Basic finite state machine in GameMaker Language.

FSM.gml

/*
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Copyright (c) 2025 orca / thennothinghappened

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*/

/**
 * An object which can exist in one or more defined states, and move between
 * those states, executing logic specific to each state.
 * 
 * States can be referred to by either strings, or enum members, depending on
 * which is preferred.
 * 
 * @param {String|Real} initialStateName Name of the state to execute first.
 */
function FSM(initialStateName) constructor {
	
	/**
	 * @ignore
	 */
	self.states = {};
	self.currentStateName = initialStateName;
	
	/**
	 * How long the current state has been running for, in frames.
	 * 
	 * @type {Real}
	 * @ignore
	 */
	self.timeInState = 0;
	
	/**
	 * Define a state with a given name. If this state is the specified initial state, the `enter`
	 * event for this state is immediately executed.
	 * 
	 * An `enter` event may return the name of another state. This is useful in such a case to define
	 * a "transition" between states, where an intermediate state only defines an `enter` method, which
	 * produces any necessary transitional side effects, and this method returns the name of the desired
	 * "end" state.
	 * 
	 * The `leave` event is passed the name of the next target state, so it may apply any relevant logic.
	 * This is an alternative option to transitions, where it makes sense to keep logic contained to the
	 * state itself.
	 * 
	 * ### Exceptions
	 * 
	 * Throws if the name is taken.
	 * 
	 * @param {String|Real} name
	 * @param {Struct} struct
	 */
	static state = function(name, state) {
		
		if (variable_struct_exists(self.states, name)) {
			throw new Err(string("State name `{0}` is taken", name));
		}
		
		// Bind state methods to the caller, instead of the struct.
		var eventNames = variable_struct_get_names(state);
		
		for (var i = 0; i < array_length(eventNames); i ++) {
			var key = eventNames[i];
			state[$ key] = method(other, state[$ key]);
		}
		
		if (name == self.currentStateName) {
			var enter = state[$ "enter"];
			
			if (!is_undefined(enter)) {
				enter();
			}
		}
		
		self.states[$ name] = state;
		
	};
	
	/**
	 * Run the event of the given name, for the current state.
	 * 
	 * If the event executed has the name `step`, `event`, or `tick` - or any others you may wish to
	 * add to this list, it will increment the event's duration every time it is executed.
	 * 
	 * @param {String|Real} name
	 * @returns {String}
	 */
	static run = function(name) {
		
		var currentState = self.states[$ self.currentStateName];
		var event = currentState[$ name];
		
		if (is_undefined(event)) {
			return self.currentStateName;
		}
		
		var newStateName = event(self.timeInState);

		if (name == "step" || name == "tick" || name == "update") {
			self.timeInState ++;
		}
		
		if (is_undefined(newStateName)) {
			return self.currentStateName;
		}
		
		return self.change(newStateName);
		
	};
	
	/**
	 * Change to a new state.
	 * 
	 * ### Exceptions
	 * 
	 * Throws if the new state does not exist.
	 * 
	 * @param {String|Real} newStateName
	 */
	static change = function(newStateName) {
		
		if (newStateName == self.currentStateName) {
			return self.currentStateName;
		}
		
		if (!variable_struct_exists(self.states, newStateName)) {
			throw new Err(string("Cannot change to non-existent state `{0}`", newStateName));
		}
		
		var currentState = self.states[$ self.currentStateName];
		
		self.currentStateName = newStateName;
		self.timeInState = 0;
		
		var leave = currentState[$ "leave"];
		
		if (!is_undefined(leave)) {
			leave(newStateName);
		}
		
		var newState = self.states[$ self.currentStateName];
		var enter = newState[$ "enter"];
		
		if (!is_undefined(enter)) {

			var potentialNewStateName = enter();

			if (potentialNewStateName != undefined) {
				return self.change(potentialNewStateName);
			}

		}
		
		return newStateName;
		
	}
	
}

USAGE.md

FSM.gml

Simple implementation of a finite state machine in GML.

Below is an abridged excerpt from gml-simpledraw, which uses this FSM class. There's not really much special here, but I thought it may be worthwhile sharing as this is a pretty simple, usable, and uncomplicated FSM.

The state machine does not implement a special mechanism for multiple methods of transitioning between states. If this behaviour is desired, the "transition" can simply be a state specifying an enter method which transitions to another state, once it has produced any necessary side effects. This also gives the freedom to transition to one of multiple target states via one transition, so logic can be structured in this way if desirable.

fsm = new FSM("none");

fsm.state("none", {
	
	/**
	 * @param {Real} duration How long we've been in this state.
	 * @returns {String|undefined}
	 */
	step: function(duration) {
		
		if (mouse_check_button(mb_left)) {
			return "toolStroke";
		}
		
		if (oCameraCtrl.fsm.run("step") != "none") {
			return "cameraMove";
		}
		
	},

	/**
	 * Draw the tool's path as it is now.
	 * @param {Real} duration How long we've been in this state.
	 */
	draw: function(duration) {
		tool.draw(oCameraCtrl.camera.fromScreen(mouse.pos[X], mouse.pos[Y]));
	}
	
});

fsm.state("cameraMove", {
	step: function() {
		if (oCameraCtrl.fsm.run("step") == "none") {
			return "none";
		}
	}
});

fsm.state("toolStroke", {
	
	enter: function() {
		ts.colour = make_color_hsv(irandom(255), 255, 255);
		tool.beginStroke(oCameraCtrl.camera.fromScreen(mouse.pos[X], mouse.pos[Y]));
	},
	
	/**
	 * @param {Real} duration How long we've been in this state.
	 */
	step: function(duration) {
		
		if (!mouse_check_button(mb_left)) {
			return "none";
		}
		
		if (mouse.moved) {
			tool.updateStroke(oCameraCtrl.camera.fromScreen(mouse.pos[X], mouse.pos[Y]));
		}

	},
	
	/**
	 * Draw the tool's path as it is now.
	 * @param {Real} duration How long we've been in this state.
	 */
	draw: function(duration) {
		tool.draw(oCameraCtrl.camera.fromScreen(mouse.pos[X], mouse.pos[Y]));
	},
	
	/**
	 * Complete the stroke.
	 */
	leave: function() {
		tool.endStroke();
	}

});