如何从列表中删除当前播放索引项，但变量“ i”现在为1而不是0？

Question

In a script :

using System;
using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.AI;

public class NaviDialogue : MonoBehaviour
{
    public ObjectsManipulation op;
    public bool scaling = true;
    public Scaling scale;
    public ConversationTrigger conversationTrigger;

    private bool ended = false;
    private bool startConversation = false;

    private void Update()
    {
        if (scaling == true && DOFControl.hasFinished == true)
        {
            DOFControl.hasFinished = false;
            scaling = false;
            op.Scaling();
            PlayerController.disablePlayerController = true;
            ConversationTrigger.conversationsToPlay.Add(0);
            ConversationTrigger.conversationsToPlay.Add(1);
            ConversationTrigger.conversationsToPlay.Add(2);
            StartCoroutine(conversationTrigger.PlayConversations());
        }
}

And in the top of ConversationTrigger :

public static List<int> conversationsToPlay = new List<int>();

In the method PlayConversations :

public IEnumerator PlayConversations()
    {
        for (int i = 0; i < conversationsToPlay.Count; i++)
        {
            yield return StartCoroutine(PlayConversation(conversationsToPlay[i]));
        }
    }

And the Play Conversation method :

public IEnumerator PlayConversation(int index)
    {
        isRunning = true;

        if (conversations.Count > 0 &&
            conversations[index].Dialogues.Count > 0)
        {
            for (int i = 0; i < conversations[index].Dialogues.Count; i++)
            {
                if (dialoguemanager != null)
                {
                    dialoguemanager.StartDialogue(conversations[index].Dialogues[i]);
                }

                while (DialogueManager.dialogueEnded == false)
                {
                    yield return null;
                }
            }

            conversationIndex = index;
            conversationEnd = true;
            canvas.SetActive(false);
            Debug.Log("Conversation Ended");
            conversationsToPlay.Remove(index);
        }
    }

In the last method Play Conversation I'm removing the current played item :

conversationsToPlay.Remove(index);

The problem is that in the PlayConversations method now I value is 1 so it will play next the last item. So if there are 3 items it will play the first and the last but the middle one will not be played.

Answer 1

You should never modify a collection you are currently iterating over (the problem you encountered is one of the reasons for that). In your case, there are a few options, a simple solution could be to copy the list of conversations and at the same time clear the original list:

public IEnumerator PlayConversations()
{
    var conversations = conversationsToPlay.ToArray(); // Copy the list
    conversationsToPlay.Clear(); // Immediately clear the original list

    for (int i = 0; i < conversations.Length; i++) // iterate over the array
    {
        // Now you also don't need to remove items anymore, 
        // since you already cleared the list
        yield return StartCoroutine(PlayConversation(conversations[i]));
    }
}

The array you create stays local to the coroutine, so you can clear the original list and work with the copy.

Alternatively, you could just change the loop to a while-loop and process the list from the start until it's empty:

public IEnumerator PlayConversations()
{
    while (conversationsToPlay.Count > 0)
    {
        // Better remove the item right here, close to the loop condition. 
        // Makes things easier to understand.
        var conversationIndex = conversationsToPlay[0];
        conversationsToPlay.RemoveAt(0);
        yield return StartCoroutine(PlayConversation(conversationIndex));

    }
}

When going with the second example, you might just as well use a Queue<T> instead of a List<T> for the conversations, as a queue is designed specifically with first-in, first-out access in mind.

Answer 2

If you dont have a business requirment to maintain order, then Always iterate the collection in reverse order when you are planning to remove items. This you can remove items without breaking the sequence of the array. So here

public IEnumerator PlayConversations()
{
    for (int i = conversationsToPlay.Count-1; i >=0;  i++)
    {
        yield return StartCoroutine(PlayConversation(conversationsToPlay[i]));
    }
}

This method works in general for all the situations where we remove something from the collection. However on a side note, Removing the conversation in Playconversation method is just a bad practice. you will end up with hard to maintain code. Remove it in some method which is specifically for this purpose. otherwise you are violating SRP