# readabilityHandler The block to use for reading the contents of the file handle asynchronously. ``` var readabilityHandler: (@Sendable (FileHandle) -> Void)? { get set } ``` ## Discussion The default value of this property is `nil`. Assigning a valid [Block object](https://developer.apple.com/library/archive/documentation/General/Conceptual/DevPedia-CocoaCore/Block.html#//apple_ref/doc/uid/TP40008195-CH3) to this property creates a dispatch source for reading the contents of the file or socket. Your block is submitted to the file handle’s dispatch queue when there is data to read. When reading a file, your handler block is typically executed repeatedly until the entire contents of the file have been read. When reading data from a socket, your handler block is executed whenever there is data on the socket waiting to be read. The block you provide must accept a single parameter that is the current file handle. The return type of your block should be `void`. To stop reading the file or socket, set the value of this property to `nil`. Doing so cancels the dispatch source and cleans up the file handle’s structures appropriately. --- 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)