[Unity] Use UnityWebRequest with async/await

UnityWebRequestAwaiter.cs

public class UnityWebRequestAwaiter : INotifyCompletion
{
	private UnityWebRequestAsyncOperation asyncOp;
	private Action continuation;

	public UnityWebRequestAwaiter(UnityWebRequestAsyncOperation asyncOp)
	{
		this.asyncOp = asyncOp;
		asyncOp.completed += OnRequestCompleted;
	}

	public bool IsCompleted { get { return asyncOp.isDone; } }

	public void GetResult() { }

	public void OnCompleted(Action continuation)
	{
		this.continuation = continuation;
	}

	private void OnRequestCompleted(AsyncOperation obj)
	{
		continuation();
	}
}

public static class ExtensionMethods
{
	public static UnityWebRequestAwaiter GetAwaiter(this UnityWebRequestAsyncOperation asyncOp)
	{
		return new UnityWebRequestAwaiter(asyncOp);
	}
}

/*
// Usage example:
UnityWebRequest www = new UnityWebRequest();
// ...
await www.SendWebRequest();
Debug.Log(req.downloadHandler.text);
*/

To reduce allocations i suggest to make WebRequestAwaiter as struct and asyncOp.completed += continuation subscribtion only in OnCompleted call. If IsCompleted = true, when asyc state machine will not call OnCompleted but just get the result.
Order of operators in OnCompleted is important, because struct subscribtion make copy of struct in delegate field.

A couple errors there

1. struct can't compile if continuation isn't assigned,
2. asyncOp.completed can't be set to continuation, parameter mismatch. It should be set to OnRequestCompleted (unless changing Action continuation to Action<AsyncOperation> continuation, but that wicked limits the callbacks you'd be able to apply,
3. if continuation is null, it's possible it could be called while null, so OnRequestComplete should accommodate a nullcheck

Updated code (including library imports):

using System;
using UnityEngine.Networking;
using System.Runtime.CompilerServices;
using UnityEngine;

/// <summary>
/// Async awaitable UnityWebRequest <br/><br/>
/// Usage example: <br/><br/>
/// UnityWebRequest www = new UnityWebRequest(); <br/>
/// // do unitywebrequest setup here here... <br/>
/// await www.SendWebRequest(); <br/>
/// Debug.Log(req.downloadHandler.text); <br/>
/// </summary>
public struct UnityWebRequestAwaiter : INotifyCompletion
{
	private UnityWebRequestAsyncOperation asyncOp;
	private Action continuation;

	public UnityWebRequestAwaiter(UnityWebRequestAsyncOperation asyncOp)
	{
		this.asyncOp = asyncOp;
		continuation = null;
	}

	public bool IsCompleted { get { return asyncOp.isDone; } }

	public void GetResult() { }

	public void OnCompleted(Action continuation)
	{
		this.continuation = continuation;
		asyncOp.completed += OnRequestCompleted;
	}

	private void OnRequestCompleted(AsyncOperation obj)
	{
		continuation?.Invoke();
	}
}

public static class ExtensionMethods
{
	public static UnityWebRequestAwaiter GetAwaiter(this UnityWebRequestAsyncOperation asyncOp)
	{
		return new UnityWebRequestAwaiter(asyncOp);
	}
}

if continuation is null, it's possible it could be called while null, so OnRequestComplete should accommodate a nullcheck.

It is no possible, because subscription made after set of parameter in OnCompleted.

this.continuation = continuation;
asyncOp.completed += OnRequestCompleted;

After performance and allocation`s tests we made this minimalistic version:

[ DebuggerNonUserCode ]
    public readonly struct AsyncOperationAwaiter : INotifyCompletion
    {
        private readonly AsyncOperation _asyncOperation;
        public bool IsCompleted => _asyncOperation.isDone;

        public AsyncOperationAwaiter( AsyncOperation asyncOperation ) => _asyncOperation = asyncOperation;

        public void OnCompleted( Action continuation ) => _asyncOperation.completed += _ => continuation();

        public void GetResult() { }
    }

 [ DebuggerNonUserCode ]
    public readonly struct UnityWebRequestAwaiter : INotifyCompletion
    {
        private readonly UnityWebRequestAsyncOperation _asyncOperation;

        public bool IsCompleted => _asyncOperation.isDone;

        public UnityWebRequestAwaiter( UnityWebRequestAsyncOperation asyncOperation ) => _asyncOperation = asyncOperation;

        public void OnCompleted( Action continuation ) => _asyncOperation.completed += _ => continuation();

        public UnityWebRequest GetResult() => _asyncOperation.webRequest;
    }

This code and other for async operations in unity could be found in https://github.com/CrazyPandaLimited/SimplePromises