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Version: 4.3

Mock testing in BT.CPP

Sometimes, in particular when implementing integration and unit tests, it is desirable to have a mechanism that allows us to quickly substitute a specific Node, or an entire class of Nodes, with a "Test" version (mocking).

Since version 4.1, we introduced a new mechanism called "substitution rules" which make this process easier.

It consists of additional methods in the BehaviorTreeFactory class that should be called after the nodes are registered and before the actual tree is instantiated.

For example, given the XML:

<SaySomething name="talk" message="hello world"/>

we may want to substitute this node with another one, called TestMessage:

The corresponding substitution is done with the command:

factory.addSubstitutionRule("talk", "TestMessage");

The first argument contains the wildcard string that will be matched with the TreeNode::fullPath.

For details about fullPath, check the previous tutorial.

The TestNode

The TestNode is an Action that can be configured to:

  • return a specific status, either SUCCESS or FAILURE
  • be synchronous or asynchronous; in the latter case, a timeout should be specified.
  • a post-condition script, generally used to simulate an OutputPort.

This simple dummy Node will not cover 100% of the cases, but can be the default solution for many substitution rules.

Full example

In this example we will see how:

  • We can use the substitution rule to substitute a node with another one.
  • How to use the built-in TestNode.
  • Examples of wildcard matching.
  • How to pass these rules using a JSON file, at run-time.

We will use this XML:

<root BTCPP_format="4">
<BehaviorTree ID="MainTree">
<SaySomething name="talk" message="hello world"/>
<AlwaysFailure name="failing_action"/>
<SubTree ID="MySub" name="mysub"/>
<SaySomething message="before last_action"/>
<Script code="msg:='after last_action'"/>
<AlwaysSuccess name="last_action"/>
<SaySomething message="{msg}"/>

<BehaviorTree ID="MySub">
<AlwaysSuccess name="action_subA"/>
<AlwaysSuccess name="action_subB"/>

The C++ code:

int main(int argc, char** argv)
BT::BehaviorTreeFactory factory;

// We use lambdas and registerSimpleAction, to create
// a "dummy" node, that we want to substitute to a given one.

// Simple node that just prints its name and return SUCCESS
factory.registerSimpleAction("DummyAction", [](BT::TreeNode& self){
std::cout << "DummyAction substituting: "<< << std::endl;
return BT::NodeStatus::SUCCESS;

// Action that is meant to substitute SaySomething.
// It will try to use the input port "message"
factory.registerSimpleAction("TestSaySomething", [](BT::TreeNode& self){
auto msg = self.getInput<std::string>("message");
if (!msg)
throw BT::RuntimeError( "missing required input [message]: ", msg.error() );
std::cout << "TestSaySomething: " << msg.value() << std::endl;
return BT::NodeStatus::SUCCESS;

// pass "no_sub" as first argument to avoid adding rules
bool skip_substitution = (argc == 2) && std::string(argv[1]) == "no_sub";

// we can use a JSON file to configure the substitution rules
// or do it manually
bool const USE_JSON = true;

else {
// Substitute nodes which match this wildcard pattern with TestAction
factory.addSubstitutionRule("mysub/action_*", "TestAction");

// Substitute the node with name [talk] with TestSaySomething
factory.addSubstitutionRule("talk", "TestSaySomething");

// This configuration will be passed to a TestNode
BT::TestNodeConfig test_config;
// Convert the node in asynchronous and wait 2000 ms
test_config.async_delay = std::chrono::milliseconds(2000);
// Execute this postcondition, once completed
test_config.post_script = "msg ='message SUBSTITUED'";

// Substitute the node with name [last_action] with a TestNode,
// configured using test_config
factory.addSubstitutionRule("last_action", test_config);


// During the construction phase of the tree, the substitution
// rules will be used to instantiate the test nodes, instead of the
// original ones.
auto tree = factory.createTree("MainTree");

return 0;

The JSON format

The JSON file, equivalent to the branch executed when USE_JSON == false is:

"TestNodeConfigs": {
"MyTest": {
"async_delay": 2000,
"return_status": "SUCCESS",
"post_script": "msg ='message SUBSTITUED'"

"SubstitutionRules": {
"mysub/action_*": "TestAction",
"talk": "TestSaySomething",
"last_action": "MyTest"

As you can see, there are two main sections:

  • TestNodeConfigs, where the parameters and name of one or multiple TestNode are set.

  • SubstitutionRules where the actual rules are specified.