A button that call a function when clicked

ButtonCallback.hx

class Button
{
    public var x:Int;
	public var y:Int;
	public var width:Int;
	public var height:Int;
	public var onClick:Void->Void;
	
    public function new(x:Int, y:Int, width:Int, height:Int)
    {
        this.x = x;
		this.y = y;
		this.width = width;
		this.height = height;
    }
    
    public function checkMouseDown(x:Int, y:Int):Bool
    {
        if (x >= this.x && x <= this.x + this.width && y >= this.y && y <= this.y + this.height) 
        {
            if (this.onClick != null)
                onClick();
            
            return true;
        }
        
        return false;
    }
}

class Project
{
    var stateButton:Button;
    var state:String;
    
    public function new()
    {
        stateButton = new Button(10, 10, 100, 50);
        stateButton.onClick = changeState;
    }
    
    public function changeState()
    {
        state = 'menu';
    }
    
    public function onMouseDown(button:Int, x:Int, y:Int)
    {
        if (button == 0)
            stateButton.checkMouseDown(x, y);
    }
}