Trigger event on variable change

Question

Hi there, how to make a custom event that triggers when a variables value is changed.

thanks!

Answer 1

Best Answer

Answer by jdean300 · Jun 22, 2016 at 05:31 PM

Using simple variables, the best you can do is constantly check in update whether or not is has changed:

 private int m_MyVar = 0;
 public int myVar = 0;
 
 public delegate void OnVariableChangeDelegate(int newVal);
 public event OnVariableChangeDelegate OnVariableChange;
 
 private void Update()
 {
     if (myVar != m_MyVar && OnVariableChange != null)
     {
         m_MyVar = myVar;
         OnVariableChange(myVar);
      }
 }

However, if you can use properties this is a little cleaner because you know when a variable is set:

 private int m_MyVar = 0;
 public int MyVar
 {
     get {return m_MyVar;}
     set {
         if (m_MyVar == value) return;
         m_MyVar = value;
         if (OnVariableChange != null)
             OnVariableChange(m_MyVar);
     }
 }
 public delegate void OnVariableChangeDelegate(int newVal);
 public event OnVariableChangeDelegate OnVariableChange;

To subscribe and listen to either of these events:

 private void Start()
 {
     componentWithEvent.OnVariableChange += VariableChangeHandler;
 }
 
 private void VariableChangeHandler(int newVal)
 {
     //do whatever
 }

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Zwusel · Jun 22, 2016 at 05:45 PM 0

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Thanks for the fast answer. Constantly checking for my variables is what i am doing at the moment but i think doing it with events will be more efficient ;) So i will try your second option...seems that is exactly what i need.

ty very much

Zwusel · Jun 22, 2016 at 06:43 PM 0

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Hm i tried it but i wont get it to work:

 using UnityEngine;
 using UnityEngine.UI;
 
 public class DamageUI : $$anonymous$$onoBehaviour
 {
     public static float damage;
     public static float $$anonymous$$Damage;
     public static float maxDamage;
 
     [SerializeField]
     private Slider slider;
 
     Text damageText;                      // Reference to the Text component.
 
 
     void Awake()
     {
         // Set up the reference.
         damageText = GetComponent<Text>();
     }
 
 
     void Start()
     {
         slider.$$anonymous$$Value = $$anonymous$$Damage;
         slider.maxValue = maxDamage;
         slider.value = maxDamage / 2;
     }
 
 
     void Update()
     {
         damage = slider.value;
         damage = $$anonymous$$athf.Round(damage);
 
         // Set the displayed text to be the word "Score" followed by the score value.
         damageText.text = "Damage: " + damage;
     }
 
 
     public float Damage
     {
         get
         {
             return damage;
         }
 
         set
         {
             if (damage == value)
                 return;
 
             damage = value;
 
             if (OnChangeDamage != null)
                 OnChangeDamage(damage);
         }
     }
 
     public delegate void ChangeDamage(float _damage);
     public static event ChangeDamage OnChangeDamage;
 }

When i try to get a Debug.Log on it from another class:

 void Start()
     {
         ChangeDamageEvent.OnChangeDamage += DamageChanged;
     }
 
 
 void DamageChanged(float _damage)
     {
         Debug.Log("Damage changed to: " + _damage);
     }

Nothing shows up on the console when i drag the slider around so the damage value changes

jdean300 Zwusel · Jun 22, 2016 at 06:46 PM 0

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Did you ever subscribe to the event? you need something like: OnChangeDamage += objectThatRespondsToEvent.FunctionThatHandlesEvent;

jdean300 Zwusel · Jun 22, 2016 at 06:52 PM 0

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Okay I think I see what you are trying to do:

 void Start()
 {
     //Add the following to the end of your start
     OnChangeDamage += UpdateDamageText;
 }
 
 private void UpdateDamageText(float dmg)
 {
     damageText.text = "Damage: " + dmg;
 }

although you could just handle this inside the property setter:

 public float Damage
      {
          get
          {
              return damage;
          }
  
          set
          {
              if (damage == value)
                  return;
  
              damage = value;
  
              damageText.text = "Damage: " + damage;
          }
      }

Events are better suited for communicating changes in one script to another script.

Zwusel · Jun 22, 2016 at 07:18 PM 0

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Isnt that exactly what i do here?

 void Start()
      {
          ChangeDamageEvent.OnChangeDamage += DamageChanged;
      }
  
  
  void DamageChanged(float _damage)
      {
          Debug.Log("Damage changed to: " + _damage);
      }

Handling it inside the property setter is not an option... i need to do that in another class.

There are more locations that need to know the dmg has changed. Thats what i need the event for ;)

Zwusel · Jun 22, 2016 at 07:22 PM 0

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wait a $$anonymous$$ute...i might have the solution...

Zwusel · Jun 22, 2016 at 07:24 PM 0

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darn no...i thought i found my mistake...but no

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Trigger event on variable change

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