如何在 ListView 中选择一条记录并在代码隐藏方法中引用它? [英] How can I select a record in a ListView and reference it in the code behind method?

问题描述

我在 ListView 的 SQLite 数据库中列出了我的 ListItem 表中的记录，每条记录都是它自己的按钮，这会导致 EditItem 方法.我想要在这个方法中做的是将 ListItem 的布尔 Large 属性设置为 false，但是我在获取相应的ListItem 作为变量.

I'm listing records from my ListItem tables in my SQLite database in a ListView, and each record is its own button, which leads to an EditItem method. All I want to do in this method is set the boolean Large attribute for the ListItem to false, but I'm having trouble getting the corresponding ListItem as a variable.

使用我当前的代码，我得到的错误是:

With my current code, the error I'm getting is:

错误 CS0266 无法将类型对象"隐式转换为'Myapp.Models.ListItem'.存在显式转换(您是缺少演员表?)

Error CS0266 Cannot implicitly convert type 'object' to 'Myapp.Models.ListItem'. An explicit conversion exists (are you missing a cast?)

在我的代码后面获取和更新所选记录的正确方法是什么?

What's the correct way to get and update the selected record in my code behind?

我尝试在我的按钮上使用命令，现在我没有收到任何错误，但是甚至没有调用 EditItem 方法.我现在知道如何使用 Command 属性传递参数，但我不确定如何使用它调用方法.我知道如何使用 Clicked 属性调用方法，但我不知道如何使用它传递参数.

I tried using a Command with my button, and now I don't get any errors, but the EditItem method isn't even called. I now know how to pass parameters with the Command property, but I'm not sure how to call a method with it. I know how to call a method with the Clicked property, but I don't know how to pass parameters with it.

Xaml:

    <ListView>
        <ListView.ItemTemplate>
            <DataTemplate>
                <ViewCell>
                    <StackLayout>
                        <Button Text="{Binding Name}" Command="EditItem" CommandParameters="{Binding}" VerticalOptions="Start"/>
                    </StackLayout>
                </ViewCell>
            </DataTemplate>
        </ListView.ItemTemplate>
    </ListView>

背后的C#代码:

    public async void EditItem(object sender, EventArgs e)
    {
        ListItem item = (ItemList)sender as ItemList;
        item.Large = false;
        ...
    }

我在 Xamarin.forms 应用中使用 sqlite-net-pcl Nuget 包.

I'm using the sqlite-net-pcl Nuget package in a Xamarin.forms app.

推荐答案

Try ListItem item = (ListItem)e.SelectedItem;.

现在你正试图将一个通用的 object 分配给一个 ListItem 类型，你应该告诉它它是什么类型.

Right now you are trying to assign a generic object to a ListItem type, you should tell it what type it is.

您可能需要一些保护措施以确保它实际上是一个 ListItem 对象.即:

You probably want some safeguards in there to make sure it is actually a ListItem object. I.e.:

var selectedListItem = e.SelectedItem as ListItem;

if (selectedListItem == null)
{
    // TODO inform user or something
    return;
}

根据您的评论，我会根据您的要求进行详细说明.对于您想要执行的操作，最好使用 Button 上的 Command 和 CommandParameter.这里有一点需要注意.对于 Button 上的 Command，您可能希望从您的视图模型中执行代码.但是一个 ViewCell 内的范围是一个 ListItem 的范围.要确保在正确的位置搜索 Command，请为 Page 命名，例如:<ContentPage xmlns="..." x:Name="MyPage">.

In accordance with what you are looking for according to your comments, I will elaborate a bit more. For what you want to do, its best to use the Command and CommandParameter on the Button. There is one thing to note here. For the Command on the Button, you probably want to execute code from your view model. But the scope within one ViewCell is that of a ListItem. To make sure the Command is being searched for in the right place, give the Page a name, i.e. like this: <ContentPage xmlns="..." x:Name="MyPage">.

现在从 Button 开始:<Button Text="{Binding Name}" Command="{Binding Source={x:Reference MyPage}, Path=BindingContext.EditItem}" CommandParameters="{Binding .}" VerticalOptions="Start"/>.这告诉按钮将 Command 的绑定源设置为名称为 MyPage 的页面.然后将绑定的实际来源设置为 BindingContext.EditItem 属性.如果您不使用单独的视图模型而只是使用代码隐藏，请省略 BindingContext. 部分.

Now from the Button go like this: <Button Text="{Binding Name}" Command="{Binding Source={x:Reference MyPage}, Path=BindingContext.EditItem}" CommandParameters="{Binding .}" VerticalOptions="Start"/>. This tells the button to set the source of the binding for the Command to the page with name MyPage. The actual source of the binding is then set to the BindingContext.EditItem property. If you're not using a separate view model but just use the code-behind, omit the BindingContext. part.

您需要做的最后一件事是添加实际的Command.在您的视图模型中(或在代码隐藏中)添加一个属性: public Command EditItem { get;设置;} 并在构造函数中添加可执行代码或您想如何初始化它:

The last thing you need to do is add the actual Command. In your view model (or in the code-behind) add a property: public Command EditItem { get; set; } and add the executable code in the constructor or how else you would like to initialize it:

public MyPage()
{
    EditItem = new Command((parameter) => {
        ListItem item = (ItemList)parameter as ItemList;

        if (item == null)
            return; // TODO error handling

        item.Large = false;
    });
}

因为 Button 上的 BindingParameter 设置为 .，这意味着它绑定到后面的实际对象ViewCell，您将检索ListItem 作为参数.

Because of the BindingParameter on your Button, which is set to ., meaning that it is bound to the actual object that is behind that ViewCell, you will retrieve the ListItem as the parameter.

您可能希望将 Command 代码移动到一个单独的方法中以提高可读性.希望这是有道理的.

You probably want to move the Command code into a separate method for readability. Hope this makes sense.

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