The Legacy Widget block allows users to use a WordPress widget as if it were a block. Notably, it is used by the Widgets block editor screen to allow adding and editing widgets using the block editor.
Your typical Legacy Widget block looks like this:
<!-- wp:legacy-widget {"idBase": "search", "instance": {} /-->
This will output a search widget with a title.
idBase is the "base ID" of the widget. It corresponds to what the widget sets
WP_Widget::$id_base to when it is registered.
instance contains the widget's settings. These settings are ultimately passed
to WP_Widget::widget. Because widget settings can contain any PHP value,
including values that have no representation in JSON, the instance settings are
represented using a base64 encoded string in instance.encoded. A hash of this
data in instance.hash verifies that the data has not been tampered with.
If a widget has set WP_Widget::$show_instance_in_rest to true, then its raw
unencoded settings are kept in instance.raw. This is useful for transforming a
Legacy Widget block into e.g. a Search block.
WordPress supports a legacy style of widget that does not extend WP_Widget.
These are registered using wp_register_sidebar_widget(). Widgets registered
this way do not support being inserted multiple times. That is, there can only
be one widget of this type per website.
For these widgets, the Legacy Widget block looks like this:
<!-- wp:legacy-widget {"id":"my_widget"} /-->
Note that, in this case, there is no instance attribute. All data handling is
done by the widget via form submission. No state is stored in the block editor,
which means that, for example, pressing Undo and Redo will have no effect on
changes made to a single widget.
