At the highest level a behavior tree is a collection of tasks. Tasks have a similar API to Unity’s MonoBehaviour so it should be really easy to get started writing your own tasks. The task class has the following API:

// OnAwake is called once when the behavior tree is enabled. Think of it as a constructor.
void OnAwake();

// OnStart is called immediately before execution. It is used to setup any variables that need to be reset from the previous run.
void OnStart();

// OnUpdate runs the actual task.
TaskStatus OnUpdate();

// OnFixedUpdate executes during the FixedUpdate loop. The TaskStatus must be returned within OnUpdate.
void OnFixedUpdate();

// OnEnd is called after execution on a success or failure. 
void OnEnd();

// OnPause is called when the behavior is paused or resumed.
void OnPause(bool paused);

// Returns the priority of the task, used by the Priority Selector.
float GetPriority();

// Returns the utility of the task, used by the Utility Selector for Utility Theory.
float GetUtility();

// OnBehaviorComplete is called after the behavior tree finishes executing.
void OnBehaviorComplete();

// OnReset is called by the inspector to reset the public properties
void OnReset();

// Allow OnDrawGizmos to be called from the tasks.
void OnDrawGizmos();

// Keep a reference to the behavior that owns this task.
Behavior Owner;

Tasks have three exposed properties: name, comment, and instant. Instant is the only property that isn’t obvious in what it does. When a task returns success or fail it immediately moves onto the next task within the same update tick. If you uncheck the instant task it will now wait a update tick before the next task gets executed. This is an easy way to throttle the behavior tree.

The following flow chart is used when executing the task: