(attributes)=
# Attributes
## Which problem is addressed by attributes?
To parametrize simulation models, different types of parameters are needed. A 
BIM model and specifically an IFC model offers multiple sources for the 
requested information. Let's assume that we need the `net_area` of a wall. This 
information can either be obtained from:
* the official IFC specified `QuantitySet` `Qto_WallBaseQuantities/NetSideArea`
* the unofficial, but typical in ArchiCAD used `BaseQuantities'/NetSideArea`
* any other PropertySet where the BIM modeller might have stored the information
* the calculation of the Space Boundary area of the wall
* nowhere, because the information simply does not exist in the model (might not be the case for this example)

To resolve this problem of multiple available sources and even parameters which
will be not included in the BIM model at all the `Attribute` system provides a solution.

## Concept / How does it work?
Simulation required informations that are potentially available in the BIM model
are stored in an [Attribute](attribute.md) at the corresponding [element](element).

E.g. the already mentioned `net_area` is  implemented by the following lines in 
the the class `OuterWall`:

```python

net_area = attribute.Attribute(
    default_ps=("Qto_WallBaseQuantities", "NetSideArea"),
    functions=[BPSProduct.get_net_bound_area],
    unit=ureg.meter ** 2
)
```
This way bim2sim knows where to look for in the IFC file and which function to 
use if no information exists in the IFC file. Also the unit in which the value 
should be stored is provided as m².  See
[_get_value function of attribute](_get_value) documentation for more detailed 
description which other ways are implemented to obtain a value.

## How are the attributes managed? 
To manage the different attributes every bim2sim [element](element) instance 
owns an instance of the [AttributeManager](AttributeManager) which is stored under `element.attributes`.
This manager manages the values and current status of the [Attributes](attribute.md). 

If an information can't be obtained by the implemented ways in
[_get_value function of attribute](_get_value) of attribute its status will be
changed to `NOT_AVAILABLE`. As some information are mandatory to parametrize a
simulation model, we implemented the concept of [Decisions](Decision)
which is basically a structured way to obtain information from the user. To make 
sure that all relevant information exists, a parameter can be 
[requested](request). This way a [Decisions](Decision) will be created if the 
status of the corresponding attribute is `NOT_AVAILABLE`.

// todo: flow chart with mermaid. To use mermaid: https://github.com/mgaitan/sphinxcontrib-mermaid#markdown-support

The [AttributeManager](AttributeManager) uses the status of each attribute to
store if an attribute was already requested. To trigger all decisions for the 
already requested attributes to finally obtain the corresponding values the 
final tasks before exporting to a simulation model should always trigger the
[](get_pending_attribute_decisions) function of [element](element). This
implementation was made to bundle the decisions at the end of the process if 
possible.