Drupal 7 Module Development

Creating New Fields
}
}
}
}
}
);
),
'%max' => $instance['settings'][$max_key],
''x)
Just as all fields can be validated individually, so can all form elements. However,
recall that fields can be saved from anywhere in code. We may not be using a
form at all. We therefore must validate the field data itself, before we save it to the
database. In this case, we're checking to make sure that if the dimension has a value,
and if a maximum was set for it, it is within that maximum limit. If it's not, then
we set an error in the $errors array, which is passed in by reference. That error
consists of, naturally, an array of possible errors. It is up to the calling code to decide
how to handle that error condition. It could show a message on screen if the error
happens from a user form, or could send an invalid message object back over an
SOAP connection if the field (and the entity it's attached to) is being saved by code
triggered by a remote server.
For more extensive information on each of the parameters to the
Field API callback functions, see the examples in the field.api.
php file in the field module.
Another important point to note here is that field is passed an array of items, not an
individual item. From a code perspective, fields in Drupal are always multi-value.
Even if there is only one value, even if the field is configured to only allow one value,
it is still multi-value as far as our code is concerned. "One" is simply a special case of
"many". That actually greatly simplifies most of our logic, as we don't need to handle
two different possible cases. We can simply iterate with a foreach() loop over our
data, and we will handle one or a hundred values equally well.
Remember that fields in Drupal are always a multi-value array
in code. That array may have only one entry, but it can still be
treated as an arbitrarily large number of values.
Again, notice that nowhere in the field type definition or supporting code do we
actually save data. In fact, there's not a single SQL query. We are simply describing
the data. Saving the data itself, and deciding where to save it, is the responsibility
of the core system. That allows a great deal of flexibility, as our dimension field can
now be used to store data in a local SQL database or a remote SOAP server without
any code changes on our part.
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