Xamarin Picker绑定

Question

是我错过了什么，还是有更多我没有得到的东西。我正在开发一个移动应用程序，必须使用选择器从数据表中进行选择。首先，我有很多这样的基于键/值的选择器。我有一个内部ID和一个对应的Show值。ID并不总是具有1、2、3个值，例如源自查找表，并且可能具有以下内容

KeyID / ShowValue
27 = Another Thing
55 = Many More
12 = Some Item

像这样简单地检索

select * from LookupTable where Category = 'demo'

下面有一个类，用于通过记录列表绑定选择器

public class CboIntKeyValue
{
    public int KeyID { get; set; } = 0;
    public string ShowValue { get; set; } = "";
}

现在，我试图绑定到的数据记录只有ID列与查找相关联。而不会被xaml埋没，但总的来说，我有我的ViewModel。在这上面，我有一个数据记录的实例，它有ID列。

public class MyViewModel : BindableObject
{
    public MyViewModel()
    {
        // Sample to pre-load list of records from data server of KVP
        PickerChoices = GetDataFromServerForDemo( "select * from LookupTable where Category = 'demo'" );


        ShowThisRecord = new MyDataRec();
        // for grins, I am setting the value that SHOULD be defaulted 
        // in picker.  In this case, ID = 12 = "Some Item" from above
        ShowThisRecord.MyID = 12;   
    }

    // this is the record that has the "ID" column I am trying to bind to
    public MyDataRec ShowThisRecord {get; set;}

    // The picker is bound to this list of possible choices
    public List<CboIntKeyValue> PickerChoices {get; set;}
}

我不能绑定到列表的索引，因为当我期望相应的"ID“作为列表中正确记录的基础时，它会给我0，1，2。

在过去的WPF中，我可以声明屏幕的SHOW值，也可以将绑定值声明为ID列。因此，我的"ShowThisRecord“上的INT属性的绑定将驱动并正确地刷新。

我可以看到SelectedItem的绑定，但这是KVP类的全部内容，它不是MyDataRec的一部分。只有ID是它们之间的公共元素。

有没有人能帮我找到合适的绑定，让它正常工作。

<Picker ItemDisplayBinding="{Binding ShowValue}" 
   SelectedItem="{Binding ???}" />

为了确认我的记录绑定是合法的，我的页面有绑定到MyViewModel的上下文，因为我可以通过添加到页面的示例文本条目正确地看到ID。

<Entry Text="{Binding Path=ShowThisRecord.MyID}"/>

谢谢

Answer 1

经过大量工作，我最终编写了自己的独立类和模板样式，以满足我的需求。我在CodeProject上发布了源代码，供任何人使用、审查和评估。

https://www.codeproject.com/Articles/5266341/Xamarin-Picker-with-ID-based-Binding

希望这对其他有类似问题的人有所帮助。同样，我遇到的主要问题是，如果我有一个来自数据源的整型键ID，则选择器不会仅根据给定的ID (int或string)自动刷新。

Answer 2

我创建了一个演示来测试你的代码，它的工作properly.The完整的演示是here。我还添加了一个函数来验证所选的项。

如果您想同步获取SelectedItem对象，MyViewModel应该是implement INotifyPropertyChanged，我为SelectedItem创建了一个selectedRecord字段，所以您可以这样做：

  public class MyViewModel :ViewModelBase
{
    public MyViewModel()
    {
        // Sample to pre-load list of records from data server of KVP
        //PickerChoices = GetDataFromServerForDemo("select * from LookupTable where Category = 'demo'");
       PickerChoices = new ObservableCollection<TestModel>() {
            new TestModel{ MyID = 5,ShowValue="test1" }, new TestModel{ MyID = 9,ShowValue="test2" },
            new TestModel{ MyID = 18,ShowValue="test18" }, new TestModel{ MyID = 34,ShowValue="test4" }
        };
       // set the default selected item 
       // foreach (TestModel model in PickerChoices) {
       //     if (model.MyID == 18) {// default value
       //         SelectedRecord = model;
       //         break;
       //     }
       // }


        ShowThisRecord = new TestModel();
        // for grins, I am setting the value that SHOULD be defaulted 
        // in picker.  In this case, ID = 12 = "Some Item" from above
        ShowThisRecord.MyID = 12;
    }

    // this is the record that has the "ID" column I am trying to bind to
    public TestModel ShowThisRecord { get; set; }

    //*****************************************

    TestModel selectedRecord; //selected item object
    public TestModel SelectedRecord
    {
        get { return selectedRecord; }
        set
        {
            if (selectedRecord != value)
            {
                selectedRecord = value;
                OnPropertyChanged();
            }
        }
    }

    //*****************************************

    // The picker is bound to this list of possible choices
    public ObservableCollection<TestModel> PickerChoices { get; set; }
}

ViewModelBase类

 public  class ViewModelBase: INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
    {
        PropertyChangedEventHandler handler = PropertyChanged;
        if (handler != null)
        {
            handler(this, new PropertyChangedEventArgs(propertyName));
        }
    }
}

和xaml

 <Picker  Title="Select a value" x:Name="mypicker"
    ItemsSource="{Binding Path= PickerChoices}"
    SelectedItem="{Binding SelectedRecord}"         
    ItemDisplayBinding="{Binding MyID}"/>

the xaml.cs

 public partial class MainPage : ContentPage
{

    ObservableCollection<TestModel> items = new ObservableCollection<TestModel>();
    MyViewModel testModel = null;
    public MainPage()
    {
        InitializeComponent();

        testModel = new MyViewModel();
        BindingContext = testModel;

        //This will also work
        //if (testModel!=null && testModel.PickerChoices!=null) {
        //  for (int index=0;index< testModel.PickerChoices.Count;index++ ) {
        //        TestModel temp = testModel.PickerChoices[index];
        //    if (18 == temp.MyID) {
        //            mypicker.SelectedIndex = index;
        //            break;
        //     }
        //   }
        //}

        foreach (TestModel model in testModel.PickerChoices)
        {
            if (model.MyID == 18)
            {// default value
                testModel.SelectedRecord = model;
                break;
            }
        }
    }


    // to show the selected item
    private void Button_Clicked(object sender, EventArgs e)
    {
        if (testModel.SelectedRecord!=null) {
        DisplayAlert("Alert", "selected Item  MyID : " + testModel.SelectedRecord.MyID + "<--> ShowValue: " + testModel.SelectedRecord.ShowValue, "OK");
        }
    }

}

结果是：

​

​

Answer 3

您需要将ItemsSource属性设置为您的CboIntValue项目列表：

<Picker Title="Select a value"
        ItemsSource="{Binding PickerChoices}"
        ItemDisplayBinding="{Binding ShowValue}" />