Add value-setter for Robot class

Fixes #305

gpiozero/boards.py

@@ -776,6 +776,20 @@ class Robot(SourceMixin, CompositeDevice):
                 right_motor=Motor(*right),
                 _order=('left_motor', 'right_motor'))
 
+    @property
+    def value(self):
+        """
+        Represents the motion of the robot as a tuple of (left_motor_speed,
+        right_motor_speed) with ``(-1, -1)`` representing full speed backwards,
+        ``(1, 1)`` representing full speed forwards, and ``(0, 0)``
+        representing stopped.
+        """
+        return super(Robot, self).value
+
+    @value.setter
+    def value(self, value):
+        self.left_motor.value, self.right_motor.value = value
+
     def forward(self, speed=1):
         """
         Drive the robot forward by running both motors forward.

tests/test_boards.py

@@ -617,20 +617,34 @@ def test_robot():
         assert (
             [device.pin for device in robot.left_motor] +
             [device.pin for device in robot.right_motor]) == pins
+        assert robot.value == (0, 0)
         robot.forward()
         assert [pin.state for pin in pins] == [1, 0, 1, 0]
+        assert robot.value == (1, 1)
         robot.backward()
         assert [pin.state for pin in pins] == [0, 1, 0, 1]
+        assert robot.value == (-1, -1)
         robot.forward(0.5)
         assert [pin.state for pin in pins] == [0.5, 0, 0.5, 0]
+        assert robot.value == (0.5, 0.5)
         robot.left()
         assert [pin.state for pin in pins] == [0, 1, 1, 0]
+        assert robot.value == (-1, 1)
         robot.right()
         assert [pin.state for pin in pins] == [1, 0, 0, 1]
+        assert robot.value == (1, -1)
         robot.reverse()
         assert [pin.state for pin in pins] == [0, 1, 1, 0]
+        assert robot.value == (-1, 1)
         robot.stop()
         assert [pin.state for pin in pins] == [0, 0, 0, 0]
+        assert robot.value == (0, 0)
+        robot.value = (-1, -1)
+        assert robot.value == (-1, -1)
+        robot.value = (0.5, 1)
+        assert robot.value == (0.5, 1)
+        robot.value = (0, -0.5)
+        assert robot.value == (0, -0.5)
 
 def test_ryanteck_robot():
     pins = [MockPWMPin(n) for n in (17, 18, 22, 23)]