EbaySearchViewModel Class

The viewmodel uses the IEbayClient instance to query the eBay OData service. The viewmodel exposes two commands:

SearchCommand—Triggered by entering text into a TextBox in the view, this command calls the Search method, which calls the IEbayClient’s SearchItemsAsync method.

FetchMoreDataCommand—This command allows the search results to be paged.

The commands are initialized in the EbaySearchViewModel constructor, which is shown in the following excerpt:

Click here to view code image

public EbaySearchViewModel(
    Func<IEbayClient> getEbayClientFunc,
    INetworkConnectionMonitor networkConnectionMonitor)
{
    this.networkConnectionMonitor = ArgumentValidator.AssertNotNull(
                    networkConnectionMonitor, "networkConnectionMonitor");
    this.getEbayClientFunc = ArgumentValidator.AssertNotNull(
                                getEbayClientFunc, "getEbayClientFunc");

    networkConnectionMonitor.NetworkConnectionChanged
        += (sender, args) => UpdateCommands();

    SearchText = "star wars action figure";

    searchCommand = new DelegateCommand<string>(
        query =>
        {
            if (!string.IsNullOrEmpty(query))
            {
                Search(query);
            }
        }, obj => networkConnectionMonitor.Connected);

    fetchMoreDataCommand = new DelegateCommand(
        obj =>
        {
            if (!string.IsNullOrEmpty(lastQuery))
            {
                Search(lastQuery, true);
            }
        }, obj => networkConnectionMonitor.Connected);
}

The enabled state of each command depends on the Connected property of the INetworkConnectionMonitor. When the NetworkConnectionChanged event is raised, the viewmodel’s UpdateCommands method is called, which calls the RaiseCanExecuteChanged method on each command, causing the Enabled state to be reevaluated:

Click here to view code image

void UpdateCommands()
{
    searchCommand.RaiseCanExecuteChanged();
    fetchMoreDataCommand.RaiseCanExecuteChanged();
}

The viewmodel’s Search method uses a Func named getEbayClientFunc to retrieve an IEbayClient (see Listing 27.5). The Search method sets the viewmodel’s Busy property to true, causing a ProgressBar in the view to be made visible.

To reduce the amount of data retrieved from the OData service, the number of records retrieved by the IEbayClient is restricted to 10.

LISTING 27.5. EbaySearchViewModel.Search Method and Related Members

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const int chunkSize = 10;
string lastQuery;
bool appendResult;

void Search(string query, bool append = false)
{
    if (lastQuery == query && Busy)
    {
        return;
    }

    lastQuery = query;
    appendResult = append;

    IEbayClient ebayClient = getEbayClientFunc();
    ebayClient.SearchItemsComplete -= HandleEbayClientSearchItemsComplete;
    ebayClient.SearchItemsComplete += HandleEbayClientSearchItemsComplete;
    int beginIndex = appendResult ? Items.Count : 0;

    try
    {
        Busy = true;
        ebayClient.SearchItemsAsync(query, beginIndex, chunkSize);
    }
    catch (Exception ex)
    {
        Busy = false;
        Console.WriteLine("Unable to perform search." + ex);
        MessageService.ShowError("Unable to perform search.");
    }
}

If the call to SearchItemsAsync fails immediately, the ViewModelBase class’s MessageService is used to present a dialog to the user.

For more information on the MessageService, see Chapter 2, “Fundamental Concepts in Windows Phone Development.”

When the search completes, the HandleEbayClientSearchItemsComplete handler is called. Old requests are ignored. The method returns immediately if the search query does not match the last query (see Listing 27.6).

If the search was not a new search but a request to fetch the next page of the current search results, the results are added to the end of the Items collection.

LISTING 27.6. EbaySearchViewModel.HandleEbayClientSearchItemsComplete Method

Click here to view code image

void HandleEbayClientSearchItemsComplete(
    object sender, SearchItemsCompleteEventArgs args)
{
    /* Ignore old query results. */
    if (args.Query != lastQuery)
    {
        return;
    }

    Busy = false;
    if (args.Error == null)
    {
        if (args.Result.Count > 0)
        {
            if (appendResult)
            {
                foreach (var item in args.Result)
                {
                    Items.Add(item);
                }
            }
            else
            {
                Items = new ObservableCollection<Item>(args.Result);
            }
        }
        else
        {
            MessageService.ShowMessage("No match found.");
        }
    }
    else
    {
        MessageService.ShowError(
            "An error occured while attempting to search.");
    }
}