CustomYieldInstruction

Description

Base class for custom yield instructions to suspend coroutines.

CustomYieldInstruction lets you implement custom yield instructions to suspend coroutine execution until an event happens. Under the hood, custom yield instruction is just another running coroutine. To implement it, inherit from CustomYieldInstruction class and override keepWaiting property. To keep coroutine suspended return true. To let coroutine proceed with execution return false. keepWaiting property is queried each frame after MonoBehaviour.Update and before MonoBehaviour.LateUpdate.

This class requires Unity 5.3 or later.

To keep coroutine suspended, return true. To let coroutine proceed with execution, return false.

// Example showing how a CustomYieldInstruction script file
// can be used.  This waits for the left button to go up and then
// waits for the right button to go down.
using System.Collections;
using UnityEngine;

public class ExampleScript : MonoBehaviour
{
    void Update()
    {
        if (Input.GetMouseButtonUp(0))
        {
            Debug.Log("Left mouse button up");
            StartCoroutine(waitForMouseDown());
        }
    }

    public IEnumerator waitForMouseDown()
    {
        yield return new WaitForMouseDown();
        Debug.Log("Right mouse button pressed");
    }
}

The following script implements the overridable version of keepWaiting. This c# implementation can be used by JS. In this case make sure this c# script is in a folder such as Plugins so it is compiled before the JS script example above.

using UnityEngine;

public class WaitForMouseDown : CustomYieldInstruction
{
    public override bool keepWaiting
    {
        get
        {
            return !Input.GetMouseButtonDown(1);
        }
    }

    public WaitForMouseDown()
    {
        Debug.Log("Waiting for Mouse right button down");
    }
}

using UnityEngine;

// Implementation of WaitWhile yield instruction. This can be later used as:
// yield return new WaitWhile(() => Princess.isInCastle);
class WaitWhile : CustomYieldInstruction
{
    Func<bool> m_Predicate;

    public override bool keepWaiting { get { return m_Predicate(); } }

    public WaitWhile(Func<bool> predicate) { m_Predicate = predicate; }
}

To have more control and implement more complex yield instructions you can inherit directly from System.Collections.IEnumerator class. In this case, implement MoveNext() method the same way you would implement keepWaiting property. Additionaly to that, you can also return an object in Current property, that will be processed by Unity's coroutine scheduler after executing MoveNext() method. So for example if Current returned another object inheriting from IEnumerator, then current enumerator would be suspended until the returned one has completed.

// Same WaitWhile implemented by inheriting from IEnumerator.
class WaitWhile : IEnumerator
{
    Func<bool> m_Predicate;

    public object Current { get { return null; } }

    public bool MoveNext() { return m_Predicate(); }

    public void Reset() {}

    public WaitWhile(Func<bool> predicate) { m_Predicate = predicate; }
}

Variables