---
title: SecTransformActionBlock
framework: security
role: symbol
role_heading: Type Alias
path: security/sectransformactionblock
---

# SecTransformActionBlock

A block that overrides the default behavior of a custom transform.

## Declaration

```swift
typealias SecTransformActionBlock = () -> Unmanaged<CFTypeRef>?
```

## Return Value

Return Value A dictionary of the custom items to be exported if this block is used to override the kSecTransformActionExternalizeExtraData action or  NULL for any other action. Alternatively, the block returns a CFError object if an error occurs.

## Discussion

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 - Determine if the transform has all of the data needed to run. kSecTransformActionStartingExecution - Called just before running ProcessData. kSecTransformActionFinalize - Called just before deleting the custom transform. kSecTransformActionExternalizeExtraData - Called to allow for writing out custom data to be exported. For example: 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);