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Like most API components, FieldTypes use the Symfony 2 service tag mechanism.
The principle is quite simple: a service can be assigned one or several tags, with specific parameters. When the dependency injection container is compiled into a PHP file, tags are read by CompilerPass implementations that add extra handling for tagged services. For FieldTypes, each Each service tagged as ezpublish.fieldType is added to a registry using the alias argument as its unique identifier (ezstring, ezxmltext…). Each FieldType must also inherit from the abstract ezpublish.fieldType service. This ensures that the initialization steps shared by all fieldtypes are executed.
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