feat: add _onStart precondition for script execution

Add a new precondition type _onStart that allows executing scripts
before a node starts its tick. This provides symmetry with the
existing _post functionality.

Resolves BehaviorTree#895

include/behaviortree_cpp/tree_node.h

@@ -49,6 +49,7 @@ enum class PreCond
   SUCCESS_IF,
   SKIP_IF,
   WHILE_TRUE,
+  ALWAYS,
   COUNT_
 };
 

src/tree_node.cpp

@@ -208,24 +208,28 @@ Expected<NodeStatus> TreeNode::checkPreConditions()
     // Some preconditions are applied only when the node state is IDLE or SKIPPED
     if(_p->status == NodeStatus::IDLE || _p->status == NodeStatus::SKIPPED)
     {
-      // what to do if the condition is true
-      if(parse_executor(env).cast<bool>())
+      auto execute_result = parse_executor(env);
+      // always
+      if(preID == PreCond::ALWAYS)
       {
-        if(preID == PreCond::FAILURE_IF)
-        {
-          return NodeStatus::FAILURE;
-        }
-        else if(preID == PreCond::SUCCESS_IF)
-        {
-          return NodeStatus::SUCCESS;
-        }
-        else if(preID == PreCond::SKIP_IF)
-        {
-          return NodeStatus::SKIPPED;
-        }
+        return nonstd::make_unexpected("");
+      }
+      auto bool_result = execute_result.cast<bool>();
+      // what to do if the condition is true and the precondition is xx_IF
+      if(bool_result && preID == PreCond::FAILURE_IF)
+      {
+        return NodeStatus::FAILURE;
+      }
+      if(bool_result && preID == PreCond::SUCCESS_IF)
+      {
+        return NodeStatus::SUCCESS;
+      }
+      if(bool_result && preID == PreCond::SKIP_IF)
+      {
+        return NodeStatus::SKIPPED;
       }
-      // if the conditions is false
-      else if(preID == PreCond::WHILE_TRUE)
+      // if the condition is false and the precondition is WHILE_TRUE, skip the node
+      if(!bool_result && preID == PreCond::WHILE_TRUE)
       {
         return NodeStatus::SKIPPED;
       }
@@ -476,6 +480,8 @@ std::string toStr<PreCond>(const PreCond& pre)
       return "_skipIf";
     case PreCond::WHILE_TRUE:
       return "_while";
+    case PreCond::ALWAYS:
+      return "_onStart";
     default:
       return "Undefined";
   }

tests/gtest_preconditions.cpp

@@ -397,3 +397,32 @@ TEST(Preconditions, WhileCallsOnHalt)
   ASSERT_EQ(status, BT::NodeStatus::SKIPPED);
   ASSERT_EQ(tree.rootBlackboard()->get<int>("B"), 69);
 }
+
+TEST(Preconditions, OnStart)
+{
+  BehaviorTreeFactory factory;
+  std::array<int, 3> counters;
+  RegisterTestTick(factory, "Test", counters);
+
+  static constexpr auto xml_text = R"(
+  <root BTCPP_format="4">
+    <BehaviorTree ID="Main">
+      <Sequence>
+        <AlwaysSuccess _onStart="B:=69"/>
+        <TestA/>
+        <TestB _successIf="B==69"/>
+        <TestC _onStart="C:=70" _post="B=70"/>
+      </Sequence>
+    </BehaviorTree>
+  </root>
+  )";
+
+  auto tree = factory.createTreeFromText(xml_text);
+  const auto status = tree.tickWhileRunning();
+  ASSERT_EQ(status, NodeStatus::SUCCESS);
+  ASSERT_EQ(tree.rootBlackboard()->get<int>("B"), 70);
+  ASSERT_EQ(tree.rootBlackboard()->get<int>("C"), 70);
+  ASSERT_EQ(counters[0], 1);
+  ASSERT_EQ(counters[1], 0);
+  ASSERT_EQ(counters[2], 1);
+}