jme-state-machine Github
jMonkeyEngine state machine helps to define complex behaviour using states. This way, one can avoid cluttered code with lots of if-else branches checking if the model is walking or running, then do x or y… This way complicated physics/input handling that are tight to some states, need only there be implemented.
Implementation and architecture are based on the book “Game Programming Patterns”, chapter State.
In the examples directory on Github, a fully working example is shown using the Sinbad model. States used in that example are found here: example states.
The State Machine consists of four entities: ModelStateMachine, Layer, State, StateChange.
One creates a state machine in jMonkeyEngine using the following:
ModelStateMachine modelStateMachine = new ModelStateMachine();
spatial.addControl(modelStateMachine);
To add a layer, one needs to set the default state.
Layer modelBaseLayer = new Layer();
modelStateMachine.addLayer(modelBaseLayer);
modelBaseLayer.setInitialState(new IdleState());
The idea is to go to a new state by one of the following methods:
handleActionInputhandleAnalogInputcontrolUpdate
There are multiple ways to change state. return on one of the above methods a StateChange instance by using either:
StateChange.to(new SomeOtherState())StateChange.push(new TemporaryState())StateChange.pop()-
nullif the state does not need to be changed.
The state machine is a pushdown automaton, meaning it is possible to push a state temporarily on the stack (and afterwards pop it from that state).
In the video this happens, when Sinbad runs into the sword, it picks it up, and then returns back to the state it was before.
Create a class that extends the abstract State class, and you are good to go.
When a state is entered, automatically the onEnter method is called by the state machine. Likewise, on leaving a state, the onExit method is called.
In these methods, attach (and detach) bindings to physics tick listeners and start (and stop) animations.
Layers are used when different states need to be defined for different parts of the model, for instance equipment versus torso behavior. In the example in this project, a layer is created for the “look at sword behavior”. It uses states from the other layer to know when the sword is attached to the model using:
if (getLayer()
.getLayers()
.stream()
.anyMatch(l -> l.hasState(PickUpSwordState.class))
) {
return StateChange.to(new IdleState());
}
If anyone is interested in improving this functionality, please let me know. @sgold if this a shot of making a pull request into the engine, that would be awesome 
.
Thanks for reading. 
I’ll keep the development mostly to myself and will answer to feature requests and maintenance.