Unity3D Coroutines and Problems With Them

If you are familiar with Unity3D, I’m sure you have used or at least heard about yield statements before. Normally, yield statements are not used in coroutines as there is no such thing as Coroutine in C# .NET. Instead, they are used to define iterators without having to create a class which implements IEnumerable/IEumerator or IEnumerable<T>/IEnumerator<T>. If you are not familiar with IEnumerator and IEnumerable interfaces or iterators, you can read the .NET Documentation, this StackOverflow thread and Yield Reference. Even though the yield statements are great to create iterators, they have some limitations:

  • You can’t pass any ref or out parameters to an iterator method.
  • Return type should be IEnumerator/IEnumerable or generic versions of these interfaces.
  • You can’t surround a yield statement with Try-Catch block, but you can write yield statement in Try-Finally block.

If you are using Unity Coroutines, you have to keep these restrictions in mind because the coroutine mechanism is implemented using iterators. As a result of this custom coroutine implementation, more items are added to the standard iterator limitations:

  • Coroutines run concurrently on single (main) thread.
  • Return value of a coroutine should be IEnumerator and it can yield null, YieldInstruction or CustomYieldInstruction.
  • If you are using nested coroutines,
    • When you stop the outer coroutine, it may not stop the inner coroutine. So you may have a dangling coroutine which causes problems during the runtime.
    • When you stop the inner coroutine, outer coroutine may not continue its execution from where it left off.

Concurrency

As you may already know, Unity Coroutines run concurrently. So in theory, you can do your heavy work such as AI calculations, WWW operations or other game logic without blocking the UI. But the Unity API is not thread safe, therefore coroutines are implemented to run concurrently in the same (main) thread. So, you can still access Unity API in a coroutine but if you do your heavy work without yielding, it will eventually block the UI and other operations.

Although they are working on the main thread, you can get help from them to start tasks in a new thread. You can write a tool that lets you to start a coroutine on background thread while simulating a coroutine internally. You can listen MoveNext calls from Unity and whenever coroutine yields to something, you can pause, resume or stop background thread by using an internal state machine. This way, you can switch from background to main thread whenever you want to access Unity API by yielding a certain command. There are also ready to use plugins to do this like Thread Ninja.

Returning a Value

Generally, you should not return a value from a coroutine unless it is nullYieldInstruction or CustomYieldInstruction. Even if you do, it will be regarded as null with a little performance loss. On each frame MoveNext is called for each coroutine, so yielding a value or null from a coroutine means that the coroutine has completed its work for the current frame and it can continue its execution on next frame. Therefore yielding a value or null means waiting for a frame. You can also yield other YieldInstruction implementations such as WaitForSeconds, WaitForEndOfFrame etc. or you can yield your own CustomYieldIntructor implementation.

The problem is, whatever the return value is, Unity engine receives it and you can’t see or use it.

private void Start()
{
    // Return type is Coroutine, so you can't access the return value here.
    Coroutine coroutine = StartCoroutine(TestCoroutine());
}

private IEnumerator TestCoroutine()
{
    // Try to return a custom value.
    yield return "value";
}

But there are some ways to receive values from coroutines.

Iterating Coroutine Manually

As I said before, Unity Coroutines are actually iterators. So you can iterate a coroutine manually and use its return value like this:

private void Start()
{
    // Return type is Coroutine, so you can't access the return value here.
    Coroutine coroutine = StartCoroutine(TestCoroutineRunner());
}

private IEnumerator TestCoroutineRunner()
{
    IEnumerator enumerator = TestCoroutine();
    while (enumerator.MoveNext())
    {
        // We can access the return value here.
        var value = enumerator.Current;
        yield return value;
    }
}

private IEnumerator TestCoroutine()
{
    // Try to return a custom value.
    yield return "value";
}

Using Nasty Callbacks

As we can’t pass ref or out parameters to the coroutines, we can pass a callback and wait for it to yield something:

private void Start()
{
    // Return type is Coroutine, so you can't access the return value here.
    Coroutine coroutine = StartCoroutine(TestCoroutine(OnNextvalue));
}

private void OnNextvalue(string value)
{
    // We can process the value in here.
}

private IEnumerator TestCoroutine(UnityAction&lt;string&gt; onNext)
{
    const string value = "value";

    // Use callback to inform caller that we are yielding a value.
    onNext("value");
    yield return value;
}

This solution might look innocent at first but as you get used to pass callbacks to coroutines, soon or later your code will become a mess. Even you won’t be able to follow the flow in your own code when you connect multiple coroutines to each other.

Using CustomYieldInsturcton

As of Unity 5.3, you can extend CustomYieldInstruction class to access return/result values. For example, in the example below we can access the Result value after the yield operation completes:

public class MyCustomYield : CustomYieldInstruction
{
    public override bool keepWaiting
    {
        get { return DoWork(); }
    }

    public string Result { get; set; }
    private bool DoWork()
    {
        // Do work for 2 seconds
        return Time.time <= 2;
    }
}

public class CoroutineTest : MonoBehaviour
{
    private IEnumerator Start()
    {
        var coroutine = A();
        yield return coroutine;

        Debug.Log(coroutine.Result);
    }

    private MyCustomYield A()
    {
        return new MyCustomYield();
    }
}

With this approach, you can create a class that extends CustomYieldInstruction, do your work and then access the custom result value. This lets you to write modular code for your custom coroutines but this doesn’t change the fact that you have to write a new class whenever you want to create a custom coroutine. In my next post, I’ll be talking about UniRx and how can we easily return values, filter and manipulate results without using extra classes.

Nested Coroutines

Have you ever tried to start or stop a coroutine in another coroutine? If the answer is yes, you’ve probably encountered strange problems. In Unity, coroutines are started using StartCoroutine method, and they can be stopped using yield break statement or StopCoroutine method. Wrong usage of these methods can cause problems while using nested coroutines.

Stopping The Outer Coroutine

Sometimes you need to start a coroutine in another coroutine, and when you want to stop them you naturally stop the outer coroutine and hope for inner coroutine to stop. Let’s consider the following example:

private IEnumerator Start()
{
    var coroutineA = StartCoroutine(A());

    yield return new WaitForSeconds(2f);

    StopCoroutine(coroutineA);
    Debug.Log("Stopped A");
}

private IEnumerator A()
{
    yield return StartCoroutine(B());
}

private IEnumerator B()
{
    while (true)
    {
        Debug.Log("Tick");
        yield return null;
    }
}

In this example, we start coroutine A when the game is started, and then it starts coroutine B. B immediately starts to print “Tick” to the console on each frame. And then we wait for 2 seconds and stop coroutine A by calling the StopCoroutine method by passing coroutine reference as argument. What do you think will happen when we stop coroutine A, do you think coroutine B will also stop?

The answer is No. Here is the output:

Ticking continues after A is stopped

It is really easy to believe that B will stop when we stop A if we don’t know how coroutines work. When we start coroutine A with StartCoroutine, Unity registers A to its internal coroutine list and calls MoveNext of it on each frame. The same is true for B, no matter who started it. So when you stop A with StopCoroutine, Unity does not remove B from its internal list and keeps calling MoveNext on B.

We can solve this problem in several ways. The first and ugly solution might be to move the code in B to A. But this might huge, monolithic coroutines which we don’t want to deal with. Another solution is starting and iterating B manually. We can do this in two ways:

private IEnumerator A()
{
    var enumerator = B();
    while (enumerator.MoveNext())
    {
        yield return enumerator.Current;
    }
}

or just (if you are using Unity 5.3 or later):

private IEnumerator A()
{
    yield return B();
}

The code above will not work if you are using an older version of Unity (older than 5.3). Previously, you were not able to start a coroutine without using StartCoroutine. Even though there is no documentation about this change in anywhere (changelogs, documentations etc.), this code should work without a problem. Now the output will be:

There are no ticks after the A is stopped

Stopping Inner Coroutine

Let’s consider the opposite of the example above, where we want to stop inner coroutine instead of the outer. Let’s have a look at this example:

public class CoroutineTest : MonoBehaviour
{
    private Coroutine _coroutineB;

    private IEnumerator Start()
    {
        StartCoroutine(A());

        yield return new WaitForSeconds(2f);

        StopCoroutine(_coroutineB);
        Debug.Log("Stopped A");
    }

    private IEnumerator A()
    {
        _coroutineB = StartCoroutine(B());
        yield return _coroutineB;

        Debug.Log("A continues...");
    }

    private IEnumerator B()
    {
        while (true)
        {
            Debug.Log("Tick");
            yield return null;
        }
    }
}

Again, we are starting coroutine A when the game starts and then it starts coroutine B. We wait for 2 seconds and stop coroutine B with StopCoroutine method. Do you think “A continues…” log will be printed?

The answer is again, No:

“A continues…” is never printed to console

Coroutine A is scheduled to run after coroutine B is finished, in other words when MoveNext of B returns false. There is a better explanation in here, but the problem is basically stopping coroutine B with StopCoroutine method. When we call StopCoroutine to stop B, Unity removes B from its internal coroutines list as soon as it yields to something. And to resume coroutine A, Unity waits for B to finish gracefully. This will never happen because we interrupted its execution before its MoveNext returns false properly.

Better Way To Stop Coroutines

Another (and elegant) way to stop the inner or outer coroutine is letting it stop gracefully by setting a flag or passing a cancellation token to the coroutine:

public class CancellationToken
{
    public bool IsCancellationRequested { get; set; }
}

public class CoroutineTest : MonoBehaviour
{
    private IEnumerator Start()
    {
        var cancellationToken = new CancellationToken();
        StartCoroutine(A(cancellationToken));

        yield return new WaitForSeconds(2f);

        cancellationToken.IsCancellationRequested = true;
        Debug.Log("Stopped A");
    }

    private IEnumerator A(CancellationToken cancellationToken)
    {
        try
        {
            yield return StartCoroutine(B(cancellationToken));
        }
        finally
        {
            Debug.Log("Finalize A...");
        }
    }

    private IEnumerator B(CancellationToken cancellationToken)
    {
        try
        {
            while (!cancellationToken.IsCancellationRequested)
            {
                Debug.Log("Tick");
                yield return null;
            }
        }
        finally
        {
            Debug.Log("Finalize B...");
        }
    }
}

Output will be like this:

Finalized Coroutines

Using this approach, we are letting the coroutine to finish gracefully. Most of the time, we want to finalize the coroutine, release the resources we use such as network sockets, disposable types etc. If we were using StopCoroutine to stop the coroutine, it was going to be interrupted so we could not see “Finalize A” and “Finalize B” logs.

Conclusion

Unity Coroutines may seem great for running concurrent tasks. The problem is, they have some limitations that you have to overcome, and sometimes you may have to write cryptic and ugly code to solve these problems. Also, since Unity’s source code is private, in some cases you may need to write a test code and see the results in order to understand what you will do while using coroutines. As an alternative to Unity Coroutines (or you can use with Unity Coroutines), you can use UniRx (Reactive Extensions for Unity) and overcome these limitations, and I’m planning to write my next post about it.

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