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This documentation covers files located at: Questionnaire Item String // todo link to source createExtensionAccessor

Extensions

In FHIR, every extension requires a unique URL that serves as its identifier. While FHIR provides standard definitions for many features, it does not cover every requirement. Other renderers (such as Aidbox) may use different URLs, and custom functionalities require the definition of proprietary URLs.

Supported Extension URLs

The following URLs are supported and parsed by our renderer:

Feature	Extension URL	Key / Value Type
Translation	http://hl7.org/fhir/StructureDefinition/translation	Complex Type
Min Value	http://hl7.org/fhir/StructureDefinition/minValue	valueDecimal
Max Value	http://hl7.org/fhir/StructureDefinition/maxValue	valueDecimal
Min Length	http://hl7.org/fhir/StructureDefinition/minLength	valueDecimal
Max Length	CUSTOM/maxLength	valueDecimal
Min Occurrences	http://hl7.org/fhir/StructureDefinition/questionnaire-minOccurs	valueInteger
Max Occurrences	http://hl7.org/fhir/StructureDefinition/questionnaire-maxOccurs	valueInteger
Item Control	http://hl7.org/fhir/StructureDefinition/questionnaire-itemControl	valueCodeableConcept
Choice Orientation	http://hl7.org/fhir/StructureDefinition/questionnaire-choiceOrientation	valueCode
Unit	http://hl7.org/fhir/StructureDefinition/questionnaire-unit	valueUnit
Regex Pattern	http://hl7.org/fhir/StructureDefinition/regex	valueString
Entry Format	http://hl7.org/fhir/StructureDefinition/entryFormat	valueString
Display Logic	http://hl7.org/fhir/StructureDefinition/display	valueString
Variable	http://hl7.org/fhir/StructureDefinition/variable	valueExpression
SDC Enable When	http://hl7.org/fhir/uv/sdc/StructureDefinition/sdc-questionnaire-enableWhenExpression	valueExpression

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Implementation Notes

• Standardization: Most URLs follow the official HL7 FHIR StructureDefinitions.
• Custom Extensions: Features like MAX_LENGTH currently use a custom path (CUSTOM/maxLength) and should be verified against R4 standards if high interoperability is required.
• SDC Support: We support the Structured Data Capture (SDC) implementation guide for advanced logic like enableWhenExpression.

Requirements

Some extensions require additional configuration to be active. For example, the minOccurs extension requires the item to be required by FHIR Definition.

Extension Accessor

To simplify interactions with FHIR extensions, we utilize an ExtensionAccessor. This utility provides a streamlined way to extract the values defined in our supported extensions list.

Initialization & Usage

The accessor must be initialized for each item using the respective questionnaireItem. Once initialized, the extension values are exposed as Angular Signals, allowing for reactive and efficient UI updates.

Key Features

• Reactive Access: Values are retrieved as Signals, ensuring the UI stays in sync with the underlying data model.
• Key Overriding: While the accessor uses the defined value keys passed via a map (e.g., valueDecimal), you can manually override the key if a different data type is expected.
• Collection Handling: Ability to retrieve all occurrences of an extension if multiple instances are present.
• Full Extension Access: Provides access to the entire extension object for complex calculations that require metadata beyond the raw value.

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Example Usage (Conceptual)

questionnaireItem: Signal<QuestionnaireItem>>

extensionAccessor = createExtensionAccessor(this.questionnaireItem, FHIR_QUESTIONNAIRE_EXTENSION_VALUE_MAP);

private minOccurExtension: Signal<number | undefined> = this.extensionAccessor(FHIR_EXTENSIONS.QUESTIONNAIRE.MIN_OCCURS);