# Physical Quantities and Unit Conversions

This package defines units of measurement and quantities with an API for quantities operations and unit conversions.

The core concepts are `PhysicalProperty`, `SystemOfMeasurent`, `Unit` and `Quantity`, explained at the bottom of this README.

Defines most common units for Basic Physical Properties:

• Length
• Mass
• Temperature
• Time
• ElectricalCurrent
• LuminousIntensity
• AmountOfSubstance

And also for these Derived Physical Properties:

• Area derived from power of Length
• Volume derived from power of Length
• Speed derived from composition of Length per Time

## Using Quantities

Static methods offer a very convenient API to create quantities. Note that the `+` operation returns a new quantity using the first operand unit, this the order of the operands is significant when they have different units:

``````final raceStart = Length.kilometers(amount: 12.5);
final raceLength = Length.miles(amount: 1 / 4.0);
print('Finish line at \${raceStart + raceLength} from here');
print('Finish line at \${raceLength + raceStart} from here');
``````

Will output:

``````'Finish line at 12.902336 km from here'
'Finish line at 8.017139902966674 mi from here'
``````

You have a number of operations available for quantities, including unit conversions:

``````final myWeight = Mass.kilograms(amount: 64.0);
final herWeight = Mass.kilograms(amount: 53.0);
final herWeightInPounds = herWeight.convertTo(Mass.lb);
print('Twice my weight is \${myWeight * 2}');
print('Our weight together is \${myWeight + herWeight}');
print('Her weight in England would be \$herWeightInPounds');
print('approximately \${herWeightInPounds.print(useName: true, fractionDigits: 2)}');
``````

Will output:

``````'Twice my weight is 128.0 kg'
'Our weight together is 117.0 kg'
'Her weight in England would be 116.84499895798511 lb'
'approximately 116.84 pounds'
``````

Precision is best within the same system of measurement:

``````final galons = Volume.galons(amount: 12.0);
print('Volume of \$galons is equivalent to \${galons.convertTo(Volume.qt)}');
print('Volume of \$galons is also the same \${galons.convertTo(Volume.l)}');
``````

Will output:

``````'Volume of 12.0 gal is equivalent to 48.0 qt'
'Volume of 12.0 gal is also the same 54.55308000000001 l'
``````

## Concepts

All concepts are interchained to model our understanding of units and measurements, as follows:

### PhysicalProperty

The main concept is `PhysicalProperty`, also called kind throughout this package, it is a quantifiable physical property by an amount of an `Unit` that is part one of its `SystemOfMeasurents`.

`PhysicalProperty` is implemented as an abstract class extended by each especific property, such as `Mass` and `Length`. All instances in this hierarchy are implemented as Singletons with many static shortcuts to the respective singleton instance property.

It offers two main properties: baseUnit and units but both are actually stored in `SystemOfMeasurements` instances, thus this class is more like a coordinator of its system of measurements all created at its single instance construction.

### SystemOfMeasurement

`SystemOfMeasurement` is a system of `Units` for a certain `PhysicalProperty`, such as `InternationalSystemOfUnits` or `ImperialSystemOfUnits`.

It defines a baseUnit, a list of contained units and a especific `UnitConverter` to convert amounts from one of its units to another of its units. Since all units are of the same system, this conversion can be efficient and precise, for example `InternationalSystemOfUnits` use a `PowerOfTenUnitConverter` while `ImperialSystemOfUnits` use a `LinearUnitConverter`.

`SystemOfMeasurement` also implements quantity conversion between different system of units, such as from meters to inches with inter system conversion by means of baseUnits conversion factors.

### Unit

`Unit` has symbol and name and belongs to a `SystemOfMeasurement` and it knows it. This way we can ask an unit for a `QuantityConverter` function to convert amounts from this unit to another one of the same kind, such quantity converter could then be used for a series of conversions from the same two units.

### Quantity

`Quantity` is a container class of Unit and Amount. It is a comparable class with intrinsic unit conversion and offers an API for quantities operations (+, -, *, /, negated, etc).

## CompoundPhysicalProperty

Introduction of `CompoundPhysicalProperty` in version 0.3.0. Compound kinds combine two existing kinds as `MultipliedPhysicalProperties` or `DividedPhysicalProperties`.

`Speed` is the first compound quantity defined in the library, and it wil be easy now to define new combined physical properties as well as generic (not named) combined kinds, that shall evolve to new operations to multiply and divide two quantities, producing a quantity with a combined kind result.

qty