public static IAsyncEnumerable <TSource> WhereAsync <TSource>(this IAsyncEnumerable <TSource> source, Func <TSource, int, Task <bool> > predicate)
{
if (source == null)
{
throw new ArgumentNullException(nameof(source));
}
if (predicate == null)
{
throw new ArgumentNullException(nameof(predicate));
}
return(Create(() =>
{
var e = source.GetEnumerator();
var index = 0;
var current = default(TSource);
var cts = new CancellationTokenDisposable();
var d = Disposable.Create(cts, e);
return Create(async ct =>
{
var b = false;
bool moveNext;
do
{
moveNext = await e.MoveNext(cts.Token).ConfigureAwait(false);
if (moveNext)
{
b = await predicate(e.Current, checked (index++)).ConfigureAwait(false);
}
} while (!b && moveNext);
if (b)
{
current = e.Current;
return true;
}
return false;
},
() => current,
d.Dispose,
e
);
}));
}
