Callback function signatures.
Drupal's API sometimes uses callback functions to allow you to define how some type of processing happens. A callback is a function with a defined signature, which you define in a module. Then you pass the function name as a parameter to a Drupal API function or return it as part of a hook implementation return value, and your function is called at an appropriate time. For instance, when setting up batch processing you might need to provide a callback function for each processing step and/or a callback for when processing is finished; you would do that by defining these functions and passing their names into the batch setup function.
Callback function signatures, like hook definitions, are described by creating and documenting dummy functions in a *.api.php file; normally, the dummy callback function's name should start with "callback_", and you should document the parameters and return value and provide a sample function body. Then your API documentation can refer to this callback function in its documentation. A user of your API can usually name their callback function anything they want, although a standard name would be to replace "callback_" with the module name.
|callback_allowed_values_function||core/modules/options/options.api.php||Provide the allowed values for a 'list_*' field.|
|callback_batch_finished||core/lib/Drupal/Core/Form/form.api.php||Complete a batch process.|
|callback_batch_operation||core/lib/Drupal/Core/Form/form.api.php||Perform a single batch operation.|
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