如何從后台代碼（WP8）設置多個圖釘？

Question

我嘗試添加多個圖釘進行映射。

這是XAML：

<maps:Map ZoomLevel="8" Height="500" x:Name="map1" Width="415" HorizontalAlignment="Left" VerticalAlignment="Top" Margin="0,0,0,0">
                <toolkit1:MapExtensions.Children>
                    <toolkit1:MapItemsControl x:Name="PushpinCollection">
                        <toolkit1:MapItemsControl.ItemTemplate>
                            <DataTemplate>
                                <toolkit1:Pushpin GeoCoordinate="{Binding Coords}" Content="{Binding Name}"/>
                            </DataTemplate>
                        </toolkit1:MapItemsControl.ItemTemplate>
                    </toolkit1:MapItemsControl>
                </toolkit1:MapExtensions.Children>
            </maps:Map>

這是代碼：

protected override async void OnNavigatedTo(NavigationEventArgs e)
    {
        base.OnNavigatedTo(e);
        var xmlDataSource = new XmlDataSource();
        var sigCol = new SignalCollection(xmlDataSource);
        var allSignals = await sigCol.GetData(false, false, "UA");
        ObservableCollection<IssueDescription> signalsWithCoords = new ObservableCollection<IssueDescription>();
        foreach (var signal in allSignals)
        {
            if (signal.Coords != null)
            {
                signalsWithCoords.Add(signal);                    
            }
        }
        PushpinCollection.ItemsSource = signalsWithCoords;
    }

但是最后一個字符串拋出NullReferenceException。 怎么了？

Answer 1

正如我檢查的PushpinCollection存在（不為null），並且以下代碼應該工作：

protected override async void OnNavigatedTo(NavigationEventArgs e)
{
    base.OnNavigatedTo(e);
    var xmlDataSource = new XmlDataSource();
    var sigCol = new SignalCollection(xmlDataSource);
    var allSignals = await sigCol.GetData(false, false, "UA");
    ObservableCollection<IssueDescription> signalsWithCoords = new ObservableCollection<IssueDescription>();
    foreach (var signal in allSignals)
    {
        if (signal.Coords != null)
        {
            signalsWithCoords.Add(signal);                    
        }
    }
    MapItemsControl PushpinCol = MapExtensions.GetChildren(map1).FirstOrDefault(x => x is MapItemsControl) as MapItemsControl;

    PushpinCol.ItemsSource = signalsWithCoords;
}

---

進一步研究發現這似乎是一個小錯誤。 在源代碼中， DependencyProperty NameProperty被注冊為"ItemTemplate" -可能阻止從代碼中按名稱獲取它。

編輯 -評論后

如果在Items.Count > 0並嘗試更改ItemsSource時，在源代碼中多挖掘一點信息，就會表明MapExtension引發了此異常。 這是一種允許您更改ItemsSource的方法：

protected override async void OnNavigatedTo(NavigationEventArgs e)
{
    base.OnNavigatedTo(e);
    var xmlDataSource = new XmlDataSource();
    var sigCol = new SignalCollection(xmlDataSource);
    var allSignals = await sigCol.GetData(false, false, "UA");
    ObservableCollection<IssueDescription> signalsWithCoords = new ObservableCollection<IssueDescription>();
    foreach (var signal in allSignals)
    {
        if (signal.Coords != null)
        {
            signalsWithCoords.Add(signal);                    
        }
    }
    MapItemsControl PushpinCol = MapExtensions.GetChildren(map1).FirstOrDefault(x => x is MapItemsControl) as MapItemsControl;
    if (PushpinCol != null && PushpinCol.ItemsSource != null)
    {
        (PushpinCol.ItemsSource as IList).Clear();
        PushpinCol.ItemsSource = null;
    }
    PushpinCol.ItemsSource = signalsWithCoords;
}

另一方面，請考慮將您的Collection設為靜態，並將其設置為ItemsSource一次。