Concepts

Schema-first

A form0 schema is a JSON document with a top-level form object. The schema defines the structure, rules, and behavior of a form without any UI code.

Fields and values

• Each field has a data_name (human-readable identifier) and a key (internal identifier, typically derived from data_name).
• Engine values are stored by data_name.
• Conditions reference fields by field_id. You can use either key or data_name; using data_name is often more practical when writing schemas manually since it's more memorable.

Engine state

The engine evaluates the schema and produces a state object with:

• values: current field values
• errors: validation errors
• visible: visibility flags per field
• required: requirement flags per field
• read_only: read-only flags per field

Data flow

1. Load the schema and create the engine
2. Provide initial values (optional)
3. Fire any initialization events
4. Evaluate: calculations, conditions, and validation
5. On each value change:
   • Update the value
   • Re-evaluate (live validation)
   • Fire change events if defined
6. Read state or build a structured record for submission

Events and expressions

• Calculated fields use expressions, inside calculate property, like $age + 1 to derive values from other fields.
• Events live in form.events.code and can react to form changes at runtime (e.g., when a record is loaded or a field changes).