Within an extension how can load a custom `Civi` name-spaced class?

Question

Within my extension I can load classes from CiviCRM's core codebase like this:

$foo = new \Civi\Core\AssetBuilder();

And I can also load custom classes that I've written in my extension like this:

$foo = new CRM_MyExtension_MyClass();

Cool.

---

But if I've defined a custom class within my extension using the Civi namespace, the following does not work:

$foo = new \Civi\MyExtension\MyClass();

It fails with this error message:

Error: Class 'Civi\MyExtension\MyClass' not found in ...

How can I load this custom class from within my extension?

Answer 1

Add the following elements to your extension's info.xml file to enable PSR-4 autoloading:

<classloader>
  <psr4 prefix="Civi\" path="Civi"/>
</classloader>

Also see docs on the <classloader> element