4.6 UI How to apply OnClick handler for button generated at runtime (script)?

Question

I'm dynamically generating UI buttons based on locations the player knows about.

How do I add my function to the OnClick listener?

 foreach (string a in places)
 {
    GameObject go = (GameObject)Instantiate(buttons);
                
    go.transform.parent = panel.transform;
    go.transform.localScale = new Vector3(1, 1, 1);
    Button b = go.GetComponent<Button>();
    b.onClick.AddListener(...???...)
 }

As a work around I made a "PointerDown" trigger on the button object but I would rather do it properly and use Unity's onclick handling.

Answer 1

Best Answer

Answer by AyAMrau · Sep 16, 2014 at 12:48 PM

I'm just going to make some assumptions here; Since you said in the other post about this, that you tried my version with lambda expressions and that you always got the last button being called, my guess it that you are trying to somehow use the string from the loop in the listener. The issue you are having there is with capturing the loop iterator instead of the value.

Eg. if you are going to write it something like this:

 foreach (string a in places)
 {
    GameObject go = (GameObject)Instantiate(buttons);
  
    go.transform.parent = panel.transform;
    go.transform.localScale = new Vector3(1, 1, 1);
    Button b = go.GetComponent<Button>();
    b.onClick.AddListener(() => MyMethod(a));
 }

At the time where you click the button MyMethod is going to receive the element from places that was last used in the loop (if you executed the full loop, then this will always be the last one)

You can solve this by capturing the variable separately before adding the listener.

 foreach (string a in places)
 {
    GameObject go = (GameObject)Instantiate(buttons);
  
    go.transform.parent = panel.transform;
    go.transform.localScale = new Vector3(1, 1, 1);
    Button b = go.GetComponent<Button>();
    string captured = a;
    b.onClick.AddListener(() => MyMethod(captured));
 }

or alternatively by making the part with listener adding a separate method:

 foreach (string a in places)
     {
        GameObject go = (GameObject)Instantiate(buttons);
      
        go.transform.parent = panel.transform;
        go.transform.localScale = new Vector3(1, 1, 1);
        Button b = go.GetComponent<Button>();
        AddListener(b, a); // Using the iterator as argument will capture the variable
     }
 
 // Outside of method running the above
 
 void AddListener(Button b, string value) 
 {
    b.onClick.AddListener(() => MyMethod(value));
 }

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LordYabo · Sep 16, 2014 at 04:06 PM 0

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Works perfect. Thanks! The lambda expression was confusing me in the other example.

AyAMrau · Dec 02, 2014 at 03:44 PM 1

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@invadererik

In a statement foreach(string a in places) a is an iteration variable, which means that on every iteration of the loop it is assigned the next value in the collection you iterating over. In most cases it doesn't cause you any issues, because if you are executing statements immediately, so it is the right value. It shows it's true nature when a anonymous method is involved. A statement:

 b.onClick.AddListener(() => $$anonymous$$y$$anonymous$$ethod(a));

doesn't actually execute $$anonymous$$y$$anonymous$$ethod(a) it just adds a reference to it in the button b. Because all this happens before you can press any buttons, the loop would have worked all the way to the end constantly changing the value of a until it stopped on the very last element in the loop. (Normally a would be destroyed afterwards, but ins$$anonymous$$d it lives on because of the stored references in the buttons).

When a button is pressed it executes all the attached listeners and $$anonymous$$y$$anonymous$$ethod is called with the current value of a. This makes sense, because you can capture and object and you are interested in it's state when the button is pressed (e.g. if ins$$anonymous$$d of a string we were talking about an InputField you would probably want to read the value of that field once the button is pressed and not the one it had when you were assigning the listener).

When you add a statement:

 string captured = a;

The current value that a is stored in the variable captured, because it is executed while the loop is working. A separate variable captured is created on every iteration, so it won't be changing its value every single time.

This changes in C# 5 Once Unity upgrades to C#5 this issue will disappear, the iteration variable will be a new one on every loop iteration.

I tried my best here to explain this. If you are really interested in how C# works, how it evolved and what are the gotchas I really recommend the book *C# in Depth* by Jon Skeet.

RalliantoDeLaVega · Dec 02, 2014 at 11:30 PM 1

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This also worked for me! It's strange that, when you run trough a foreach loop, only the last method with AddListener(() => $$anonymous$$y$$anonymous$$ethod(value) is stored, and this simple work-around

      foreach (string a in places)
     {
     GameObject go = (GameObject)Instantiate(buttons);
     go.transform.parent = panel.transform;
     go.transform.localScale = new Vector3(1, 1, 1);
     Button b = go.GetComponent<Button>();
     AddListener(b, a); // Using the iterator as argument will capture the variable
     }
     // Outside of method running the above
     void AddListener(Button b, string value)
     {
     b.onClick.AddListener(() => $$anonymous$$y$$anonymous$$ethod(value));
     }

makes the code behave as you expect. Thx !!

AyAMrau · Feb 21, 2015 at 04:02 PM 1

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@E$$anonymous$$OTION THEORY from what I gathered they don't show up there because Unity considers them separately from listeners added in editor. If you add a listener in editor it's known as a persistent listener, if you add it in code it's a non persistent listener. You can see those terms mentioned in the UnityEvent doc (which is what onClick is).

Not sure if there is an easier solution than writing own inspector that will show the non persistent listeners.

Meltdown · Mar 05, 2015 at 02:02 AM 1

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Thanks! Only the outside the method version seems to work for me. Awesome answer though.

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4.6 UI How to apply OnClick handler for button generated at runtime (script)?

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