Drupal 7 Module Development

Creating New Fields
Like most "info hooks", this function returns a large definition array, defining one or
more fields. Also as we would expect, there is a corresponding hook_field_info_
alter() hook. In our case, we just have the one called dimensions. Let's look at each
property in turn:
• label and description specify the human-readable name and explanation
of this field.
• settings defines an array of configuration options for the field and their
default values. These settings are fixed and after we create an instance of a
field cannot be changed, so use with caution. Generally you only want field
settings if changing the setting would affect how data gets saved.
• instance_settings is the same as the settings array, except that it can be
changed after a field has been created. That makes it generally preferred over
field-level settings.
• default_widget and default_formatter specify what widget and
formatter Drupal should use for a given field before the user specifies one.
Like fields, widgets and formatters have unique string names. We'll talk
about how to write those later in this chapter.
The above code tells Drupal that there is a new field type called dimensions defined
by our dimfield module, and gives a little metadata about it. However, Drupal still
needs to know how that field is put together. For that, we implement a couple of
other hooks.
Defining the field structure
Actually, no, we don't. Although called hooks in the Drupal documentation, these
functions are pseudo-hooks: magically named module callbacks that are called
individually by Drupal rather than together with that hook as used by all modules.
Since our module is named dimfield, the supporting code for all of the field types
we define in the dimfield module will live together in the same magic callback. For
that reason, it's generally a good idea to not define too many field types in a single
module as the code may get unwieldy. We also use a different name for the module
and for the field type to help keep track of when we need to use which.
A magic module callback, or pseudo-hook, looks like a hook, but
is called individually rather than alongside implementations from
all other active modules.
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