# SecTransformActionBlock A block that overrides the default behavior of a custom transform. ``` typealias SecTransformActionBlock = () -> Unmanaged? ``` ## Return Value A dictionary of the custom items to be exported if this block is used to override the [`kSecTransformActionExternalizeExtraData`](/documentation/Security/kSecTransformActionExternalizeExtraData) action or `NULL` for any other action. Alternatively, the block returns a object if an error occurs. ## Discussion A block of this type is used to override the default behavior of a custom transform. This block is associated with the SecTransformOverrideTransformAction block. The behaviors that can be overridden are: - [`kSecTransformActionCanExecute`](/documentation/Security/kSecTransformActionCanExecute) - Determine if the transform has all of the data needed to run. - [`kSecTransformActionStartingExecution`](/documentation/Security/kSecTransformActionStartingExecution) - Called just before running ProcessData. - [`kSecTransformActionFinalize`](/documentation/Security/kSecTransformActionFinalize) - Called just before deleting the custom transform. - [`kSecTransformActionExternalizeExtraData`](/documentation/Security/kSecTransformActionExternalizeExtraData) - Called to allow for writing out custom data to be exported. For example: ```objc SecTransformImplementationRef ref; CFErrorRef error = NULL; error = SecTransformSetTransformAction(ref, kSecTransformActionStartingExecution, ^{ // Initialize any data needed before running CFErrorRef result = DoMyInitialization(); return result;}); SecTransformTransformActionBlock actionBlock = ^{ // Clean up any existing data before running CFErrorRef result = DoMyFinalization(); return result;}; error = SecTransformSetTransformAction(ref, kSecTransformActionFinalize,actionBlock); ``` --- 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)