Question

问题：

我正在尝试创建似乎是简单的MVVM视图设置。但是，无论我进行什么修改，我似乎都无法使PropertyChanged连接连接到.xaml，反之亦然。

这是视图：

VpicInformationPage.xaml

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:viewModels="clr-namespace:ScanditBarcodeScanner.ViewModels"
             x:Class="ScanditBarcodeScanner.Pages.VehicleOperationPages.VpicInformationPage">
    <!--<ContentPage.BindingContext>
        <viewModels:VpicInformationPageViewModel />
    </ContentPage.BindingContext>-->
    <StackLayout VerticalOptions="CenterAndExpand" Padding="5">
        <StackLayout.BindingContext>
            <viewModels:VpicInformationPageViewModel />
        </StackLayout.BindingContext>
        <Entry x:Name="VinEntry" Placeholder="VIN (Manual Entry)" />
        <Label Text="{Binding VinType.Make}" />
        <Label Text="{Binding VinType.Model}" />
        <Label Text="{Binding VinType.ModelYear}" />
        <Label Text="{Binding VinType.BodyClass}" />
        <Label Text="{Binding VinType.ErrorCode}" />
        <Button Text="Scan/Check VIN" Clicked="ScanOrCheckVin_Clicked"/>
    </StackLayout>
</ContentPage>

模型：

VpicInformationPage.xaml.cs

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Linq;
using Scandit.BarcodePicker.Unified;
using Scandit.BarcodePicker.Unified.Abstractions;
using ScanditBarcodeScanner.ViewModels;
using ScanditBarcodeScanner.ViewModels.Base;
using Xamarin.Forms;
using Xamarin.Forms.Xaml;

namespace ScanditBarcodeScanner.Pages.VehicleOperationPages
{
    [XamlCompilation(XamlCompilationOptions.Compile)]
    public partial class VpicInformationPage : ContentPage, INotifyPropertyChanged
    {
        IBarcodePicker _picker;

        VpicInformationPageViewModel ViewModel;
        public VpicInformationPage()
        {
            InitializeComponent ();

            ViewModel = BindingContext as VpicInformationPageViewModel;

            ViewModel.VinType = VehicleApi.EmptyVinType;

            _picker = ScanditService.BarcodePicker;

            SetVinSettings();

            _picker.DidScan += OnDidScan;

            VinEntry.Text = "";
        }
    //...
    }
}

ViewModel：

VpicInformationPageViewModel.cs

using ScanditBarcodeScanner.ViewModels.Base;
using System.ComponentModel;

namespace ScanditBarcodeScanner.ViewModels
{
    public class VpicInformationPageViewModel : ViewModelBase
    {
        #region VinType
        private VehicleApi.VinType _vinType;

        public VehicleApi.VinType VinType
        {
            get { return _vinType; }
            set
            {
                SetValue(ref _vinType, value, "VinType");
                Make = _vinType.Make;
                Model = _vinType.Model;
                ModelYear = _vinType.ModelYear;
                BodyClass = _vinType.BodyClass;
                ErrorCode = _vinType.ErrorCode;
            }
        }
        #endregion VinType

        #region VinType.Make
        private string _make;
        public string Make
        {
            get { return _vinType.Make; }
            private set
            {
                SetValue(ref _make, value);
            }
        }
        #endregion VinType.Make

        #region VinType.Model
        private string _model;

        public string Model
        {
            get { return _vinType.Model; }
            private set
            {
                SetValue(ref _model, value);
            }
        }
        #endregion VinType.Model

        #region VinType.ModelYear
        private string _modelYear;

        public string ModelYear
        {
            get { return _vinType.ModelYear; }
            private set
            {
                SetValue(ref _modelYear, value);
            }
        }
        #endregion VinType.ModelYear

        #region VinType.BodyClass
        private string _bodyClass;

        public string BodyClass
        {
            get { return _vinType.BodyClass; }
            private set
            {
                SetValue(ref _bodyClass, value);
            }
        }
        #endregion VinType.BodyClass

        #region VinType.ErrorCode
        private string _errorCode;

        public string ErrorCode
        {
            get { return _vinType.ErrorCode; }
            private set
            {
                SetValue(ref _errorCode, value);
            }
        }
        #endregion VinType.ErrorCode

        public VpicInformationPageViewModel()
        {
            _vinType = new VehicleApi.VinType();
        }
    }
}

ViewModelBase：

ViewModelBase.cs

using System.Collections.Generic;
using System.ComponentModel;
using System.Runtime.CompilerServices;

namespace ScanditBarcodeScanner.ViewModels.Base
{
    public class ViewModelBase : INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged;

        protected void OnPropertyChanged([CallerMemberName] string PropertyName = null)
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(PropertyName));
        }

        protected bool SetValue<T>(ref T BackingField, T Value, [CallerMemberName] string PropertyName = null)
        {
            if(EqualityComparer<T>.Default.Equals(BackingField, Value))
            {
                return false;
            }
            BackingField = Value;
            OnPropertyChanged(PropertyName);
            return true;
        }
    }

我意识到我没有加入VehicleApi类。这是该课程的重要部分：

VehicleApi.cs

namespace ScanditBarcodeScanner
{
    public static class VehicleApi
    {
        public static VinType EmptyVinType { get; } = new VinType
        {
            Make = "Make",
            Model = "Model",
            ModelYear = "Model Year",
            BodyClass = "Body Class",
            ErrorCode = "Status/Error Code"
        };

        public class VinType
        {
            public string Make { get; set; }
            public string Model { get; set; }
            public string ModelYear { get; set; }
            public string BodyClass { get; set; }
            public string ErrorCode { get; set; }
        }
}

据我了解，我已经正确实现了这些文件并将它们正确链接在一起。但是，每次运行该应用程序时，我都会得到：

[0:] Binding: 'VinType' property not found on 'ScanditBarcodeScanner.ViewModels.VpicInformationPageViewModel', target property: 'Xamarin.Forms.Label.Text'

我已将View绑定到ViewModel，并实现了INotifyPropertyChanged。

我尝试了许多解决方案，例如更改设置绑定上下文的位置（在ContentPage或StackLayout中，甚至在这两者中），尝试使用各种方法通知视图属性已更改，以及将标签绑定到VinType的基础成员，并允许VinType为其修改和引发PropertyChanged。我什至尝试过PropertyChanged.Fody，但是似乎问题不在于通知代码，而在于我将View和ViewModel绑定在一起的方式，或者也许是如何定义属性的。

问题：

绑定联播的哪一部分丢失/不正确？该文档指出，我应该能够使用我提供的代码访问VinType及其成员。

Link to sample

上述项目的问题在于，尽管它没有给出我正在谈论的相同错误，但在预期的情况下仍不会更改字段。

Answer 1

因此，解决此问题的方法实际上很简单。

我只需要更改标签定义 <Label Text="{Binding PropertyName.Member}" />至<Label Text="{Binding PropertyName.Member, StringFormat='{}{0}'}">

在格式字符串中需要使用{}的原因是由于在定义StringFormat时使用单引号''而不是双引号""时会出现错误。

编辑：尽管StringFormat问题很重要，但更大的问题是我使用的对象中的属性未定义get或set。只要类型符合以下内容：

public class TypeName
{
    public Type Property1 { get; set; }
    public Type Property2 { get; set; }
}

然后，绑定引擎将能够查看和编辑成员属性。

数据绑定看不到视图模型属性

1 个答案: