Interleave tasks in FindAllPagesAsync

[sliekens]

The FindAllPagesAsync extension method returns a collection of tasks that complete in order that they were started.

GW2.NET/src/GW2NET.Core/Common/Paginator.cs

Line 119 in 1fd0356

public static IEnumerable<Task<ICollectionPage<T>>> FindAllPagesAsync<T>(this IPaginator<T> instance, int pageSize, int pageCount, CancellationToken cancellationToken)

Instead, the tasks should be interleaved so that tasks are returned in their order of completion, using this task combinator. It is a small change that dramatically improves performance by minimizing idle CPU time between incomplete requests.

https://msdn.microsoft.com/en-us/library/hh873173(v=vs.110).aspx

static IEnumerable<Task<T>> Interleaved<T>(IEnumerable<Task<T>> tasks)
{
    var inputTasks = tasks.ToList();
    var sources = (from _ in Enumerable.Range(0, inputTasks.Count) 
                   select new TaskCompletionSource<T>()).ToList();
    int nextTaskIndex = -1;
    foreach (var inputTask in inputTasks)
    {
        inputTask.ContinueWith(completed =>
        {
            var source = sources[Interlocked.Increment(ref nextTaskIndex)];
            if (completed.IsFaulted) 
                source.TrySetException(completed.Exception.InnerExceptions);
            else if (completed.IsCanceled) 
                source.TrySetCanceled();
            else 
                source.TrySetResult(completed.Result);
        }, CancellationToken.None, 
           TaskContinuationOptions.ExecuteSynchronously, 
           TaskScheduler.Default);
    }
    return from source in sources 
           select source.Task;
}

There is also a synchronous method named FindAllPages that returns a collection of Lazy<TResult> instead of Task<TResult>. Because it is synchronous, I don't think that it can be interleaved in this way. Any method that interleaves Lazy objects would probably still use TaskCompletionSource under the hood, defeating the purpose of synchronous methods. One more reason to drop synchronous API calls.