c＃中的闭包示例中的async关键字

Question

I am going thru this blog to understand how to increment load.

http://marcominerva.wordpress.com/2013/05/22/implementing-the-isupportincrementalloading-interface-in-a-window-store-app/

I need help in understanding the following method: I think:

()  =>
{

}

mean a closure with no input argument. Why it adds 'async' before () =>? Why the method LoadMoreItemsAsync() return IAsyncOperation<>, but it calls 'Task.Run'? Did the operantion ran before it returns?

 public IAsyncOperation<LoadMoreItemsResult> LoadMoreItemsAsync(uint count)
    {
        var dispatcher = Window.Current.Dispatcher;

        return Task.Run<LoadMoreItemsResult>(
            async () =>   // why aysnc here?
            {
                uint resultCount = 0;
                var result = await source.GetPagedItems(currentPage++, itemsPerPage);

                if (result == null || result.Count() == 0)
                {
                    hasMoreItems = false;
                }
                else
                {
                    resultCount = (uint)result.Count();

                    await dispatcher.RunAsync(
                        CoreDispatcherPriority.Normal,
                        () =>
                        {
                            foreach (I item in result)
                                this.Add(item);
                        });
                }

                return new LoadMoreItemsResult() { Count = resultCount };

            }).AsAsyncOperation<LoadMoreItemsResult>();
    }

Answer 1

() => { /* ... */ } is a lambda without input parameter, in terms of a delegate it would be an Action .

To use the await keyword the method has to be marked with async . Since the body of that lambda function contains await you have to use that async lambda syntax.

The purpose of the LoadMoreItemsAsync method is to return almost immediately and continue loading items in the background, ie asynchronous with respect to the calling thread. THis is done because presumably the loading takes some time and you don't want to block the UI.

In order to do so it uses the Task.Run method which returns a Task<LoadMoreItemsResult> and AsAsyncOperation<LoadMoreItemsResult>() turns that into an object which implements the IAsyncOperation<LoadMoreItemsResult> interface.

Answer 2

A Lambda Expression which runs runs async code needs to be marked async in order to use the await keyword inside it. Similar to the idea that a Named Method needs to mark itself async .

This signals the compiler that a state machine should be generated in the execution flow.

This:

var result = await source.GetPagedItems(currentPage++, itemsPerPage);

And this:

await dispatcher.RunAsync(
           CoreDispatcherPriority.Normal,
           () =>
           {
               foreach (I item in result)
                   this.Add(item);
           });

Need the async modifier to run.