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docs: Add note about composition to the design guide.

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Scott Shawcroft 2017-06-07 14:57:55 -07:00
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docs/design_guide.rst
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@ -289,6 +289,26 @@ SPI Example
i2c.readinto(self.buf)
return self.buf[0]
Use composition
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When writing a driver, take in objects that provide the functionality you need
rather than taking their arguments and constructing them yourself or subclassing
a parent class with functionality. This technique is known as composition and
leads to code that is more flexible and testable than traditional inheritance.
.. seealso:: `Wikipedia <https://en.wikipedia.org/wiki/Dependency_inversion_principle>`_
has more information on "dependency inversion".
For example, if you are writing a driver for an I2C device, then take in an I2C
object instead of the pins themselves. This allows the calling code to provide
any object with the appropriate methods such as an I2C expansion board.
Another example is to expect a `DigitalInOut` for a pin to toggle instead of a
`microcontroller.Pin` from `board`. Taking in the `~microcontroller.Pin` object
alone would limit the driver to pins on the actual microcontroller instead of pins
provided by another driver such as an IO expander.
Lots of small modules
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