Refactor FunctionCall to enable full validation #551
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| # Extending A2UI with Custom Functions | ||
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| A2UI functions are designed to be extensible. Third-party developers can define | ||
| their own | ||
| function catalogs while preserving strict validation for the standard set. | ||
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| This guide demonstrates how to create a `custom_catalog.json` that adds a string | ||
| `trim` function and a hardware query function (`getScreenResolution`). | ||
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| ## 1. Define the Custom Catalog | ||
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| Create a JSON Schema file (e.g., `custom_catalog.json`) that defines your | ||
| function parameters. | ||
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| Use the `functions` property to define a map of function schemas. | ||
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| ```json | ||
| { | ||
| "$schema": "https://json-schema.org/draft/2020-12/schema", | ||
| "$id": "https://example.com/schemas/custom_catalog.json", | ||
| "title": "Custom Function Catalog", | ||
| "description": "Extension catalog adding string trimming and screen resolution functions.", | ||
| "functions": { | ||
| "trim": { | ||
| "type": "object", | ||
| "properties": { | ||
| "call": { "const": "trim" }, | ||
| "returnType": { "const": "string" }, | ||
| "args": { | ||
| "type": "array", | ||
| "minItems": 1, | ||
| "maxItems": 2, | ||
| "prefixItems": [ | ||
| { | ||
| "$ref": "https://a2ui.dev/specification/v0_9/common_types.json#/$defs/DynamicString", | ||
| "description": "The string to trim." | ||
| }, | ||
| { | ||
| "$ref": "https://a2ui.dev/specification/v0_9/common_types.json#/$defs/DynamicString", | ||
| "description": "Optional. A set of characters to remove. Defaults to whitespace." | ||
| } | ||
| ] | ||
| } | ||
| }, | ||
| "required": ["call", "args"] | ||
| }, | ||
| "getScreenResolution": { | ||
| "type": "object", | ||
| "properties": { | ||
| "call": { "const": "getScreenResolution" }, | ||
| "returnType": { "const": "array" }, | ||
| "args": { | ||
| "type": "array", | ||
| "minItems": 0, | ||
| "maxItems": 1, | ||
| "prefixItems": [ | ||
| { | ||
| "$ref": "https://a2ui.dev/specification/v0_9/common_types.json#/$defs/DynamicNumber", | ||
| "description": "Optional. The index of the screen to query. Defaults to 0 (primary screen)." | ||
| } | ||
| ] | ||
| } | ||
| }, | ||
| "required": ["call"] | ||
| } | ||
| } | ||
| } | ||
| ``` | ||
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| ## 2. Make the functions available | ||
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| The `FunctionCall` definition refers to a [catalog-agnostic reference](a2ui_protocol.md#the-standard-catalog). | ||
| In your catalog, you simply need to define the `anyFunctions` reference: | ||
| ```json | ||
| { | ||
| "$defs": { | ||
| "anyFunction": { | ||
| "oneOf": [ | ||
| {"$ref": "#/functions/trim"}, | ||
| {"$ref": "#/functions/getScreenResolution"} | ||
| ] | ||
| } | ||
| } | ||
| } | ||
| ``` | ||
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| If you want to incorporate functions defined in the [`standard_catalog.json`], | ||
| those can be added too: | ||
| ```json | ||
| { | ||
| "$defs": { | ||
| "anyFunction": { | ||
| "oneOf": [ | ||
| {"$ref": "#/functions/trim"}, | ||
| {"$ref": "#/functions/getScreenResolution"}, | ||
| {"$ref": "catalog.json#/$defs/anyFunction" } | ||
| ] | ||
| } | ||
| } | ||
| } | ||
| ``` | ||
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| ## How Validation Works | ||
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| When a `FunctionCall` is validated: | ||
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| 1. **Discriminator Lookup:** The validator looks at the `call` property of the | ||
| object. | ||
| 2. **Schema Matching:** | ||
| * If `call` is "length", it matches `Functions` -> `length` | ||
| and validates the arguments against the length rules. | ||
| * If `call` is "trim", it matches `CustomFunctions` -> `trim` and | ||
| validates against your custom rules. | ||
| * If `call` is "unknownFunc": | ||
| * If `GenericFunction` is NOT included, validation FAILS immediately ( | ||
| strict mode). | ||
| * If `GenericFunction` IS included, it matches the generic fallback and | ||
| PASSES (loose mode). | ||
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| This strict-by-default approach ensures typos are caught early, while the | ||
| modular structure makes it easy to add new capabilities. | ||
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For consistency with other function definitions in this document and in
standard_catalog.json, thegetScreenResolutionfunction example should require theargsproperty. Even when a function has optional or no arguments, requiring anargsproperty (which can be an empty array) makes the function call structure uniform and more predictable for implementers.