Page 141 - Hacking Roomba

122 Part I — Interfacing

With the updateSensors() manner of observing the world, there’s always going to be big
chunks of time where your program just sits and waits. For example, in the loop:
while( !done ) {
roombacomm.pause(1000);
roombacomm.updateSensors();
}
the actual time through the loop varies between 1050 milliseconds and 2000 milliseconds,
depending on whether or not the Roomba is plugged in. At the expense of having little com-
plexity, you can have an optimally responsive loop that is able to do both things not related to
Roomba and Roomba sensor-related things as soon as the sensor data is available. Listing 6-5
shows an example of such a loop.

Listing 6-5: Non-blocking Periodic Sensor Reading

int ts = 1000;
int t1 = System.currentTimeMillis();
roombacomm.sensors();
while( !done ) {
t2 = System.currentTimeMillis();
if( (t2 - t1) > ts && roombacomm.sensorsValid() ) {
actOnSensors();
roombacomm.sensors();
t1 = t2;
}
doNonSensorThings();
}

The variable ts defines the time interval that sensors should be checked. Variables t1
and t2 are used to measure when ts has occurred. Every time through the loop, it will
execute doNonSensorThings(). But if ts amount of time has happened, it will execute
actOnSensors(), resend the sensors command, and reset the time pointers. Just to be
sure, before acting on the sensors, it makes sure that sensors are valid.
This non-blocking will be used in Chapter 7 as part of the RoombaView application.

BumpTurn: Making an Autonomous Roomba

Armed with the techniques from above, you can now start creating some programs that make
an autonomous Roomba. By reading sensors, you can make Roomba know about its environ-
ment, and by commanding its motors, you can make it move around. Listing 6-6 shows the
entirety of the main() method for BumpTurn.java, a program that turns Roomba into a
simple object-avoidance robot. (The keyPressed() method isn’t shown in the listing but
implements a Java-standard way of detecting a keypress.)