WPF MVVM - 如何检测视图是否“脏”

Question

目前，如果视图上的任何字段已更改/更新，我需要通知我的应用程序用户。

例如，如果用户更改视图上的日期字段，然后尝试关闭视图，应用程序将显示一条消息，要求用户继续并放弃更改或取消，以便他们可以单击“保存”按钮。

问题是：如何检测视图中任何数据字段的更改？

希望这是有道理的，比你提前问候，

Answer 1

您可以采取的一种方法是利用 IChangeTracking< /code>和 INotifyPropertyChanged 接口。

如果您创建视图模型继承的抽象基类 (ViewModelBase)，该基类实现了 IChangeTracking 和 INotifyPropertyChanged 接口，那么您可以拥有自己的视图模型库附加到属性更改的通知（实际上表明视图模型已被修改），并且它将设置 IsChanged 属性为true 表示视图模型是“脏的”。

使用这种方法，您依靠通过数据绑定的属性更改通知来跟踪更改，并在进行任何提交后重置更改跟踪。

在您描述的情况下，您可以处理 已卸载或关闭视图事件以检查DataContext;如果 DataContext 实现了 IChangeTracking，您可以使用 IsChanged 属性来确定是否进行了任何未接受的更改。

简单示例：

/// <summary>
/// Provides a base class for objects that support property change notification 
/// and querying for changes and resetting of the changed status.
/// </summary>
public abstract class ViewModelBase : IChangeTracking, INotifyPropertyChanged
{
    //========================================================
    //  Constructors
    //========================================================
    #region ViewModelBase()
    /// <summary>
    /// Initializes a new instance of the <see cref="ViewModelBase"/> class.
    /// </summary>
    protected ViewModelBase()
    {
        this.PropertyChanged += new PropertyChangedEventHandler(OnNotifiedOfPropertyChanged);
    }
    #endregion

    //========================================================
    //  Private Methods
    //========================================================
    #region OnNotifiedOfPropertyChanged(object sender, PropertyChangedEventArgs e)
    /// <summary>
    /// Handles the <see cref="INotifyPropertyChanged.PropertyChanged"/> event for this object.
    /// </summary>
    /// <param name="sender">The source of the event.</param>
    /// <param name="e">A <see cref="PropertyChangedEventArgs"/> that contains the event data.</param>
    private void OnNotifiedOfPropertyChanged(object sender, PropertyChangedEventArgs e)
    {
        if (e != null && !String.Equals(e.PropertyName, "IsChanged", StringComparison.Ordinal))
        {
            this.IsChanged = true;
        }
    }
    #endregion

    //========================================================
    //  IChangeTracking Implementation
    //========================================================
    #region IsChanged
    /// <summary>
    /// Gets the object's changed status.
    /// </summary>
    /// <value>
    /// <see langword="true"/> if the object’s content has changed since the last call to <see cref="AcceptChanges()"/>; otherwise, <see langword="false"/>. 
    /// The initial value is <see langword="false"/>.
    /// </value>
    public bool IsChanged
    {
        get
        {
            lock (_notifyingObjectIsChangedSyncRoot)
            {
                return _notifyingObjectIsChanged;
            }
        }

        protected set
        {
            lock (_notifyingObjectIsChangedSyncRoot)
            {
                if (!Boolean.Equals(_notifyingObjectIsChanged, value))
                {
                    _notifyingObjectIsChanged = value;

                    this.OnPropertyChanged("IsChanged");
                }
            }
        }
    }
    private bool _notifyingObjectIsChanged;
    private readonly object _notifyingObjectIsChangedSyncRoot = new Object();
    #endregion

    #region AcceptChanges()
    /// <summary>
    /// Resets the object’s state to unchanged by accepting the modifications.
    /// </summary>
    public void AcceptChanges()
    {
        this.IsChanged = false;
    }
    #endregion

    //========================================================
    //  INotifyPropertyChanged Implementation
    //========================================================
    #region PropertyChanged
    /// <summary>
    /// Occurs when a property value changes.
    /// </summary>
    public event PropertyChangedEventHandler PropertyChanged;
    #endregion

    #region OnPropertyChanged(PropertyChangedEventArgs e)
    /// <summary>
    /// Raises the <see cref="INotifyPropertyChanged.PropertyChanged"/> event.
    /// </summary>
    /// <param name="e">A <see cref="PropertyChangedEventArgs"/> that provides data for the event.</param>
    protected void OnPropertyChanged(PropertyChangedEventArgs e)
    {
        var handler = this.PropertyChanged;
        if (handler != null)
        {
            handler(this, e);
        }
    }
    #endregion

    #region OnPropertyChanged(string propertyName)
    /// <summary>
    /// Raises the <see cref="INotifyPropertyChanged.PropertyChanged"/> event for the specified <paramref name="propertyName"/>.
    /// </summary>
    /// <param name="propertyName">The <see cref="MemberInfo.Name"/> of the property whose value has changed.</param>
    protected void OnPropertyChanged(string propertyName)
    {
        this.OnPropertyChanged(new PropertyChangedEventArgs(propertyName));
    }
    #endregion

    #region OnPropertyChanged(params string[] propertyNames)
    /// <summary>
    /// Raises the <see cref="INotifyPropertyChanged.PropertyChanged"/> event for the specified <paramref name="propertyNames"/>.
    /// </summary>
    /// <param name="propertyNames">An <see cref="Array"/> of <see cref="String"/> objects that contains the names of the properties whose values have changed.</param>
    /// <exception cref="ArgumentNullException">The <paramref name="propertyNames"/> is a <see langword="null"/> reference (Nothing in Visual Basic).</exception>
    protected void OnPropertyChanged(params string[] propertyNames)
    {
        if (propertyNames == null)
        {
            throw new ArgumentNullException("propertyNames");
        }

        foreach (var propertyName in propertyNames)
        {
            this.OnPropertyChanged(propertyName);
        }
    }
    #endregion
}

Answer 2

在 MVVM 中，视图绑定到视图模型，视图模型又绑定到模型。

视图不能是脏的，因为它的更改会立即反映到视图模型。

如果您希望仅在“确定”或“接受”时将更改应用于模型，
将视图绑定到不将更改应用于模型的视图模型，
直到执行ApplyCommand或AcceptCommand（您定义和实现的）。

（视图绑定到的命令由视图模型实现。）

示例 - VM：

public class MyVM : INotifyPropertyChanged
{
    public string MyText
    {
        get
        {
            return _MyText;
        }
        set
        {
            if (value == _MyText)
                return;

            _MyText = value;
            NotifyPropertyChanged("MyText");
        }
    }
    private string _MyText;

    public string MyTextTemp
    {
        get
        {
            return _MyTextTemp;
        }
        set
        {
            if (value == _MyTextTemp)
                return;

            _MyTextTemp = value;
            NotifyPropertyChanged("MyTextTemp");
            NotifyPropertyChanged("IsTextDirty");
        }
    }
    private string _MyTextTemp;

    public bool IsTextDirty
    {
        get
        {
            return MyText != MyTextTemp;
        }
    }

    public bool IsMyTextBeingEdited
    {
        get
        {
            return _IsMyTextBeingEdited;
        }
        set
        {
            if (value == _IsMyTextBeingEdited)
                return;

            _IsMyTextBeingEdited = value;

            if (!value)
            {
                MyText = MyTextTemp;
            }

            NotifyPropertyChanged("IsMyTextBeingEdited");
        }
    }
    private bool _IsMyTextBeingEdited;


    public event PropertyChangedEventHandler PropertyChanged;

    protected void NotifyPropertyChanged(string propertyName)
    {
        if (PropertyChanged != null)
            PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
    }
}

示例 - 视图：

    <Label Content="{Binding MyText}" />

    <!-- You can translate the events to commands by using a suitable framework -->
    <!-- or use code behind to update a new dependency property as in this example -->
    <TextBox
        LostFocus="TextBox_LostFocus"
        GotFocus="TextBox_GotFocus"
        Text="{Binding Path=MyTextTemp, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
        />

示例 - 视图 - 代码隐藏：

    public MainWindow()
    {
        InitializeComponent();

        SetBinding(IsTextBoxFocusedProperty,
            new Binding
            {
                Path = new PropertyPath("IsMyTextBeingEdited"),
                Mode = BindingMode.OneWayToSource,
            });
    }

    private void TextBox_LostFocus(object sender, RoutedEventArgs e)
    {
        IsTextBoxFocused = false;
    }

    private void TextBox_GotFocus(object sender, RoutedEventArgs e)
    {
        IsTextBoxFocused = true;
    }

    #region IsTextBoxFocused

    /// <summary>
    /// Gets or Sets IsTextBoxFocused
    /// </summary>
    public bool IsTextBoxFocused
    {
        get
        {
            return (bool)this.GetValue(IsTextBoxFocusedProperty);
        }
        set
        {
            this.SetValue(IsTextBoxFocusedProperty, value);
        }
    }

    /// <summary>
    /// The backing DependencyProperty behind IsTextBoxFocused
    /// </summary>
    public static readonly DependencyProperty IsTextBoxFocusedProperty = DependencyProperty.Register(
      "IsTextBoxFocused", typeof(bool), typeof(MainWindow), new PropertyMetadata(default(bool)));

    #endregion

Answer 3

想法：检查实体状态：

问题是这指的是整个视图，因此当在任何编辑之前选择新参与者（刷新表单）时，该值也是“已修改”。保存后，如果没有其他变化并且我们不切换参与者，则该值为“不变”