Scheduler

[Zabot]

WIP - Don't merge
I think it works, a forked command ends up getting wrapped something like three times, but I think the logic works out.

[rhauch]

Is this the sequence of calls?

1. always call initialize() once
2. always call firstExecute() once, and if it returns false then:
   1. repeatedly call execute() until it returns 'true'
3. always call finalize() once

What is the difference between initialize() and firstExecute()? Could initialize() return a boolean that says whether execute() should be called (or whether the command is finished, to be symmetric with execute())? If so, then this would be the sequence of calls:

1. always call initialize() once, and if it returns false then:
   1. repeatedly call execute() until it returns 'true'
2. always call finalize() once

[rhauch]

Also, would it be useful to change the signature of execute to include the integer number of milliseconds since the command was first started; e.g., execute(int elapsedMillis)? Should initialize be changed as well? That might make it very easy to implemented commands that timeout.

Or, how about if we add:

public default int maximumDurationInMillis() {
    return 0;
}

Implementations for time-limited commands could override this and return the total duration; the CommandRunner might then track the elapsed times automatically (see below).

[rhauch]

Overall, this is a really great start. We have to figure out the requirements, but the time-limited commands can probably easily be solved by having CommandRunner track the time along with the command state.

[Zabot]

I'm not sure these should be static, but CommandGroup doesn't fit the definition of Command, so we can't use the architecture WPILib uses. The best I've come up with is a separate add() in Scheduler that takes a CommandGroup. CommandGroup's constructor can be then be protected and take a CommandRunner so that any class that extends it is forced to synthesize a command group using the methods in CommandGroup. Then we can drop their visibility to protected.