Extending the TYPO3 Core

Events and hooks provide an easy way to extend the functionality of the TYPO3 Core and its extensions without blocking others to do the same.

Events are being emitted by the TYPO3 Core or an extension via the event dispatcher. The event will be received by all implemented event listeners for the event in question. Events are strongly typed. Events only allow changes to variables that are intended to be changed by the event.

Hooks are basically places in the source code where a user function will be called for processing, if such has been configured.

Changed in version 12.0

Signals and slots and all related classes have been removed from the Core. Use PSR-14 events instead.

TYPO3 extending mechanisms video

Lina Wolf: Extending Extensions @ TYPO3 Developer Days 2019

Events and hooks vs. XCLASS extensions

Events and hooks are the recommended way of extending TYPO3 compared to extending PHP classes with a child class (see XCLASS extensions). Using the XCLASS functionality only one extension of a PHP class can exist at a time while hooks and events may allow many different user-designed processor functions to be executed. With TYPO3 v10 the event dispatcher was introduced. It is a strongly typed way of extending TYPO3 and therefore recommended to use wherever available.

However, events have to be emitted and hooks have to be implemented in the TYPO3 Core or an extension before they can be used, while extending a PHP class via the XCLASS method allows you to extend any class you like.

Proposing events

If you need to extend the functionality of a class which has no event or hook yet, then you should suggest emitting an event. Normally that is rather easily done by the author of the source you want to extend:

For the TYPO3 Core create an issue on forge.typo3.org.
For a third-party extension create an issue in the according issue tracker of that extension.