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Andrew Blem

Unity How to Instantiate two Game Objects at the same time in Unity3D

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I have two empty game objects (BlueTeam and RedTeam). The BlueTeam needs to set its opponent as the RedTeam and vice versa. When a team is instantiated it as to generate its players and place them on there home regions which are created when I Instantiate the pitch. When I instantiate one team only my players appear at the correct positions with no error. When I try to instantiate both teams at the same time I have a "Null Exception The Object which you are trying to reference doe not exist". I know what the error means but I cant seem to get why that is happening.The code is in C#
Here is some code of what I am trying to do.
Here is my start function for my SoccerPitch
   // Use this for initialization
     void Start()
     //create the soccer ball
     m_pBall = Instantiate(m_pBall) as SoccerBall;
     //create the teams
     m_pRedTeam = Instantiate(m_pRedTeam) as SoccerTeam;
     m_pBlueTeam = Instantiate(m_pBlueTeam) as SoccerTeam;
     m_bGameOn = true;
     m_bGoalKeeperHasBall = false;
     m_bPaused = false;
Here is my Start function for my SoccerTeam
     // Use this for initialization
         void Start () {
         //initialize soccerball
         m_pPitch = GameObject.FindGameObjectWithTag("soccerpitch").GetComponent<SoccerPitch>();
         //initilize team home regions
         if(transform.tag == "redteam")
         m_pHomeGoal =  GameObject.FindGameObjectWithTag("redgoal").GetComponent<Goal>();
         m_pHomeGoal.m_vFacing = new Vector3(0,1,0);
         m_pOpponents = GameObject.FindGameObjectWithTag("blueteam").GetComponent<SoccerTeam>();
         m_POpponentsGoal = GameObject.FindGameObjectWithTag("bluegoal").GetComponent<Goal>();
         if(transform.tag == "blueteam")
         m_pHomeGoal = GameObject.FindGameObjectWithTag("bluegoal").GetComponent<Goal>();
         m_pHomeGoal.m_vFacing = new Vector3(0,-1,0);
    m_pOpponents = GameObject.FindGameObjectWithTag("redteam").GetComponent<SoccerTeam>()
             m_POpponentsGoal = GameObject.FindGameObjectWithTag("redgoal").GetComponent<Goal>();
         //create players for this team
         m_pReceivingPlayer = null;
Here is my start unction for my Field player. 
    // Use this for initialization
     public void Start(){
     m_vVelocity = new Vector3(0,0,0);
     m_vHeading = new Vector3(0,0, 0);
     rigidbody.velocity = new Vector3(0,0, 0);
     //m_pBall = GameObject.FindGameObjectWithTag("soccerBall");
     m_pSteering = new SteeringBehaviours(this.gameObject.GetComponent<FieldPlayer>(), GameObject.FindGameObjectWithTag("soccerBall").GetComponent<SoccerBall>());
     //initilize the statemachine
     m_pStateMachine = new StateMachine<FieldPlayer>(this);
     //intilize the states
     //set the players start position
     transform.position = (this.HomeRegion()).Pos() ;
The Null Exception error points me to the FieldPlayer's start function where I try to set the position of the FieldPlayer
(transform.position = (this.HomeRegion()).Pos() ;)

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Instantiate() should clone the entire hierarchy, so it probably isn't that.


Drop your debugger on the line and step into the function.  The value of this should never be null, and transform.position should work, unless for some reason your object does not have a Transform, which can happen if it is not placed in the scene. Based on your error I'm guessing that your HomeRegion() function is returning null. Step into the function and verify that every value -- especially the value it returns -- is what you expect. 


Also, is there any reason you aren't giving a location for them to be instantiated? If you don't say where to put them it will drop them at the origin, which probably isn't where you expect them.

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I wil do that again. the thing is when I Instantiate the RedTeam only it works well but when I try to Instantiate the blue team it does not work.The same script is attached to two dirrerent soccerteam objects. So the script test which tag the gameobject it is attached to has and populates the player's accordingly. I am not sure were the problem is coming from but i wil check

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What I do, is put my "GetComponent" calls in Awake() and then after everyone gets each other, in Start() do things like set positions, and call member functions. 60% of the time it works everytime.

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Make sure "transform.position" and "this.HomeRegion()" don't evaluate to null. It's very likely one of them does so.
The difference between instanciating the red team and the blue team is the prefab/existing GameObject you're using for the instantiation. You could also try to instanciate the red team twice and see if there are any troubles.

By the way: you also may want to use properties instead of your "Steering" and "HomeRegion" methods.
Also, please, indent your code. It would ease the reading. ;)

What I would do in a different way:
  • Instead of searching for GameObjects ("GameObject.FindGameObjectWithTag([...])"), you could assign them (SendMessage("Set[...]", [...])) right after instanciating the GameObjects. This way you wouldn't need to check, wether it's the red or the blue team within the team script.
    And regarding the team color: I would make it a variable of the script (of type of an enumeration of "all team colors" - red and blue).
  • Also if you're assigning the values this way, you could assign anything that differs between the red and the blue team, so you would only need to have a single "team" prefab.
  • are the goals contained in the scene or are the goals contained in the teams hierarchy? In the latter case: create a variable in the team script and assign the goal, otherwise you should assign it the same way as you're assigning everything else.
But keep in mind: in my opinion this would lead to a better design, but on the one hand my opinion is not necessarily the best regarding good design, and on the other hand a good structure for the game can help to keep order to the project, but it's not required to finish the game.

Oh, and what I'm doing about the "GetComponent" calls: I just use them (almost) only for components of the exact same GameObject, and assign anything else to the variables using the inspector. (The exception to this rule could be, when I need different components of the same GameObject, like the teams goal script and the corresponding transform, but that's a very, very rare case though.) Edited by Sacaldur

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For safety, I'd make this sort of relationship happen out of Start()

That way you'll avoid a lot of concurrent issues with GameObjects not being entirely ready when another GameObject hits their Start()


Perhaps have them respond to: m_bGameOn = true; ?


Or BroadcastMessage() some form of trigger when the whole stage is set?

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