• Advertisement
Sign in to follow this  

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

This topic is 1047 days old which is more than the 365 day threshold we allow for new replies. Please post a new topic.

If you intended to correct an error in the post then please contact us.

Recommended Posts

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() ;)

Share this post

Link to post
Share on other sites

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.

Share this post

Link to post
Share on other sites
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

Share this post

Link to post
Share on other sites
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.

Share this post

Link to post
Share on other sites
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

Share this post

Link to post
Share on other sites

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?

Share this post

Link to post
Share on other sites
Sign in to follow this  

  • Advertisement
  • Advertisement
  • Popular Tags

  • Advertisement
  • Popular Now

  • Similar Content

    • By Alexander Nazarov
      Hello. I'm newby in Unity and just start learning basics of this engine. I want to create a game like StackJump (links are below). And now I wondering what features do I have to use to create such my game. Should I use Physics engine or I can move objects changing transform manually in Update().
      If I should use Physics can you in several words direct me how can I implement and what I have to use. Just general info, no need for detailed description of developing process.
      Game in PlayMarket
      Video of the game
    • By GytisDev
      without going into any details I am looking for any articles or blogs or advice about city building and RTS games in general. I tried to search for these on my own, but would like to see your input also. I want to make a very simple version of a game like Banished or Kingdoms and Castles,  where I would be able to place like two types of buildings, make farms and cut trees for resources while controlling a single worker. I have some problem understanding how these games works in the back-end: how various data can be stored about the map and objects, how grids works, implementing work system (like a little cube (human) walks to a tree and cuts it) and so on. I am also pretty confident in my programming capabilities for such a game. Sorry if I make any mistakes, English is not my native language.
      Thank you in advance.
    • By Ovicior
      So I'm currently working on a rogue-like top-down game that features melee combat. Getting basic weapon stats like power, weight, and range is not a problem. I am, however, having a problem with coming up with a flexible and dynamic system to allow me to quickly create unique effects for the weapons. I want to essentially create a sort of API that is called when appropriate and gives whatever information is necessary (For example, I could opt to use methods called OnPlayerHit() or IfPlayerBleeding() to implement behavior for each weapon). The issue is, I've never actually made a system as flexible as this.
      My current idea is to make a base abstract weapon class, and then have calls to all the methods when appropriate in there (OnPlayerHit() would be called whenever the player's health is subtracted from, for example). This would involve creating a sub-class for every weapon type and overriding each method to make sure the behavior works appropriately. This does not feel very efficient or clean at all. I was thinking of using interfaces to allow for the implementation of whatever "event" is needed (such as having an interface for OnPlayerAttack(), which would force the creation of a method that is called whenever the player attacks something).
      Here's a couple unique weapon ideas I have:
      Explosion sword: Create explosion in attack direction.
      Cold sword: Chance to freeze enemies when they are hit.
      Electric sword: On attack, electricity chains damage to nearby enemies.
      I'm basically trying to create a sort of API that'll allow me to easily inherit from a base weapon class and add additional behaviors somehow. One thing to know is that I'm on Unity, and swapping the weapon object's weapon component whenever the weapon changes is not at all a good idea. I need some way to contain all this varying data in one Unity component that can contain a Weapon field to hold all this data. Any ideas?
      I'm currently considering having a WeaponController class that can contain a Weapon class, which calls all the methods I use to create unique effects in the weapon (Such as OnPlayerAttack()) when appropriate.
    • By Vu Chi Thien
      Hi fellow game devs,
      First, I would like to apologize for the wall of text.
      As you may notice I have been digging in vehicle simulation for some times now through my clutch question posts. And thanks to the generous help of you guys, especially @CombatWombat I have finished my clutch model (Really CombatWombat you deserve much more than a post upvote, I would buy you a drink if I could ha ha). 
      Now the final piece in my vehicle physic model is the differential. For now I have an open-differential model working quite well by just outputting torque 50-50 to left and right wheel. Now I would like to implement a Limited Slip Differential. I have very limited knowledge about LSD, and what I know about LSD is through readings on racer.nl documentation, watching Youtube videos, and playing around with games like Assetto Corsa and Project Cars. So this is what I understand so far:
      - The LSD acts like an open-diff when there is no torque from engine applied to the input shaft of the diff. However, in clutch-type LSD there is still an amount of binding between the left and right wheel due to preload spring.
      - When there is torque to the input shaft (on power and off power in 2 ways LSD), in ramp LSD, the ramp will push the clutch patch together, creating binding force. The amount of binding force depends on the amount of clutch patch and ramp angle, so the diff will not completely locked up and there is still difference in wheel speed between left and right wheel, but when the locking force is enough the diff will lock.
      - There also something I'm not sure is the amount of torque ratio based on road resistance torque (rolling resistance I guess)., but since I cannot extract rolling resistance from the tire model I'm using (Unity wheelCollider), I think I would not use this approach. Instead I'm going to use the speed difference in left and right wheel, similar to torsen diff. Below is my rough model with the clutch type LSD:
      speedDiff = leftWheelSpeed - rightWheelSpeed; //torque to differential input shaft. //first treat the diff as an open diff with equal torque to both wheels inputTorque = gearBoxTorque * 0.5f; //then modify torque to each wheel based on wheel speed difference //the difference in torque depends on speed difference, throttleInput (on/off power) //amount of locking force wanted at different amount of speed difference, //and preload force //torque to left wheel leftWheelTorque = inputTorque - (speedDiff * preLoadForce + lockingForce * throttleInput); //torque to right wheel rightWheelTorque = inputTorque + (speedDiff * preLoadForce + lockingForce * throttleInput); I'm putting throttle input in because from what I've read the amount of locking also depends on the amount of throttle input (harder throttle -> higher  torque input -> stronger locking). The model is nowhere near good, so please jump in and correct me.
      Also I have a few questions:
      - In torsen/geared LSD, is it correct that the diff actually never lock but only split torque based on bias ratio, which also based on speed difference between wheels? And does the bias only happen when the speed difference reaches the ratio (say 2:1 or 3:1) and below that it will act like an open diff, which basically like an open diff with an if statement to switch state?
      - Is it correct that the amount of locking force in clutch LSD depends on amount of input torque? If so, what is the threshold of the input torque to "activate" the diff (start splitting torque)? How can I get the amount of torque bias ratio (in wheelTorque = inputTorque * biasRatio) based on the speed difference or rolling resistance at wheel?
      - Is the speed at the input shaft of the diff always equals to the average speed of 2 wheels ie (left + right) / 2?
      Please help me out with this. I haven't found any topic about this yet on gamedev, and this is my final piece of the puzzle. Thank you guys very very much.
  • Advertisement