer, proposedSelectedRange proposedSelRangePtr: NSRangePointer?, originalString origString: String, originalSelectedRange origSelRange: NSRange, errorDescription error: AutoreleasingUnsafeMutablePointer?) -> Bool ``` ## Parameters `partialStringPtr` The new string to validate. `proposedSelRangePtr` The selection range that will be used if the string is accepted or replaced. `origString` The original string, before the proposed change. `origSelRange` The selection range over which the change is to take place. If the user change is a deletion, `origSelRange` contains the range of the deleted characters. `error` If non-`nil`, if validation fails contains an `NSString` object that describes the problem. ## Return Value if `partialStringPtr` is acceptable, otherwise . ## Discussion In a subclass implementation, evaluate `partialString` according to the context. Return if `partialStringPtr` is acceptable and if `partialStringPtr` is unacceptable. If you return and assign a new string to `partialStringPtr` and a new range to `proposedSelRangePtr`, the string and selection range are changed, otherwise, if no values are assigned to `partialStringPtr` or `proposedSelRangePtr`, the change is rejected. If you return , you can also return by indirection an `NSString` object (in `error`) that explains the reason why the validation failed; the delegate (if any) of the `NSControl` object managing the cell can then respond to the failure in control:didFailToValidatePartialString:errorDescription:. --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)