docs: Add note about composition to the design guide.

tannewt · tannewt · commit 9345562cc88f · 2017-06-07T14:57:55.000-07:00
diff --git a/docs/design_guide.rst b/docs/design_guide.rst
@@ -289,6 +289,26 @@ SPI Example
               i2c.readinto(self.buf)
           return self.buf[0]
 
+Use composition
+--------------------------------------------------------------------------------
+
+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
 --------------------------------------------------------------------------------
 
