[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);
*/

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