Button.onClick.AddListener() giving unexpected results

I'm creating a list of buttons at runtime and trying to add listeners to each of them. My code goes something like this:

Code (CSharp):

1.         for (int i = 0; i < moveHistory.Count; ++i)
2.         {
3.             GameObject m = (GameObject)Instantiate(movePrefab);
4.             m.transform.SetParent(transform, false);
5.             MoveHistoryItem item = m.GetComponent<MoveHistoryItem>();
6.             item.mainButton.onClick.AddListener(() => GoToMove(i));
7.         }

So I want the first button to call GoToMove(0) when clicked, the second to call GoToMove(1), etc.

In testing, all of the buttons call GoToMove with the same argument (which I believe is the final value of i when the loop ends).

Why does this happen, and how can I get my intended behavior?

 

Not entirely sure how to fix your code, but I did find something similar here: http://forum.unity3d.com/threads/how-to-addlistener-featuring-an-argument.266934/

Maybe it's helpful to you.

 

This is normal, addlistener will capture the instance of i rather than the value of i, to fix just assign it to a local variable:

Code (CSharp):

1.    for (int i = 0; i < moveHistory.Count; ++i)
2.         {
3.             int local_i = i;
4.             GameObject m = (GameObject)Instantiate(movePrefab);
5.             m.transform.SetParent(transform, false);
6.             MoveHistoryItem item = m.GetComponent<MoveHistoryItem>();
7.             item.mainButton.onClick.AddListener(() => GoToMove(local_i));
8.         }

 

Your proposed fix appears to work, but I'm having some trouble with your explanation. I am not passing i to AddListener; I am passing a lambda expression. Is that how lambdas work in general? (I've used lambda expressions in many other places and would be surprised not to have revealed my misunderstanding before now if that's the case, but I haven't specifically been looking for it, so I suppose it's possible.) If not, how is the fact that I'm passing the lambda to AddListener() changing the behavior?

 

Yes sorry it's the lamda expression that does that. I remember seeing this on the MSDN documentation so I think this is the intended behavior.

 

+10 for solution