Building Behavior Trees programmatically without XML

[sea-bass]

I've been trying out this tool and my personal preference is to build BTs on the fly by instantiating nodes and putting them together in code. It's useful to me (and potentially others) who are trying to automatically generate BTs from action sequences output by a task and motion planner. Of course, I see the merit of the XML approach as well for more standalone cases.

Would it be worthwhile for me to work on a simple tutorial for the programmatic approach, if that's useful? If so, I can submit a PR but I wanted thoughts from the maintainers first.

[facontidavide]

At some point, I should add an "advanced tutorials" session to the tutorials, to cover subjects like this.

On the other hand...
Take into account that the XML Tree creator does a lot of things that you might miss, like remapping ports in Subtrees.

If I "support" the programmatic approach with a tutorial, people will start submitting bugs with "why this doesn't work when I create trees programmatically"?

[sea-bass]

That makes sense.

So what would be you suggestion for building trees in a for-loop given an arbitrary sequence at runtime? Is it to dynamically create an XML string and then synthesize the tree out of that?

[facontidavide]

I have the feeling that you are doing it wrong ;)

Maybe reconsider the architecture of the tree.
Let me give you an example:

Imagine that you have a robot that should visit 50 places. You don't create a Sequence with 50 "GoTo" children!

In this case, you would create a "PullDestinationFromQueue" that returns FAILURE if the queue of destinations is empty or SUCCESS otherwise; in the latter case, it would also write the destination in the blackboard to would be accessed a single "GoTo" node.

Your case might be different, but I invite you to think if there isn't a simple solution :)

If you really want to go for the programmatic tree creation, you can do it, but you have the responsibility of manually create port remapping if you use Subtrees

[sea-bass]

That is very helpful, and your suggestion would indeed work for my case -- thank you for the insights!

I've turned this into a good lesson for my example, and thought I would share.

// "Naive" implementation where there is a hard-coded sequence for each location.
// Here, there is a top-level fallback node that tries locations sequentially.
// As you can see, this does not automatically scale with number of locations.
static const char* xml_text_naive = R"(
 <root main_tree_to_execute = "MainTree" >
     <BehaviorTree ID="MainTree">
        <Fallback name="root">
            <Sequence name="search_location1">
                <GoToPose       name="go_to_location1"  loc="location1" />
                <LookForObject  name="look_in_location1"/>
            </Sequence>
            <Sequence name="search_location2">
                <GoToPose       name="go_to_location2" loc="location2"/>
                <LookForObject  name="look_in_location2"/>
            </Sequence>
            <Sequence name="search_location3">
                <GoToPose       name="go_to_location3" loc="location3"/>
                <LookForObject  name="look_in_location3"/>
            </Sequence>
            <Sequence name="search_location3">
                <GoToPose       name="go_to_location4" loc="location4"/>
                <LookForObject  name="look_in_location4"/>
            </Sequence>
        </Fallback>
     </BehaviorTree>
 </root>
 )";

// A better implementation which uses a queue of location names to iterate through 
// visiting locations regardless of number of locations.
static const char* xml_text_queue = R"(
<root main_tree_to_execute = "MainTree" >
    <BehaviorTree ID="MainTree">
    <Sequence name="main_loop">
        <SetLocations              name="set_locations" num_locs="{num_locs}"/>
        <RetryUntilSuccesful       num_attempts="{num_locs}">
            <Sequence   name="search_location">
                <GetLocationFromQueue   name="get_loc"      target_location="{target_location}"/>   
                <GoToPose               name="go_to_loc"    loc="{target_location}"/>
                <LookForObject          name="look_for_obj" />
            </Sequence>   
        </RetryUntilSuccesful>
    </Sequence>
    </BehaviorTree>
</root>
)";

[facontidavide]

great!