Listview在恢复后不加载项目

Question

我有一个带有listview的应用程序，其中包含与此示例相关的itemsource pdf图像：

https://code.msdn.microsoft.com/windowsapps/PDF-viewer-showcase-sample-39ced1e8

我的问题是，一旦应用程序暂停，然后恢复，列表视图不会加载更多项目。 也就是说，一旦恢复，您只会看到列表视图中的项目已经加载（在暂停之前），但不再加载其他元素。 如何在不重新初始化itemsource的情况下修复它？

 pdfDocument = await Windows.Data.Pdf.PdfDocument.LoadFromFileAsync(pdfFile);
 PdfDocViewModel pdfDataSourceZoomedInView = new PdfDocViewModel(pdfDocument, pageSize, SurfaceType.VirtualSurfaceImageSource);
 this.listview.ItemsSource = this.pdfDataSourceZoomedInView;


<DataTemplate x:Name="zoomedInViewItemTemplate" x:DataType="model:PdfPageViewModel">
                <Grid HorizontalAlignment="Center" Margin="0" >
                    <Image Source="{x:Bind ImageSourceVsisBackground ,Mode=OneWay}" HorizontalAlignment="Center" Height="{x:Bind Height, Mode=OneWay}" Width="{x:Bind Width ,Mode=OneWay}"></Image>
                    <Image Source="{x:Bind ImageSourceVsisForeground ,Mode=OneWay}" HorizontalAlignment="Center" Height="{x:Bind Height, Mode=OneWay}" Width="{x:Bind Width ,Mode=OneWay}"></Image>
                </Grid>
            </DataTemplate>

这是PdfDocViwModel类：

// Copyright (c) Microsoft Corporation. All rights reserved

//
// PdfDocViewModel.cpp
// Definition of the PdfDocViewModel class
//

#include "pch.h"
#include "PdfDocViewModel.h"
#include "ImageSourceVSIS.h"
#include "PdfViewContext.h"
#include "PdfPageViewModel.h"

using namespace Windows::UI::Xaml::Data;
using namespace Windows::UI::Xaml::Interop;
using namespace Platform;

namespace PdfViewModel
{
    PdfDocViewModel::PdfDocViewModel(_In_ Windows::Data::Pdf::PdfDocument^ pdfDoc, _In_ Windows::Foundation::Size pageSize, _In_ PdfViewModel::SurfaceType surfaceType)
        : pdfDocument(pdfDoc)
    {
        pdfViewContext = ref new PdfViewContext(pageSize, pdfDocument->GetPage(0)->Size, surfaceType);
        storage = ref new Collections::Vector<Object^>();
        storage->VectorChanged += ref new Windows::Foundation::Collections::VectorChangedEventHandler<Object^>(this, &PdfDocViewModel::StorageVectorChanged);
        isVectorChangedObserved = false;

        for (unsigned int pg = 0; pg < pdfDocument->PageCount; ++pg)
        {
            Append(CreateEmptyItem(pg));
        }
    }

#pragma region IBindableObservableVector

    Windows::Foundation::EventRegistrationToken PdfDocViewModel::VectorChanged::add(_In_ BindableVectorChangedEventHandler^ e)
    {
        isVectorChangedObserved = true;
        return privateVectorChanged += e;
    }

    void PdfDocViewModel::VectorChanged::remove(_In_ Windows::Foundation::EventRegistrationToken t)
    {
        privateVectorChanged -= t;
    }

    void PdfDocViewModel::VectorChanged::raise(_In_ IBindableObservableVector^ vector, _In_ Object^ e)
    {
        if (isVectorChangedObserved)
        {
            privateVectorChanged(vector, e);
        }
    }

#pragma endregion

#pragma region IBindableIterator

    IBindableIterator^ PdfDocViewModel::First()
    {
        return dynamic_cast<IBindableIterator^>(storage->First());
    }

#pragma endregion

#pragma region IBindableVector

    void PdfDocViewModel::Append(_In_ Object^ value)
    {
        storage->Append(value);
    }

    void PdfDocViewModel::Clear()
    {
        storage->Clear();
    }

    Object^ PdfDocViewModel::GetAt(_In_ unsigned int index)
    {
        Object^ item = storage->GetAt(index);
        if (item == nullptr)
        {
            item = CreateEmptyItem(index);
            SetAt(index, item);
        }
        RealizeItemIfNotAvailable(index, item);
        return item;
    }

    IBindableVectorView^ PdfDocViewModel::GetView()
    {
        return safe_cast<IBindableVectorView^>(storage->GetView());
    }

    bool PdfDocViewModel::IndexOf(_In_ Object^ value, _In_ unsigned int* index)
    {
        return storage->IndexOf(value, index);
    }

    void PdfDocViewModel::InsertAt(_In_ unsigned int index, _In_ Object^ value)
    {
        storage->InsertAt(index, value);
    }

    void PdfDocViewModel::RemoveAt(_In_ unsigned int index)
    {
        storage->RemoveAt(index);
    }

    void PdfDocViewModel::RemoveAtEnd()
    {
        storage->RemoveAtEnd();
    }

    void PdfDocViewModel::SetAt(_In_ unsigned int index, _In_ Object^ value)
    {
        storage->SetAt(index, value);
    }

    unsigned int PdfDocViewModel::Size::get()
    {
        return storage->Size;
    }

#pragma endregion

    /// <summary>
    /// This function is used to create empty PdfPageViewModel items
    /// </summary>
    Object^ PdfDocViewModel::CreateEmptyItem(_In_ unsigned int index)
    {
        auto obj = ref new PdfPageViewModel(pdfDocument, index);
        obj->SetViewContext(pdfViewContext);
        return obj;
    }

    /// <summary>
    /// This function should be used to realize the item (do some initialization tasks if required).
    /// </summary>
    void PdfDocViewModel::RealizeItemIfNotAvailable(_In_ unsigned int /*index*/, _In_ Object^ /*item*/)
    {
    }

    /// <summary>
    /// Call this method when the app suspends to hint to the driver that the app is entering an idle state
    /// and that its memory can be used temporarily for other apps.
    /// </summary>
    void PdfDocViewModel::Trim()
    {
        pdfViewContext->Renderer->Trim();
    }

    /// <summary>
    /// Function to update page based on current zoomFactor
    /// </summary>
    /// <param name="currentZoomFactor">Current zoom factor of scroll viewer</param>
    void PdfDocViewModel::UpdatePages(_In_ float currentZoomFactor)
    {
        if (pdfViewContext->ZoomFactor != currentZoomFactor)
        {
            pdfViewContext->ZoomFactor = currentZoomFactor;
            if (pdfViewContext->SurfaceType == SurfaceType::VirtualSurfaceImageSource)
            {
                for (unsigned int index = 0, len = Size; index < len; index++)
                {
                    auto pageData = dynamic_cast<PdfPageViewModel^>(storage->GetAt(index));
                    if ((pageData != nullptr) && !pageData->IsUnloaded())
                    {
                        pageData->UpdateSurface();
                    }
                }
            }
        }
    }

#pragma region State

    /// <summary>
    /// Event handler for store vector changed
    /// </summary>
    /// <param name="e">IVectorChangedEventArgs</param>
    void PdfDocViewModel::StorageVectorChanged(_In_ Windows::Foundation::Collections::IObservableVector<Object^>^ /*sender*/, _In_ Windows::Foundation::Collections::IVectorChangedEventArgs^ e)
    {
        if (isVectorChangedObserved)
        {
            VectorChanged(this, e);
        }
    }
#pragma endregion
}