项目
11/17/2011

AutoCompleteBox.Populating Event

Microsoft Silverlight will reach end of support after October 2021. Learn more.

Occurs when the AutoCompleteBox is populating the drop-down with possible matches based on the Text property.

Namespace: System.Windows.Controls
Assembly: System.Windows.Controls.Input (in System.Windows.Controls.Input.dll)

Syntax

'Declaration
Public Event Populating As PopulatingEventHandler

public event PopulatingEventHandler Populating

<sdk:AutoCompleteBox Populating="eventhandler"/>

Remarks

If the event is canceled, by setting the PopulatingEventArgs.Cancel property to true, the AutoCompleteBox will not automatically populate the selection adapter contained in the drop-down. In this case, if you want possible matches to appear, you must provide the logic for populating the drop-down.

For example, when you make a call to a Web service or other long-acting process, you typically use the following steps.

Set the MinimumPrefixLength and MinimumPopulateDelay properties to values larger than the default to minimize calls to the Web service.
Handle the Populating event and set the PopulatingEventArgs.Cancel property to true.
Do the necessary processing and set the ItemsSource property to the desired item collection.
Call the PopulateComplete method to signal the AutoCompleteBox to show the drop-down.

For more information about handling events, see Events Overview for Silverlight.

Examples

The following code example demonstrates how to handle the Populating event and populate the AutoCompleteBox manually. This code example sets the PopulatingEventArgs.Cancel property to true to stop auto-population and then calls PopulateComplete when the manual population operation is complete. This code example requires a reference to the System.Windows.Controls.Input assembly.

Run this sample

   <StackPanel x:Name="LayoutRoot" Background="White" Orientation="Horizontal">
    <TextBlock Text="News Topic:" HorizontalAlignment="Left"  VerticalAlignment="Center"/>
    <sdk:AutoCompleteBox VerticalAlignment="Center" HorizontalAlignment="Right" Height="30" 
        Width="100" MinimumPopulateDelay="200" MinimumPrefixLength="3" x:Name="myACB" Populating="AutoCompleteBox_Populating" />
</StackPanel>

    ' Handle the Populating event, but cancel it. Make the call to the web service. 
    Private Sub AutoCompleteBox_Populating(ByVal sender As Object, ByVal e As PopulatingEventArgs)
        e.Cancel = True
        CallToWebService()
    End Sub

    ' Call the topic service at the live search site. 
    Private wc As WebClient
    Private Sub CallToWebService()
        wc = New WebClient()
        wc.DownloadStringAsync(New Uri("http://api.search.live.net/qson.aspx?query=" & _
            myACB.SearchText & "&sources=news"))
        AddHandler wc.DownloadStringCompleted, AddressOf wc_DownloadStringCompleted
    End Sub

    Private Sub wc_DownloadStringCompleted(ByVal sender As Object, _
            ByVal e As DownloadStringCompletedEventArgs)
        If e.[Error] IsNot Nothing Then
            Exit Sub
        End If

        Dim data As New List(Of String)()
        Try
            Dim jso As JsonObject = TryCast(DirectCast(JsonObject.Parse(e.Result),  _
                JsonObject)("SearchSuggestion"), JsonObject)
            Dim originalSearchString As String = jso("Query")
            If originalSearchString = myACB.SearchText Then
                For Each suggestion As JsonObject In DirectCast(jso("Section"), JsonArray)
                    data.Add(suggestion.Values.First())
                Next

                ' Diplay the AutoCompleteBox drop down with any suggestions 
                myACB.ItemsSource = data
                myACB.PopulateComplete()
            End If
        Catch
        End Try
    End Sub
End Class

    // Handle the Populating event, but cancel it. Make the call to the web service.
    private void AutoCompleteBox_Populating(object sender, PopulatingEventArgs e)
    {
        e.Cancel = true;
        CallToWebService();
    }

    // Call the topic service at the live search site.
    WebClient wc;
    private void CallToWebService()
    {
        wc = new WebClient();
        wc.DownloadStringAsync(new Uri
            ("http://api.search.live.net/qson.aspx?query=" + myACB.SearchText +
            "&sources=news"));
        wc.DownloadStringCompleted +=
            new DownloadStringCompletedEventHandler(wc_DownloadStringCompleted);
    }

    void wc_DownloadStringCompleted(object sender,
        DownloadStringCompletedEventArgs e)
    {
        if (e.Error != null)
        {
            return;
        }

        List<string> data = new List<string>();
        try
        {
            JsonObject jso = ((JsonObject)JsonObject.Parse(e.Result))
                ["SearchSuggestion"] as JsonObject;
            string originalSearchString = jso["Query"];
            if (originalSearchString == myACB.SearchText)
            {
                foreach (JsonObject suggestion in (JsonArray)jso["Section"])
                {
                    data.Add(suggestion.Values.First());
                }

                // Diplay the AutoCompleteBox drop down with any suggestions
                myACB.ItemsSource = data;
                myACB.PopulateComplete();
            }
        }
        catch { }
    }
}

Version Information

Silverlight

Supported in: 5, 4, 3

Platforms

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.

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