Simple Arcade Game 4: User Input and Spatial Controls

Welcome back to our simple arcade game tutorial where we look into what it takes to make a very simple, flock herding game using jMonkeyEngine. Today we are going to start adding logic to our game. We are going to start with a simple way to steer the white cube we made last time by reading user input, and applying the movements to our game objects via Controls.

First, let's look at our GameState class. Our GameState extends AbstractAppState and overrides the Initialize method. We also have a custom method to spawn our player object. We are going to add a few more lines in our Initialize method to handle user input.

First let's grab the SimpleApplications input manager. The input manager needs to have Mappings added which we will later listen for, so let's define some static mapping names so they are easy to find later. We then take our InputManager and add our mappings to specific key bindings. For keyboard keys we use a KeyTrigger and grab the keycode from the KeyInput class.

public class GameState extends AbstractAppState{
    //We make the input mappings public and static so they are easy to find later
    public static final String PLAYER_FORWARD = "Forward_Move";
    public static final String PLAYER_BACKWARD = "Backward_Move";
    public static final String PLAYER_LEFT = "Left_Move";
    public static final String PLAYER_RIGHT = "Right_Move";
    private Spatial player;
    private Node scene;

    @Override
    public void initialize(AppStateManager stateManager, Application app) {
        SimpleApplication application = (SimpleApplication)app;
        AssetManager am = application.getAssetManager();
        Node rootNode = application.getRootNode();
        scene = new Node("Game_Scene");
        rootNode.attachChild(scene);
        
        player = createPlayer(am);
        scene.attachChild(player);
        
        //configure user input
        InputManager input = application.getInputManager();
        input.addMapping(PLAYER_FORWARD, new KeyTrigger(KeyInput.KEY_UP));
        input.addMapping(PLAYER_BACKWARD, new KeyTrigger(KeyInput.KEY_DOWN));
        input.addMapping(PLAYER_LEFT, new KeyTrigger(KeyInput.KEY_LEFT));
        input.addMapping(PLAYER_RIGHT, new KeyTrigger(KeyInput.KEY_RIGHT));
    }
    
    ...
}

If you run the game now and start pressing the cursor buttons... nothing happens! We still need to register a listener for these mappings. Let's make GameState implement ActionListener so anything we want to do with player input can be done very easily.

The ActionListener interface has a single method we need to override, the onAction(String name, boolean isPressed, float tpf) method. When we register a listener to the key input, the onAction method gets called from ALL keybindings we register. Therefor, we need to sort through all possible inputs with if or switch statements. I prefer if statements because it is easier to assign blocking vs non blocking keybindings.

public class GameState extends AbstractAppState implements ActionListener{
    ...

    @Override
    public void initialize(AppStateManager stateManager, Application app) {

        ...
        
        //disable the default flycam
        application.getStateManager().getState(FlyCamAppState.class).setEnabled(false);

        //configure user input
        InputManager input = application.getInputManager();
        input.addMapping(PLAYER_FORWARD, new KeyTrigger(KeyInput.KEY_UP));
        input.addMapping(PLAYER_BACKWARD, new KeyTrigger(KeyInput.KEY_DOWN));
        input.addMapping(PLAYER_LEFT, new KeyTrigger(KeyInput.KEY_LEFT));
        input.addMapping(PLAYER_RIGHT, new KeyTrigger(KeyInput.KEY_RIGHT));
        //here we register GameState as a listener with the input manager and assign
        //it all the mappings we want to listen to
        input.addListener(this, PLAYER_FORWARD, PLAYER_BACKWARD, PLAYER_LEFT, PLAYER_RIGHT);
    }
    
    ...

    @Override
    public void onAction(String name, boolean isPressed, float tpf) {
        System.out.println(name+" was pressed");
        //We WANT the forward button press to block a backward button press, so we use if/else
        if(name.equals(PLAYER_FORWARD)){
            
        } else if(name.equals(PLAYER_BACKWARD)){
            
        }
        //we DON'T want the forward or backward buttons to block left and right buttons
        if(name.equals(PLAYER_LEFT)){
            
        } else if(name.equals(PLAYER_RIGHT)){
            
        }
    }
}

Now if you press play you can use the arrow keys to print the name of the binding you are currently pressing to the console. You may also notice the camera moving when you use the arrow keys, this is because SimpleApplication creates a FlyCam by default. It's easy enough to turn off by adding another line to our Initialize Method.

We have our user input registered with our input manager, now how do we get this to control our cube? The simple answer, Controls. Let's make a new package com.mrugames.unit and create a new class, Mob.java. Mobs in game design lingo are simply mobile entities, or anything that moves. That is exactly what our mob control is going to do, move whatever it is attached to at a specific speed every frame. Make our Mob class extend AbstractControl and override the 2 abstract methods controlUpdate(float tpf) and controlRender(RenderManager rm, ViewPort vp). We won't be using control render in our mob class, but it is a good place to put render specific code.

For our Mob class we want to store the maximum speed the mob can move as well as the current input direction for the mob. We will assume the input direction is a number between 0 and 1. Every frame that the input direction is greater than 0, we move the mob by it's maxSpeed*inputDirection*tpf (time per frame).

The Control class has a convenient "spatial" field which you can use to access the Spatial the control is attached to. Spatials also have a convenient "move" method which takes a Vector3f argument.

public class Mob extends AbstractControl{
    public float maxSpeed = 1f;
    public Vector2f dir = new Vector2f(0,0);

    @Override
    protected void controlUpdate(float tpf) {
        //every frame that dir is greater than 0, move in the direction of dir by
        //our max speed
        if(dir.lengthSquared() > 0){
            //spatial is a protected field inherited from AbstractControl
            spatial.move(dir.x*maxSpeed*tpf, dir.y*maxSpeed*tpf, 0);
        }
    }

    @Override
    protected void controlRender(RenderManager rm, ViewPort vp) {
        
    }
}

Now that we have our Mob control, we need to attach it to our player Spatial. We can then access our Mob control in our ActionListener to set the Mob.dir to match the players input.

public class GameState extends AbstractAppState implements ActionListener{    

    ...

    private Spatial createPlayer(AssetManager am){
        Mesh mesh = new Box(0.5f,0.5f,1f);
        Geometry geo = new Geometry("Player", mesh);
        Material mat = new Material(am, "Common/MatDefs/Misc/Unshaded.j3md");
        geo.setMaterial(mat);
        //Add a new Mob control to the player
        geo.addControl(new Mob());
        return geo;
    }    

    ...

    @Override
    public void onAction(String name, boolean isPressed, float tpf) {
        Mob playerMob = player.getControl(Mob.class);
        //We WANT the forward button press to block a backward button press, so we use if/else
        if(name.equals(PLAYER_FORWARD)){
            if(isPressed){
                //Called the frame the button was pressed down
                playerMob.dir.y =1 ;
            } else{
                //Called the frame the button was released
                playerMob.dir.y = 0;
            }
        } else if(name.equals(PLAYER_BACKWARD)){
            if(isPressed){
                playerMob.dir.y = -1;
            } else{
                playerMob.dir.y = 0;
            }
        }
        //we DON'T want the forward or backward buttons to block left and right buttons
        if(name.equals(PLAYER_LEFT)){
            if(isPressed){
                playerMob.dir.x = -1;
            } else{
                playerMob.dir.x = 0;
            }
        } else if(name.equals(PLAYER_RIGHT)){
            if(isPressed){
                playerMob.dir.x = 1;
            } else{
                playerMob.dir.x = 0;
            }
        }
    }

As you see there are some simple tricks we use to process the player input. The ifPressed variable will be true the frame that the player activates the mapping, and false on the frame that the mapping is deactivated. This makes on/off functionality easy. If you need to be notified every frame that a mapping is being activated, take a look at the AnalogListener class. We will cover the AnalogListener in more detail later.

Go ahead and press play! You'll see our little cube drift around the screen corresponding to your button presses.

Simple Arcade Game 3: Scenegraph

Hello again and welcome to another jMonkey Engine tutorial! Today we are going to take a detailed look at probably the most important aspect of jMonkey Engine, it's Scenegraph.

A great overview of how jme uses it's scenegraph is available here, but I'll run a brief overview for those who want to dive right in.

The Scene in jme is made up of Spatials, mostly Nodes and Geometries. Nodes do not have any visual elements, but can have children elements. Geometries have meshes and materials and will be rendered, these are the elements you actually see! All spatials must be connected to the RootNode, which is the top most node in the scene. You can easily make complex or large scale models with many moving parts by mixing Nodes and Geometries.

The most important thing to remember: DO NOT EXTEND NODE, GEOMETRY OR SPATIAL. Instead you should store your game data seperately, in the Spatial's UserData container, or as a Control which is attached to a spatial.

UserData can be any Object and is easily written by using Spatial.setUserData(String key, Object data), and later retrieved using Spatial.getUserData(String key).

Controls are a lot like AppStates, except they only work on the spatial they are attached to. We will go over Controls in a later tutorial, but if you are interested in making your own controls now they are easily attached to a spatial using Spatial.addControl(Control control), and retrieved by using Spatial.getControl(Class<T> controlType) where T extends Control.

We are going to make a new appstate for our game, let's call it "GameState". We need to override the "initialize" method as this is where we will be modifying our scene.

public class GameState extends AbstractAppState{
    private Spatial player;
    private Node scene;

    @Override
    public void initialize(AppStateManager stateManager, Application app) {
        SimpleApplication application = (SimpleApplication)app;
        AssetManager am = application.getAssetManager();
        Node rootNode = application.getRootNode();
        scene = new Node("Game_Scene");
        rootNode.attachChild(scene);
    }
    
}

Lets go ahead and cast the Application app to a SimpleApplication so we can access our RootNode and the AssetManager. We should also define a Spatial object that will reference the Players spatial so we can easily find it later. We also define a Node for the Scene. Create a new Node with a string argument for the node's name and attach it to the root node via the Node.attachChild() method. We will be attaching all the game objects to the game scene instead of the root node to keep them separated and easy to work with!

Now let's define a new method to create our player object. We will eventually be using a Dragon model, but for now let's simple spawn a box to represent our player. First we define a new method, createPlayer(AssetManager am). We are going to pass the assetManager from our Application object to this method to allow the use of pre-defined material definitions which really speed this process.

public class GameState extends AbstractAppState{

    @Override
    public void initialize(AppStateManager stateManager, Application app) {
        SimpleApplication application = (SimpleApplication)app;
        AssetManager am = application.getAssetManager();
        Node rootNode = application.getRootNode();
        scene = new Node("Game_Scene");
        rootNode.attachChild(scene);

        player = createPlayer(am);
        scene.attachChild(player);
    }

    private Spatial createPlayer(AssetManager am){
        Mesh mesh = new Box(0.5f,0.5f,1f);
        Geometry geo = new Geometry("Player", mesh);
        Material mat = new Material(am, "Common/MatDefs/Misc/Unshaded.j3md");
        geo.setMaterial(mat);
        return geo;
    }
}

Spawning a model is as simple as creating a geometry, giving it a mesh and a material. Here we use the Box shape to create our mesh. With 3 floating point arguments we construct a box that extends into the x, y and z axis by the given amount. This will give us a box that is actually 1x1x2. Simply pass the mesh to the Geometry as an argument along with a String name to attach the mesh to the geometry.

Next we create the material. Here we are using a Material Definition which is how jmonkey defines what pixel and vertex shaders control the material. We are using the built-in material definition for Unshaded objects by simply defining the path to the j3md file. You can view other material definitions included in the jme3 library in the jme3-core.jar/Common/MatDefs. Call Geometry.setMaterial() and we are finished.

In the initialize method add the 2 lines player = createPlayer(am) and scene.attachChild(player).

This is enough to have our GameState spawn a player object and attach it to the GameScene node, which in turn is attached to the RootNode and will be rendered. Now we need to attach the GameState to the AppStateManager, let's do this in our MainMenu state so in the future we can have a button start the game.

Open our MainMenu appstate and add the following lines.

public class MainMenu extends AbstractAppState {
    private AppStateManager stateManager;
    private float countDown = 3f;
    private BitmapText text;

    @Override
    public void initialize(AppStateManager stateManager, Application app) {
        super.initialize(stateManager, app);
        this.stateManager = stateManager;
        
        ...
    }
    
    @Override
    public void update(float tpf) {
        if((countDown -= tpf) <= 0){
            stateManager.detach(this);
            stateManager.attach(new GameState());
        }
    }
    
    ...
}

First we need to define 2 new fields, stateManager and countDown. The state manager is passed via the initialize() method when the AppState is attached to the AppStateManager. Go ahead and assign this via "this.stateManager = stateManager" so we can access it later.

Then in our update loop we are going to count down from 3. Once our countdown is equal to or less than 0, we will remove MainMenu from our AppStateManager and attach a new GameState. Do this by simple subtracting tpf from countDown once a frame and checking with a basic if statement. Remember, in a video game you NEVER want to use wait() as this will completely lock your thread from doing other things. Making simple timers like the one above is easy and guarantees that your other appstates have a chance to  run their updates while we wait.

Now if you press Play you will see our lovely Main Menu for approximately 3 seconds before being taken to our game state, and spawning in a simple, white cube!