Quantity Formatting And Parsing

• Quantity Formatting And Parsing
  • Terms and Concepts
    • Common Terms
    • FormatProps
    • Concepts
      • Formats Provider
      • Units Provider
      • Unit Conversion
  • Persistence
    • FormatSet
  • Using KindOfQuantities to Retrieve Formats
  • Examples of Usage
    • Numeric Format
    • Composite Format
    • Parsing Values
    • Using a FormatsProvider
    • Using a MutableFormatsProvider
    • Registering a SchemaFormatsProvider on IModelConnection open
    • Mathematical Operation Parsing
      • Limitations

The @itwin/core-quantity package contains classes for quantity formatting and parsing. For detailed API documentation, see our iTwin.js reference documentation.

If you're developing a frontend application that takes advantage of the core quantity APIs, also check out the iTwin.js frontend quantity formatting learning section.

Terms and Concepts

Common Terms

• Unit - A named unit of measure which can be located by its name or label. The definition of any unit is represented through its UnitProps.
• UnitsProvider - An interface that locates the UnitProps for a unit given name or label. This interface also provides methods for UnitConversion to allow converting from one unit to another.
• Unit Family/Phenomenon - A physical quantity that can be measured (e.g., length, temperature, pressure). Only units in the same phenomenon can be converted between.
• Persistence Unit - The unit used to store the quantity value in memory or to persist the value in an editable iModel. iModels define the persistence unit through KindOfQuantity objects.
• KindOfQuantity - An object that defines a persistence unit and presentation formats.
• Format - The display format for the quantity value. For example, an angle may be persisted in radians but formatted and shown to user in degrees.
• CompositeValue - An addition to the format specification that allows the explicit specification of a unit label, it also allows the persisted value to be displayed as up to 4 sub-units. Typical multi-unit composites are used to display feet'-inches" and degree°minutes'seconds".
• FormatterSpec - Holds the format specification as well as the UnitConversion between the persistence unit and all units defined in the format. This is done to avoid any async calls by the UnitsProvider during the formatting process.
• ParserSpec - Holds the format specification as well as the UnitConversion between the persistence unit and all other units in the same Phenomenon. This is done to avoid async calls by the UnitsProvider and to allow users to input quantities in different unit systems than specified. For instance, if a metric unit system is set, a user could enter 43in and have the result properly converted to meters.
• Formatter - A class that holds methods to format a quantity value into a text string. Given a FormatterSpec object — which includes one or more unit definitions, each with their own conversion information and a specified Format — and a single magnitude number, the Formatter can convert this number into a text string, adhering to the properties specified in formatTraits.
• Parser - A class that holds methods to parse a text string into a single number. Given a ParserSpec object containing a Format Units and their unit conversions, as well as an input string, the Parser can either return an object QuantityParseResult that contains the magnitude of type number, or an object ParseQuantityError.

FormatProps

For a detailed description of all the setting supported by FormatProp see the EC documentation on Format.

Concepts

Formats Provider

The FormatDefinition interface is an extension of FormatProps to help identify formats.

A FormatsProvider interface helps provide all the necessary Formats for displaying formatted quantity values, while also enabling users to add formats of their own.

A MutableFormatsProvider interface extends the read-only FormatsProvider above by allowing adding or removing Formats to the provider.

The SchemaFormatsProvider takes in a SchemaContext, to provide Formats coming from schemas. The SchemaFormatsProvider also requires a UnitSystemKey passed in to filter the FormatDefinition returned, according to the current unit system set in the SchemaFormatsProvider. When getting a format, the SchemaFormatsProvider will throw an error if it receives a non-valid EC full name.

Units Provider

To appropriately parse and output formatted values, a units provider is used to define all available units and provides conversion factors between units. There are several implementations of the UnitsProvider across iTwin.js:

The BasicUnitsProvider holds many common units and their conversions between each other.

The SchemaUnitProvider is used to load unit definitions of schemas from an iModel. This holds more extensive units through the Units schema, while also allowing users to define their own units.

The AlternateUnitLabelsProvider interface allows users to specify a set of alternate labels which may be encountered during parsing of strings. By default only the input unit label and the labels of other units in the same Unit Family/Phenomenon, as well as the label of units in a Composite format are used.

Unit Conversion

Unit conversion is performed through a UnitConversionSpec. These objects are generated by a UnitsProvider, with the implementation determined by each specific provider. During initialization, a ParserSpec or FormatterSpec can ask for UnitConversionSpec objects provided via the UnitsProvider. During parsing and formatting, the specification will retrieve the UnitConversionSpec between the source and destination units to apply the unit conversion.

Persistence

We expose APIs and interfaces to support persistence of formats. Different from KindOfQuantity, which enables persistence of formats at the schema level, this section covers persistence at the application level.

FormatSet

FormatSet defines properties necessary to support persistence of a set of Formats.

Each Format defined in a FormatSet need to be mapped to a valid ECName for a KindOfQuantity. During an application's runtime, the Format associated to a KindofQuantity within a FormatSet would take precedence and be used over the default presentation formats of that KindOfQuantity.

The naming convention for a valid format within a FormatSet is : .

Using KindOfQuantities to Retrieve Formats

Building off of FormatSet, Tools and components that format quantities across applications should be linked to a KindOfQuantity and a Persistence Unit. See Domains for available schemas, including AecUnits and RoadRailUnits, which define many KindOfQuantity values.

The table below lists common measurements with their typical KindOfQuantity and Persistence Unit. This allows tools to request a default KindOfQuantity from IModelApp.formatsProvider and a Persistence Unit from IModelApp.quantityFormatter to create a FormatterSpec for quantity formatting.

Examples of Usage

Numeric Format

The example below uses a simple numeric format and generates a formatted string with 4 decimal place precision. For numeric formats there is no conversion to other units; the unit passed in is the unit returned with the unit label appended if showUnitLabel trait is set.

Composite Format

For the composite format below, we provide a unit in meters and produce a formatted string showing feet and inches to a precision of 1/8th inch.

Parsing Values

Using a FormatsProvider

The example below uses the SchemaFormatsProvider, an implementation of a FormatsProvider, found in ecschema-metadata to format values associated with the length of an object.

The example below uses the SchemaFormatsProvider, an implementation of a FormatsProvider, found in ecschema-metadata to parse values associated with the length of an object.

When retrieving a format from a schema, users might want to ensure the format they get matches the unit system they are currently using. They can either pass in the unit system on initialization, or change them after initialization, like so:

Using a MutableFormatsProvider

The example below is of a MutableFormatsProvider that lets you add/remove formats during runtime.

Registering a SchemaFormatsProvider on IModelConnection open

The simplest way to get formats from schemas into an iTwin application is to register a new SchemaFormatsProvider through a IModelConnection.onOpen event listener, passing IModelConnection.schemaContext to the provider. The example below illustrates how that can be done.

Mathematical Operation Parsing

The quantity formatter supports parsing mathematical operations. The operation is solved, formatting each value present, according to the specified format. This makes it possible to process several different units at once.

Limitations

Only plus(+) and minus(-) signs are supported for now. Other operators will end up returning a parsing error or an invalid input result. If a Format uses a spacer that conflicts with one of the operators above, additional restrictions will apply:

1. Mathematical operations only apply when the operator is in front of whitespace. So -2FT 6IN + 6IN is equal to -2FT-6IN + 6IN, and -2FT-6IN - 6IN is not equal to -2FT-6IN- 6IN.

2. For a value like 2FT 6IN-0.5, the - sign will be treated as a spacer and not subtraction. However, the 0.5 value will use the default unit conversion provided to the parser, because it's not a part of the composite unit when that composite is made up of only 2 units - FT and IN.