Windows Phone，多个HTTP请求并行，多少？

Question

In my Windows Phone 8 app, Im fetching list of items from web api. After that I loop all items and get details for each Item.

Right now my code is something like this:

List<plane> planes = await planeService.getPlanes(); // Get all planes from web api

foreach(Plane plane in planes)
{
    var details = await planeService.getDetails(plane.id); // Get one plane details from web api

    drawDetails(details);
}

How can I improve this to make multiple request in parallel and what is resonable number of request running parallel? The planes list can be anything from 0 to 100 objects, typically max 20.

Answer 1

How can I improve this to make multiple request in parallel?

You can do the parallel processing like below (untested). It uses SemaphoreSlim to throttle getDetails requests.

async Task ProcessPlanes()
{
    const int MAX_REQUESTS = 50;

    List<plane> planes = await planeService.getPlanes(); // Get all planes from web api

    var semaphore = new SemaphoreSlim(MAX_REQUESTS);

    Func<string, Task<Details>> getDetailsAsync = async (id) =>
    {
        await semaphore.WaitAsync();
        try 
        {           
            var details = await planeService.getDetails(id);
            drawDetails(details);
            return details;
        }
        finally
        {
            semaphore.Release();
        }
    };

    var tasks = planes.Select((plane) => 
        getDetailsAsync(plane.id));

    await Task.WhenAll(tasks.ToArray());
}

what is resonable number of request running parallel? The planes list can be anything from 0 to 100 objects, typically max 20.

It largely dependents on the server, but I don't think there's an ultimate answer to this. For example, check this question:

A reasonable number of simultaneous, asynchronous ajax requests

As far as the WP8 client goes, I believe it can spawn 100 parallel requests without a problem.

Answer 2

I don't know what the limit is for network connections, but there will be one.

If there wasn't, the only problem would be the amount of memory used to keep that many requests alive.

So, assuming the underlying operating system will handle throttling properly, I would do something this:

List<plane> planes = await planeService.getPlanes();

var allDetails = Task.WhenAll(from plane in plains
                              select planeService.getDetails(plane.id));

foreach(var details in allDetails)
{
    drawDetails(details);
}

NOTE: You should follow common naming conventions to help others understand your code. Asynchronous methods should be suffixed Async and, in *C# , method names are always CamelCase.

Answer 3

You should check the ServicePoint , this will provides connection management for HTTP connections. The default maximum number of concurrent connections allowed by a ServicePoint object is 2. So if you need to increase it you can use ServicePointManager.DefaultConnectionLimit property . Just check the link in MSDN there you can see a sample. And set the value you need. This might help you..