Programming with Coroutines

A simple example

Here’s a simple application, from SimpleApp:

from BrickPython.CommandLineApplication import CommandLineApplication

class SimpleApp(CommandLineApplication):
    def __init__(self):
        CommandLineApplication.__init__(self)
        self.addSensorCoroutine( self.doActivity() )  #A

    def doActivity(self):
        'Coroutine to rotate a motor forward and backward'
        motorA = self.motor('A')
        motorA.zeroPosition()                         #B
        while True:
            for i in motorA.moveTo( 2*90 ):           #C
                yield                                 #D

            for i in motorA.moveTo( 0 ):
                yield

if __name__ == "__main__":
    SimpleApp().mainloop()

It uses the simplest scheduler: the CommandLineApplication.

Line A creates a coroutine from the coroutine method doActivity() and adds it to the scheduler. A coroutine method must have one or more yield calls, such as the one at line D, so that calling the method returns a coroutine which the scheduler can invoke using next(). For more about such ‘generator functions’, see https://wiki.python.org/moin/Generators

To get the most benefit from coroutines we want to be able to call other coroutines, and to wait until they’ve finished. Lines C,D show how this is done. motorA.moveTo(2*90) is itself a coroutine - one that implements the Servo motor PID algorithm - and thus we can invoke it using Python’s iteration syntax:

for i in coroutine():
    yield

The dummy variable i is ignored.

We could, of course, add extra code between C and D - for example to check a sensor. It will be executed for each work call. DoorControl shows how this can work.

Composing Coroutines

We can also combine coroutines.

The coroutine method Scheduler.runTillFirstCompletes() creates a new coroutine from a list of coroutines and terminates them all (and itself) when the first completes. The coroutine method Scheduler.runTillAllComplete() similarly runs until all of a list of coroutines complete.

The Scheduler.withTimeout() coroutine method adds a timeout to a coroutine, terminating it if it hasn’t completed after the given timeout.

Ready-made coroutines

Scheduler.waitFor() answers a coroutine that terminates when a given function returns true.

Motor.moveTo() and Motor.setSpeed() are coroutines that control a motor.