Calling child's method from parent's implementation method in golang

Question

I'm trying to implement a behavior tree in go, and I'm struggling with its composition features. Basically, I need Tick() implemented below to call the method defined by wherever it was embedded.

Here is behavior.go:

type IBehavior interface {
  Tick() Status
  Update() Status
}

type Behavior struct {
  Status Status
}

func (n *Behavior) Tick() Status {
  fmt.Println("ticking!")
  if n.Status != RUNNING { n.Initialize() }
  status := n.Update()
  if n.Status != RUNNING { n.Terminate(status) }

  return status
}

func (n *Behavior) Update() Status {
  fmt.Println("This update is being called")
  return n.Status
}

And here is the Behavior struct being embedded:

type IBehaviorTree interface {
  IBehavior
}

type BehaviorTree struct {
  Behavior

  Root IBehavior
}

func (n *BehaviorTree) Update() Status {
  fmt.Printf("Tree tick! %#v\n", n.Root)
  return n.Root.Tick()
}

A few more files to make this example make sense:

type ILeaf interface {
  IBehavior
}

type Leaf struct {
  Behavior
}

And this one:

type Test struct {
  Leaf

  Status Status
}

func NewTest() *Test {
    return &Test{}
}

func (n Test) Update() Status {
    fmt.Println("Testing!")
    return SUCCESS
}

And here is an example of its usage:

tree := ai.NewBehaviorTree()
test := ai.NewTest()
tree.Root = test

tree.Tick()

I was expecting the tree to tick normally by printing this:

ticking!
Tree tick!

But instead I'm getting:

ticking!
This update is being called

Could anyone help me with this issue?

Edit: Added a few extra files to illuminate the issue. Also, I don't understand the downvotes. I have an honest go question. Am I only supposed to ask questions that make sense to me already?

Answer 1

Your issue here is that Tick() is not defined on your BehaviorTree structure. As a result, when you call tree.Tick(), there's no direct method defined, so it calls the promoted Tick() method of the embedded Behavior struct. That Behavior struct has no idea what a BehaviorTree is! In Go’s embedding style of pseudo-inheritance, “child” types have no concept of their "parents", nor any reference or access to them. Embedded methods are called with the embedded type as their receiver, not the embedding struct.

If you want your expected behavior, you need to define a Tick() method on your BehaviorTree type, and have that method to call its own Update() method (and then call sub Tick() or Update() methods if you wish). For example:

type BehaviorTree struct {
  Behavior

  Root IBehavior
}

func (n *BehaviorTree) Tick() Status {
    n.Update() // TODO: do you want this status or the Root.Tick() status?
    return n.Root.Tick()
}

func (n *BehaviorTree) Update() Status {
  fmt.Printf("Tree tick! %#v\n", n.Root)
  return nil
}

Answer 2

As Volker said

Go has absolutely no notion of inheritance (embedding is not inheritance) and you simply cannot do parent/child stuff in Go. Redesign

What you want as far as I can tell is a function which uses an interface to perform the same task multiple times.

func Tick(n IBehavior) Status {
  fmt.Println("ticking!")
  if n.Status != RUNNING { n.Initialize() }
  status := n.Update()
  if n.Status != RUNNING { n.Terminate(status) }
  return status
}

Of course Initialize will then have to be in the interface.