c++ - 使用 UWP 和 C++/WinRT 应用处理从 ListView 拖放到 ListView 的拖放事件

Question

我正在开发一个在 Windows 10 下用 C++/WinRT 编写的简单 UWP 应用程序，其中包含两个ListView控件。此应用程序的目标是学习如何从一个ListView控件中选择一个项目，将其拖动到另一个ListView控件，然后放下该项目，以便将其从源ListView控件复制到目标ListView控件。

ListView到目前为止，我发现的所有示例都使用 C#，其中一些使用 C++/CX 而不是 C++/WinRT 和本机 C++，但是我已经设法坚持到从源代码中选择项目的基本机制可以正常工作的地步拖放到目的地ListView。但是，当尝试从 drop 事件中获取信息以更新目标ListView时，我遇到了异常。

问题：我需要进行哪些更改才能将源ListView控件中的选定文本拖放到目标ListView控件上，然后将文本添加到目标ListView控件中？

Visual Studio 2017 的输出窗口显示以下文本，我将其解释为错误地址异常：

Unhandled exception at 0x0259DC3C (Windows.UI.Xaml.dll) in TouchExperiment_01.exe: 0xC000027B: An application-internal exception has occurred (parameters: 0x05F5E3D8, 0x00000005).

Unhandled exception at 0x74ECE61D (combase.dll) in TouchExperiment_01.exe: 0xC0000602:  A fail fast exception occurred. Exception handlers will not be invoked and the process will be terminated immediately.

Unhandled exception at 0x74F9D7D9 (combase.dll) in TouchExperiment_01.exe: Stack cookie instrumentation code detected a stack-based buffer overrun.

Unhandled exception at 0x74F9D7D9 (combase.dll) in TouchExperiment_01.exe: Stack cookie instrumentation code detected a stack-based buffer overrun.

当函数中的以下源代码行（void MainPage::OnListViewDrop()MainPage.cpp 源文件中的最后一个函数）执行时引发异常：

auto x = e.DataView().GetTextAsync();

附加信息 A：使用调试器，我发现与异常相关的错误消息意味着方法提供的数据中存在错误OnListViewDragItemsStarting()。异常错误消息的文本是：

{m_handle={m_value=0x05550330 L"DataPackage does not contain the specified format. Verify its presence using DataPackageView.Contains or DataPackageView.AvailableFormats." } }

我还在 Visual Studio 首次引发和捕获异常的站点上发现，停止应用程序base.h（来自 C++/WinRT 模板），错误文本0x8004006a : Invalid clipboard format表明我对数据格式缺乏一致意见拖动开始创建并且拖放尝试消耗。

源代码概述

我在 MainPage.xml、MainPage.cpp、MainPage.h 和 pch.h 区域中修改了标准 C++/WinRT 应用程序模板。我还为一个新类 DataSource 添加了类文件，它使用 astd::vector<>来包含一些测试数据。此内存驻留数据在App构造函数中使用一些虚拟数据进行初始化：

App::App()
{
    InitializeComponent();
    DataSource::InitializeDataBase();
    Suspending({ this, &App::OnSuspending });
    //  … other code

首先，我必须在 pch.h 文件中添加一行以提供拖放模板：

#include "winrt/Windows.ApplicationModel.DataTransfer.h"    // ADD_TO:  need to add to allow use of drag and drop in MainPage.cpp

XAML 源文件包含两个ListView控件的源以及一个TextBlock显示源中所选项目的完整描述的控件ListView：

 <Page
    x:Class="TouchExperiment_01.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:TouchExperiment_01"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d">

    <StackPanel Orientation="Horizontal" HorizontalAlignment="Center" VerticalAlignment="Center" Width="1130" Margin="0,0,0,0">
        <ListView x:Name="myList" HorizontalAlignment="Left" Height="100" VerticalAlignment="Top" Width="300" SelectionChanged="OnSelectionChanged"
                  CanDragItems="True" DragItemsStarting="OnListViewDragItemsStarting" BorderBrush="AliceBlue" BorderThickness="3">
        </ListView>
        <TextBlock x:Name="myTextBlock" Height="200" Width="200" Text="this is temp text to replace." TextWrapping="WrapWholeWords" Margin="5"/>
        <ListView x:Name="myList2" HorizontalAlignment="Right" Height="100" VerticalAlignment="Top" Width="300" SelectionChanged="OnSelectionChanged" AllowDrop="True"
                  DragOver="OnListViewDragOver" Drop="OnListViewDrop"  BorderBrush="DarkGreen" BorderThickness="5">
        </ListView>
    </StackPanel>
</Page>

的类声明DataSource很简单。类定义如下：

#pragma once
class DataSource
{
public:
    DataSource();
    ~DataSource();

    static int InitializeDataBase();

    struct DataSourceType
    {
        std::wstring  name;
        std::wstring  description;
    };

    static std::vector<DataSourceType> myDataBase;

}

;

和 的初始化vector，这是App在应用程序启动时的构造时完成的：

int DataSource::InitializeDataBase()
{
    myDataBase.clear();

    for (int i = 0; i < 50; i++) {
        DataSourceType x;
        wchar_t  buffer[256] = { 0 };

        swprintf_s(buffer, 255, L"Name for %d Item", i);
        x.name = buffer;
        swprintf_s(buffer, 255, L"Description %d. Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam.", i);
        x.description = buffer;
        myDataBase.push_back(x);
    }

    return 0;
}

XAML 页面背后的 MainPage.cpp 源代码是：

#include "pch.h"
#include "MainPage.h"
#include "DataSource.h"

using namespace winrt;
using namespace Windows::UI::Xaml;


namespace winrt::TouchExperiment_01::implementation
{
    MainPage::MainPage()
    {
        InitializeComponent();

        // load up the source ListView with the name field from out
        // in memory database.
        auto p = myList().Items();
        for (auto a : DataSource::myDataBase) {
            p.Append(box_value(a.name));
        }

        // add a single ListViewItem to the destination ListView so that we
        // know where it is.
        p = myList2().Items();
        p.Append(box_value(L"list"));
    }

    int32_t MainPage::MyProperty()
    {
        throw hresult_not_implemented();
    }

    void MainPage::MyProperty(int32_t /* value */)
    {
        throw hresult_not_implemented();
    }

    void MainPage::OnSelectionChanged(Windows::Foundation::IInspectable const & target, Windows::UI::Xaml::RoutedEventArgs const & )
    {
        // the user has selected a different item in the source ListView so we want to display
        // the associated description information for the selected ListViewItem.
        winrt::Windows::UI::Xaml::Controls::ListView p = target.try_as<winrt::Windows::UI::Xaml::Controls::ListView>();
        if (p) {
            int iIndex = p.SelectedIndex();
            myTextBlock().Text(DataSource::myDataBase[iIndex].description);
        }
    }

    void MainPage::OnListViewDragItemsStarting(Windows::Foundation::IInspectable const & target, Windows::UI::Xaml::Controls::DragItemsStartingEventArgs  const & e)
    {
        // provide the data that we have in the ListView which the user has selected
        // to drag to the other ListView. this is the data that will be copied from
        // the source ListView to the destination ListView.
        auto p = target.try_as<winrt::Windows::UI::Xaml::Controls::ListView>();
        if (p) {
            int iIndex = p.SelectedIndex();
            e.Items().SetAt(0, box_value(iIndex));
        }
    }

    void MainPage::OnListViewDragOver(Windows::Foundation::IInspectable const & target, Windows::UI::Xaml::DragEventArgs  const & e)
    {
        // indicate that we are Copy of data from one ListView to another rather than one of the other
        // operations such as Move. This provides the operation type informative user indicator when the
        // user is doing the drag operation.
        e.AcceptedOperation(Windows::ApplicationModel::DataTransfer::DataPackageOperation::Copy);
    }

    void MainPage::OnListViewDrop(Windows::Foundation::IInspectable const & target, Windows::UI::Xaml::DragEventArgs const & e)
    {
        // update the destination ListView with the data that was dragged from the
        // source ListView.
        auto p = target.try_as<winrt::Windows::UI::Xaml::Controls::ListView>();

        auto x = e.DataView().GetTextAsync();  // ** this line triggers exception on drop.
    }

}

在开始拖动之前在源中选择项目的应用程序的屏幕截图ListView如下所示。源ListView控件在左侧，目标ListView控件在右侧。

附录：参考资料和文档

Microsoft Docs - Windows.ApplicationModel.DataTransfer 命名空间

Microsoft Docs - DragItemsStartingEventArgs 类，其中包含指向此示例项目的链接，该示例项目看起来正在使用 C++/CX 将示例拖放到 GitHub 上，其中包含Windows-universal-samples/Samples/XamlDragAndDrop/cpp/Scenario1_ListView.xaml.cpp，它有一个有用的例子。

Answer 1

异常的原因是误用了该GetTextAsync()方法，该方法是一种异步方法，需要使用线程、任务、协程或其他一些并发功能。

我找到了示例源代码Windows-universal-samples/Samples/XamlDragAndDrop/cpp/Scenario1_ListView.xaml.cpp ，它提供了关于我做错了什么的提示。另请参阅https://github.com/Microsoft/cppwinrt/blob/master/Docs/Using%20Standard%20C%2B%2B%20types%20with%20C%2B%2B%20WinRT.md上的文章

    // We need to take a Deferral as we won't be able to confirm the end
    // of the operation synchronously
    auto def = e->GetDeferral();
    create_task(e->DataView->GetTextAsync()).then([def, this, e](String^ s)
    {
        // Parse the string to add items corresponding to each line
        auto wsText = s->Data();
        while (wsText) {
            auto wsNext = wcschr(wsText, L'\n');
            if (wsNext == nullptr)
            {
                // No more separator
                _selection->Append(ref new String(wsText));
                wsText = wsNext;
            }
            else
            {
                _selection->Append(ref new String(wsText, wsNext - wsText));
                wsText = wsNext + 1;
            }
        }

        e->AcceptedOperation = DataPackageOperation::Copy;
        def->Complete();
    });

为纠正问题所做的更改概述

我决定使用协程，GetTextAsync()因为我使用的是最新版本的 Visual Studio 2017 社区版。为此，需要对方法返回类型进行一些更改，void同时winrt::Windows::Foundation::IAsyncAction对解决方案属性进行一些更改，并添加一些包含文件以使协程更改能够正确编译和运行。

请参阅有关几种不同并发方法的答案和注释，以及 Visual Studio 2017 解决方案属性更改以在C++11 线程中co_await使用协程和运算符来更新 MFC 应用程序窗口。需要 SendMessage()、PostMessage() 吗？

在 MainPage.cpp 的顶部，我添加了以下两个包含指令：

#include <experimental\resumable>
#include <pplawait.h>

我将OnListViewDragItemsStarting()方法修改为：

void MainPage::OnListViewDragItemsStarting(Windows::Foundation::IInspectable const & target, Windows::UI::Xaml::Controls::DragItemsStartingEventArgs  const & e)
{
    // provide the data that we have in the ListView which the user has selected
    // to drag to the other ListView. this is the data that will be copied from
    // the source ListView to the destination ListView.
    auto p = target.try_as<winrt::Windows::UI::Xaml::Controls::ListView>();
    unsigned int n = e.Items().Size();
    if (p) {
        int iIndex = p.SelectedIndex();
        e.Data().Properties().Title(hstring (L"my Title"));
        e.Data().SetText(DataSource::myDataBase[iIndex].name.c_str());
        e.Data().RequestedOperation(winrt::Windows::ApplicationModel::DataTransfer::DataPackageOperation::Copy);
    }
}

最后我重写了OnListViewDrop()使用协程的方法如下（还要求类声明中声明的返回类型要更改为与新的返回类型一致）：

winrt::Windows::Foundation::IAsyncAction MainPage::OnListViewDrop(Windows::Foundation::IInspectable const & target, Windows::UI::Xaml::DragEventArgs const & e)
{
    // update the destination ListView with the data that was dragged from the
    // source ListView. the method GetTextAsync() is an asynch method so
    // we are using coroutines to get the result of the operation.

    // we need to capture the target ListView before doing the co_await
    // in a local variable so that we will know which ListView we are to update.
    auto p = target.try_as<winrt::Windows::UI::Xaml::Controls::ListView>();

    // do the GetTextAsync() and get the result by using coroutines.
    auto ss = co_await e.DataView().GetTextAsync();

    // update the ListView control that originally triggered this handler.
    p.Items().Append(box_value(ss));
}