c# - ObservesProperty 方法不在 Prism 6 上观察模型的属性

Question

我正在尝试学习 Prism MVVM，我正在制作一个包含 2 个字段和一个按钮的窗口，当这两个字段不为空时该按钮将启用。

问题是我找不到使方法 ObservesProperty() 对对象(在那种情况下为 Pessoa)起作用的方法。 CanExecuteAtualizar() 方法仅在应用程序启动时被调用，当我编辑文本字段 Nome 或 Sobrenome 时，按钮和方法未被触发...

我尝试在没有模型的情况下工作，将 Nome、Sobrenome 和 UltimaAtualizacao 属性直接放在 ViewModel 中，它工作正常，禁用根据方法 CanExecuteAtualizar 返回的按钮，但我想将它与模型一起使用。有办法做到这一点吗？

ViewAViewModel.cs

public class ViewAViewModel : BindableBase
{
    private Pessoa _pessoa;

    public Pessoa Pessoa
    {
        get { return _pessoa; }
        set { SetProperty(ref _pessoa, value); }
    }

    public ICommand CommandAtualizar { get; set; }

    public ViewAViewModel()
    {
        Pessoa = new Pessoa();
        Pessoa.Nome = "Gabriel";
        CommandAtualizar = new DelegateCommand(ExecuteAtualizar, CanExecuteAtualizar).ObservesProperty(() => Pessoa.Nome).ObservesProperty(() => Pessoa.Sobrenome);
    }

    public bool CanExecuteAtualizar()
    {
        return !string.IsNullOrWhiteSpace(Pessoa.Nome) && !string.IsNullOrWhiteSpace(Pessoa.Sobrenome);
    }

    public void ExecuteAtualizar()
    {
        Pessoa.UltimaAtualizacao = DateTime.Now;
    }
}

Pessoa.cs

public class Pessoa : BindableBase
{
    private string _nome;

    public string Nome
    {
        get { return _nome; }
        set { SetProperty(ref _nome, value); }
    }

    private string _sobrenome;

    public string Sobrenome
    {
        get { return _sobrenome; }
        set { SetProperty(ref _sobrenome, value); }
    }

    private DateTime? _ultimaAtualizacao;

    public DateTime? UltimaAtualizacao
    {
        get { return _ultimaAtualizacao; }
        set { SetProperty(ref _ultimaAtualizacao, value); }
    }
}

ViewA.xaml

<UserControl x:Class="PrismDemo.Views.ViewA"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
             xmlns:local="clr-namespace:PrismDemo.Views"
             mc:Ignorable="d"
             d:DesignHeight="100" d:DesignWidth="500">
    <Grid Background="White">
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="*" />
            <ColumnDefinition Width="2*" />
        </Grid.ColumnDefinitions>
        <Grid.RowDefinitions>
            <RowDefinition Height="*" />
            <RowDefinition Height="*" />
            <RowDefinition Height="*" />
            <RowDefinition Height="*" />
        </Grid.RowDefinitions>

        <Label Content="Nome:"  Grid.Column="0" Grid.Row="0" HorizontalAlignment="Left" VerticalAlignment="Center" />
        <TextBox Grid.Column="1" Grid.Row="0"  Margin="3" TabIndex="0" Text="{Binding Pessoa.Nome, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" />
        <Label Content="Sobrenome:"  Grid.Column="0" Grid.Row="1" HorizontalAlignment="Left" VerticalAlignment="Center" />
        <TextBox Grid.Column="1" Grid.Row="1"  Margin="3" TabIndex="1" Text="{Binding Pessoa.Sobrenome, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" />
        <Label Content="Última atualização:"  Grid.Column="0" Grid.Row="2" HorizontalAlignment="Left" VerticalAlignment="Center" />
        <Label Grid.Column="1" Grid.Row="2"  Margin="3" HorizontalAlignment="Left" Content="{Binding Pessoa.UltimaAtualizacao, Mode=TwoWay}" />
        <Button Content="Atualizar" Grid.Column="1" Grid.Row="3" Width="70" Margin="2,2,3,2" HorizontalAlignment="Right" Command="{Binding CommandAtualizar}" />
    </Grid>
</UserControl>

Answer 1

DelegateCommand.ObservesPropery 不支持复杂的对象属性。它只支持命令中定义的 ViewModel 上存在的属性。这是因为复杂对象的生命周期是未知的，如果创建了很多对象实例就会造成内存泄漏。我的建议是像这样定义您的属性:

private Pessoa _pessoa;
public Pessoa Pessoa
{
    get { return _pessoa; }
    set 
    {
        if (_pessoa != null)
            _pessoa.PropertyChanged -= PropertyChanged; 

        SetProperty(ref _pessoa, value);


        if (_pessoa != null)
            _pessoa.PropertyChanged += PropertyChanged;
    }
}

然后在PropertyChanged方法中，调用DelegateCommand.RaiseCanExecuteChanged

编辑:Prism for Xamarin.Forms 7.0 现已提供复杂属性支持。