Xamarin：Picker标题不会以编程方式更改

Question

我正在使用Ca，Xamarin编写的iOS应用程序。我在MVVM体系结构中使用了选择器，并希望更改选择器标题。但是当我使用OnAddMaterial更改Pickers标题时，标题不会更改。

The ViewModel：

private void OnAddNewMaterial()
        {
            SelectedMaterialIndex = -1;
            MaterialPickerTitle = "New Material";
        }

    private string _materialPickerTitle { get; set; }
            public string MaterialPickerTitle
            {
                get { return _materialPickerTitle; }
                set
                {
                    _materialPickerTitle = value;
                    OnPropertyChanged();

                }
            }

视图：

<Picker Title="{Binding MaterialPickerTitle}" Margin="12,4,4,4" Grid.Row="0" Grid.Column="0" HorizontalOptions="FillAndExpand" ItemsSource="{Binding Materials}" ItemDisplayBinding="{Binding Name}" SelectedItem="{Binding SelectedMaterial}" SelectedIndex="{Binding SelectedMaterialIndex}" />

我使用Visual Studio 2019。

编辑：当我初始化视图时，我从Picker中设置标题。效果很好。之后，我从Picker将对象分配给ItemSource。当我尝试设置选择器标题之后，它不起作用。

Answer 1

我编写了一个演示，在更改selectedItem之后可以更改Picker的标题。这是您可以参考的代码：

public partial class MainPage : ContentPage
{
    List<string> monkeyList = new List<string>();

    TestModel model = new TestModel();
    public MainPage()
    {
        InitializeComponent();

        monkeyList.Add("Baboon");
        monkeyList.Add("Capuchin Monkey");
        monkeyList.Add("Blue Monkey");
        monkeyList.Add("Squirrel Monkey");
        monkeyList.Add("Golden Lion Tamarin");
        monkeyList.Add("Howler Monkey");
        monkeyList.Add("Japanese Macaque");

        picker.ItemsSource = monkeyList;

        model.MaterialPickerTitle = "123";
        model.SelectedMaterialIndex = 2;
        BindingContext = model;
    }

    private void Button_Clicked(object sender, EventArgs e)
    {
        monkeyList.Add("Baboonww");

        model.SelectedMaterialIndex = -1;
        model.MaterialPickerTitle = "456";

    }
}

class TestModel : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    public TestModel()
    {

    }

    string materialPickerTitle;
    public string MaterialPickerTitle
    {
        set
        {
            if (materialPickerTitle != value)
            {
                materialPickerTitle = value;

                if (PropertyChanged != null)
                {
                    PropertyChanged(this, new PropertyChangedEventArgs("MaterialPickerTitle"));
                }
            }
        }
        get
        {
            return materialPickerTitle;
        }
    }

    int selectedMaterialIndex;
    public int SelectedMaterialIndex
    {
        set
        {
            if (selectedMaterialIndex != value)
            {
                selectedMaterialIndex = value;

                if (PropertyChanged != null)
                {
                    PropertyChanged(this, new PropertyChangedEventArgs("SelectedMaterialIndex"));
                }
            }
        }
        get
        {
            return selectedMaterialIndex;
        }
    }
}

在xaml中：

<StackLayout>
    <!-- Place new controls here -->
    <Button Clicked="Button_Clicked" Text="click to change title" 
       HorizontalOptions="Center"
       VerticalOptions="CenterAndExpand" />

    <Picker x:Name="picker"
    Title="{Binding MaterialPickerTitle}"
    SelectedIndex="{Binding SelectedMaterialIndex}"
    TitleColor="Red">
    </Picker>
</StackLayout>

请检查您在项目中的绑定。如果标题更改，请添加一些断点以进行调试。我还上传了示例项目here。