The MBS Xojo MacOSX Plugin 25.3 introduces a powerful new class: DispatchIOMBS. This macOS and iOS-only class provides asynchronous I/O operations using Apple's Dispatch I/O API. Whether you want to stream or randomly access file data, DispatchIOMBS gives you fine control over reads and writes—with event-driven or delegate-based callbacks.

Let’s explore how to use DispatchIOMBS to copy a file asynchronously.

Why use dispatch I/O?

• It runs I/O in the background, avoiding UI freezes.
• Built-in memory management avoids unnecessary copying.
• It monitors system pressure for optimized performance.
• Supports stream (kTypeStream) or random-access (kTypeRandom) modes.
• We got a file copy example for Xojo for you

Our example file copies a large file from one location to another asynchronously using stream mode. It uses event handlers rather than delegates for simplicity.

But let's start with a simple async read operation:

As you see we open a file for reading and request the whole file to be read by passing a huge maximum size. Alternatively you can of course pass InputFile.length here. Then we get the delegate called multiple times with chunks:

We log the calls and see we get three calls for a 1.3 MB big file. First call gives 1 MB, then second call provides the rest and last call sets the done flag.

Test Launched

Window1.ReadCompleted
offset: 0
data len: 1048576
done: False

Window1.ReadCompleted
offset: 0
data len: 253015
done: False

Window1.ReadCompleted
offset: 0
data len: 0
done: True

Test Ended

While we read the file in the background, the application do other things in-between.

e.g. you could have the user pick a file and start reading it. Then the user could enter maybe some metadata or set some options while the file is read in the background.

Check out the example file coming with the plugin to copy. Compared to using binarystream, you can have read and write not blocking the main thread and actually have the application doing all at the same time: read, write and user interface.