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Question by SherlockHolmes · Oct 09, 2014 at 05:17 AM · javascriptmovement

Can't get speed of a rigidbody.

Hello!

My first script in Unity. I have a block that I can move, alright. Now I thought I'd fetch the speed of the rigidbody I have on that object, and if the speed is lower than 3, make it 10. Then it would obviously speed up quickly and then speed down again.

The moving part works. Everything works fine, but when I do this:

 #pragma strict
 
     var moveSpeed : float = 3; //speed of the player
     var maxSpeed : float = 3;
     var currentSpeed = rigidbody.velocity.magnitude;
 
     //called at start
     function Start () {  
 
 }
      //called every frame
     function Update () {
         //Fetch the input from the player, and convert
         //it into realtime movement
         
         if(currentSpeed < maxSpeed){
             moveSpeed = 10;
         
         }
         
         //translates the input into movement
         var translationZ : float = Input.GetAxis ("Horizontal") * moveSpeed; 
         var translationX : float = Input.GetAxis ("Vertical") * moveSpeed;
         //We don't want to let the player speed be dependent
         //on the frames per second, so let's fix the speed to
         //Time.deltaTime. By multiplying it, we basically
         //make it frame-independent. (At least, that's the plan).
         translationZ *= Time.deltaTime;
         translationX *= Time.deltaTime;
             
         //move desired creature along Z+X axis
         transform.Translate (translationX, 0, translationZ);
     
 }


This is the code. But as soon as I type the

 if(currentSpeed < maxSpeed){
         moveSpeed = 10;
         }

line, it gives me this:

UnityException: You are not allowed to call this function when declaring a variable. Move it to the line after without a variable declaration.

get_rigidbody can only be called from the main thread.

Constructors and field initializers will be executed from the loading thread when loading a scene. Don't use this function in the constructor or field initializers, instead move initialization code to the Awake or Start function. PlayerMovement..ctor () (at Assets/Scripts/PlayerMovement.js:5)

Any help would be extremely appreciated. Thank you.

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Answer by Baste · Oct 09, 2014 at 01:36 PM

You cannot assign the currentSpeed variable like that. Even if it was possible, it would be constant from the moment the script started, and your code wouldn't make sense. You need to move the assignment to currentSpeed inside the Update method:

 function Update () {
     //Fetch the input from the player, and convert
     //it into realtime movement
     currentSpeed = rigidbody.velocity.magnitude;
     if(currentSpeed < maxSpeed){
         moveSpeed = 10;
     }
     ...

There is still a big problem with your code! When you move a transform by setting it's position, that doesn't effect the physics engine in the way you would expect. In particular, the rigidbody does not register transform movement as movement. In essence, moving the transform directly means that the rigidbody's velocity is always zero.

To check speed by getting the rigidbody's velocity, you either have to move the rigidbody by setting it's velocity (making checking it redundant), or by adding force to the rigidbody by using AddForce. Hope that helps!

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avatar image Bunny83 · Oct 09, 2014 at 01:40 PM 0
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perfect summary

+1

avatar image SherlockHolmes · Oct 09, 2014 at 06:57 PM 0
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That's a great answer. Thank you.

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