# isPartialStringValid(_:newEditingString:errorDescription:) Returns a Boolean value that indicates whether a partial string is valid. ``` func isPartialStringValid(_ partialString: String, newEditingString newString: AutoreleasingUnsafeMutablePointer?, errorDescription error: AutoreleasingUnsafeMutablePointer?) -> Bool ``` ## Parameters `partialString` The text currently in a cell. `newString` If `partialString` needs to be modified, upon return contains the replacement string. `error` If non-`nil`, if validation fails contains an `NSString` object that describes the problem. ## Return Value if `partialString` is an acceptable value, otherwise . ## Discussion This method is invoked each time the user presses a key while the cell has the keyboard focus—it lets you verify and edit the cell text as the user types it. In a subclass implementation, evaluate `partialString` according to the context, edit the text if necessary, and return by reference any edited string in `newString`. Return if `partialString` is acceptable and if `partialString` is unacceptable. If you return and `newString` is `nil`, the cell displays `partialString` minus the last character typed. 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:. The selection range will always be set to the end of the text if replacement occurs. This method is a compatibility method. If a subclass overrides this method and does not override [`isPartialStringValid(_:proposedSelectedRange:originalString:originalSelectedRange:errorDescription:)`](/documentation/Foundation/Formatter/isPartialStringValid(_:proposedSelectedRange:originalString:originalSelectedRange:errorDescription:)), this method will be called as before ([`isPartialStringValid(_:proposedSelectedRange:originalString:originalSelectedRange:errorDescription:)`](/documentation/Foundation/Formatter/isPartialStringValid(_:proposedSelectedRange:originalString:originalSelectedRange:errorDescription:)) just calls this one by default). --- 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)