bacpypes/doc/source/tutorial/capability.rst

.. BACpypes capability tutorial

Capabilities
============

The `capabilty` module is used to mix together classes that provide both
separate and overlapping functionality.  The original design was motivated
by a component architecture where collections of components that needed to be
mixed together were specified outside the application in a database.

The sample applications in this section are available in the unit test.  Start
out importing the classes in the module::

    >>> from bacpypes.capability import Capability, Collector

Transforming Data
-----------------

Assume that the application needs to transform data in a variety of different
ways, but the exact order of those functions isn't specified, but all of the
transformation functions have the same signature.

First, create a class that is going to be the foundation of the transformation
process::

    class BaseCollector(Collector):

        def transform(self, value):
            for fn in self.capability_functions('transform'):
                value = fn(self, value)

            return value

If there are no other classes mixed in, the `transform()` function doesn't
do anything::

    >>> some_transformer = BaseCollector()
    >>> some_transformer.transform(10)
    10

Adding A Transformation
-----------------------

Create a `Capability` derived class that transforms the value slightly::

    class PlusOne(Capability):

        def transform(self, value):
            return value + 1

Now create a new class that mixes in the base collector::

    class ExampleOne(BaseCollector, PlusOne):
        pass

And our transform function incorporates the new behavior::

    >>> some_transformer = ExampleOne()
    >>> some_transformer.transform(10)
    11

Add Another Transformation
--------------------------

Here is a different transformation class::

    class TimesTen(Capability):

        def transform(self, value):
            return value * 10

And the new class works as intended::

    class ExampleTwo(BaseCollector, TimesTen):
        pass

    >>> some_transformer = ExampleTwo()
    >>> some_transformer.transform(10)
    100

And the classes can be mixed in together:

    class ExampleThree(BaseCollector, PlusOne, TimesTen):
        pass

    >>> some_transformer = ExampleThree()
    >>> some_transformer.transform(10)
    110

The order of the classes makes a difference::

    class ExampleFour(BaseCollector, TimesTen, PlusOne):
        pass

    >>> some_transformer = ExampleFour()
    >>> some_transformer.transform(10)
    101