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Unity Engine

CustomYieldInstruction

Base class for custom yield instructions to suspend coroutines.
Read time 2 minutesLast updated 4 days ago

Definition

public abstract class CustomYieldInstruction : IEnumerator

Remarks

Inherit from 
CustomYieldInstruction
to create custom yield instructions that suspend coroutine execution until a specified event happens.
The CustomYieldInstruction.keepWaiting property determines whether the coroutine must keep waiting or can resume execution. Unity queries CustomYieldInstruction.keepWaiting each frame after MonoBehaviour.Update() and before MonoBehaviour.LateUpdate(). To keep the coroutine suspended, return 
true
. To resume 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
:
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 System.Collections;using UnityEngine;using System;// Implementation of <see cref="UnityEngine.WaitWhile"></see> yield instruction. This can be later used as:// yield return new WaitWhile(() => Princess.isInCastle);class WaitWhile1 : CustomYieldInstruction{    Func<bool> m_Predicate;    public override bool keepWaiting { get { return m_Predicate(); } }    public WaitWhile1(Func<bool> predicate) { m_Predicate = predicate; }}
To have more control and implement more complex yield instructions you can inherit directly from the 
System.Collections.IEnumerator
class. In this case, implement the 
MoveNext
method as you would implement the CustomYieldInstruction.keepWaiting property. You can also return an object in the 
Current
property, which is processed by Unity's coroutine scheduler after executing 
MoveNext
. For example, if 
Current
returns another object inheriting from 
IEnumerator
, then the current enumerator is suspended until the returned one has completed.
using System;using System.Collections;// Same WaitWhile implemented by inheriting from IEnumerator.class WaitWhile2 : IEnumerator{    Func<bool> m_Predicate;    public object Current { get { return null; } }    public bool MoveNext() { return m_Predicate(); }    public void Reset() {}    public WaitWhile2(Func<bool> predicate) { m_Predicate = predicate; }}

Properties

Property

Description

keepWaitingIndicates if coroutine should be kept suspended.