3 ms·

Given your expertise, why use behaviour trees when coroutines are available? What value do they provide in languages like javascript where generators work well?

ifyalciner 3y ago
Behaviour Trees are a bit more than that. They also effect/advice how you structure your project (not component based but behaviour based: exp. you don't have "controller" component in your architecture to handle all locomation but "go_to_pose" or "turn_around" behaviour) plus there are tooling provided around those architectures. Design of Behaviour Trees were not an answer to lack of coroutines/generators (although they were missing from C++ standards until C++20) but an alternative to state machines. Although I agree that "tick" based BT implementation is outdated and should completely be replaced by coroutines and mandatory yields (thats what I did in my implementation). There is still value in behaviour based architectures in robotics. They make building complex/reactive behaviours much easier than component based ones.
jeffomatic 3y ago
I'm writing a game that uses both behavior trees and coroutines (via C# generators), and I have found that they are not quite interchangeable. The main thing is that the execution model is different. I think of the behavior tree as being evaluated from the root on every tick, whereas with coroutines, you are moving linearly through a sequence of steps, with interruptions. When you resume a coroutine, it picks up exactly where it left off. The program does not attempt to re-evaluate any preceding code in the coroutine in order to determine whether it's still the right thing to be executing. By contrast, a behavior tree will stop in the middle of a task if some logic higher up in the tree decides that you need to be on a different branch. What we end up doing is composing behavior trees with coroutines as leaf nodes. It works quite nicely, although I wish there was a way to express the structure of the behavior tree in a more elegant way. We do the obvious thing: each node in the tree is some subclass of a Node base class, representing a logical operation like "if" or "do these in parallel". I very much feel OP's angst about creating a pseudo-programming language-within-a-language by creating what are basically ad hoc AST nodes, but I haven't come up with a better solution. Maybe something like React, where you use basically imperative code to describe a structure, and there is some ambient state that gets properly reconciled by a runtime that you don't touch directly.

ifyalciner 3y ago

Behaviour Trees are a bit more than that. They also effect/advice how you structure your project (not component based but behaviour based: exp. you don't have "controller" component in your architecture to handle all locomation but "go_to_pose" or "turn_around" behaviour) plus there are tooling provided around those architectures. Design of Behaviour Trees were not an answer to lack of coroutines/generators (although they were missing from C++ standards until C++20) but an alternative to state machines. Although I agree that "tick" based BT implementation is outdated and should completely be replaced by coroutines and mandatory yields (thats what I did in my implementation). There is still value in behaviour based architectures in robotics. They make building complex/reactive behaviours much easier than component based ones.

jeffomatic 3y ago

I'm writing a game that uses both behavior trees and coroutines (via C# generators), and I have found that they are not quite interchangeable. The main thing is that the execution model is different. I think of the behavior tree as being evaluated from the root on every tick, whereas with coroutines, you are moving linearly through a sequence of steps, with interruptions. When you resume a coroutine, it picks up exactly where it left off. The program does not attempt to re-evaluate any preceding code in the coroutine in order to determine whether it's still the right thing to be executing. By contrast, a behavior tree will stop in the middle of a task if some logic higher up in the tree decides that you need to be on a different branch. What we end up doing is composing behavior trees with coroutines as leaf nodes. It works quite nicely, although I wish there was a way to express the structure of the behavior tree in a more elegant way. We do the obvious thing: each node in the tree is some subclass of a Node base class, representing a logical operation like "if" or "do these in parallel". I very much feel OP's angst about creating a pseudo-programming language-within-a-language by creating what are basically ad hoc AST nodes, but I haven't come up with a better solution. Maybe something like React, where you use basically imperative code to describe a structure, and there is some ambient state that gets properly reconciled by a runtime that you don't touch directly.