无法更改按钮的 IsEnabled 状态

Question

我已经尝试了无数种方法到星期天来让它工作，但我显然缺少一些东西，因为我的“Register”按钮不是无论我做什么都会启用。

在我的 XAML 中：

<Button Text="Register" 
        Style="{StaticResource RegularButtonStyle}" 
        WidthRequest="280"
        x:Name="RegisterButton">
    <!-- I HAD the following, but it wasn't working so I finally decided to try DataTriggers -->
    <!-- bindings:Bi.nd="Clicked RegisterButtonClickedCommand;IsEnabled IsRegisterButtonEnabled"> -->
    <Button.Triggers>
        <DataTrigger TargetType="Button"
                     Binding="{Binding IsRegisterButtonEnabled}"
                     Value="False">
            <Setter Property="IsEnabled" Value="False"></Setter>
        </DataTrigger>
        <DataTrigger TargetType="Button"
                     Binding="{Binding IsRegisterButtonEnabled}"
                     Value="True">
            <Setter Property="IsEnabled" Value="True"></Setter>
        </DataTrigger>
    </Button.Triggers>
</Button>

在 ViewModel 中：

public class RegisterViewModel : MvxViewModel
{
    ...

    private bool _isRegisterButtonEnabled;
    public bool IsRegisterButtonEnabled
    {
        get => ShouldEnableRegisterButton();

        set
        {
            _isRegisterButtonEnabled = value;
            SetProperty(ref _isRegisterButtonEnabled, value);
        }
    }

    ...
    public IMvxCommand RegisterButtonClickedCommand { get; private set; }
    ...

    private void InitializeCommands()
    {
        ...
        RegisterButtonClickedCommand = new MvxCommand(RegisterUser);
    }

    ...

    private bool ShouldEnableRegisterButton()
    {
        var isValidUser = _userName.Validate();
        var isValidPass = _password.Validate();
        var isValidConfirmedPass = _confirmedPassword.Validate();

        var shouldEnable = isValidUser && isValidPass && isValidConfirmedPass;

        _mvxLogger.Log(MvxLogLevel.Trace, () => $"RegisterViewModel : ShouldEnableRegisterButton() called. Returning: {shouldEnable}");

        return shouldEnable;
    }

    ...
}

我已经阅读了 MvvmCross“文档”，但它都是对话式的，我找不到任何特定的绑定到按钮的 IsEnabled 属性的示例，并且具有足够的特异性让我到达那里。

当然会感谢一些帮助。 :)

家政信息： 我正在使用以下 NuGet 包 (/libraries)

• MvvmCross & MvvmCross.Forms v6.3.1
• Xamarin.Forms v4.0.0.497661
• Xamarin.Android.* 库都是 v28.0.0.1

然后想我应该更新，因为有更新可用，所以我更新到以下；

• MvvmCross & MvvmCross.Forms v6.4.1
• Xamarin.Forms v4.2.0.848062
• Xamarin.Android.* 库现在都是 v28.0.0.3

还使用 .Net Standard v2.0.3 来共享内容。

Answer 1

你可以试试这个：

public class RegisterViewModel : INotifyPropertyChanged
{

    public event PropertyChangedEventHandler PropertyChanged;
    private bool _isRegisterButtonEnabled;

    public bool IsRegisterButtonEnabled
    {
        set { _isRegisterButtonEnabled = value; PropertyChanged?.Invoke(this, new 
        PropertyChangedEventArgs(nameof(IsRegisterButtonEnabled))); }
        get { return _isRegisterButtonEnabled; }
    }
}

然后在 Xaml 中

<Button Text="Register" 
        Style="{StaticResource RegularButtonStyle}" 
        IsEnabled = "{Binding IsRegisterButtonEnabled}"
        WidthRequest="280"
        x:Name="RegisterButton">
</Button>

Answer 2

最终的答案原来是多个问题的组合，其中一个大问题可能需要在 SO 上发布新的帖子/问题。

所以这就是最终的工作；

我将 Xaml 更新为以下内容；

<Button Text="Register" 
        Style="{StaticResource RegularButtonStyle}" 
        WidthRequest="280"
        x:Name="RegisterButton"
        Command="{Binding RegisterButtonClickedCommand}">
    <Button.Triggers>
        <DataTrigger TargetType="Button"
                     Binding="{Binding IsRegisterButtonEnabled}"
                     Value="False">
            <Setter Property="IsEnabled" Value="False"></Setter>
        </DataTrigger>
        <DataTrigger TargetType="Button"
                     Binding="{Binding IsRegisterButtonEnabled}"
                     Value="True">
            <Setter Property="IsEnabled" Value="True"></Setter>
        </DataTrigger>
    </Button.Triggers>
</Button>

还将 RegisterViewModel.cs 更新为以下内容；

public class RegisterViewModel : MvxViewModel, INotifyPropertyChanged
{
    ...

    // Added this (and the inheriting from INotify.. [per @Adlorem]
    public event PropertyChangedEventHandler PropertyChanged;

    private bool _isRegisterButtonEnabled;
    public bool IsRegisterButtonEnabled
    {
        get
        {
            _mvxLogger.Log(MvxLogLevel.Trace,
                () => $"RegisterViewModel : IsRegisterButtonEnabled property get called. Returning: {_isRegisterButtonEnabled}");

            return _isRegisterButtonEnabled;
        } 

        set
        {
            _isRegisterButtonEnabled = value;

            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(IsRegisterButtonEnabled)));

            _mvxLogger.Log(MvxLogLevel.Trace,
                () => $"RegisterViewModel : IsRegisterButtonEnabled property set called. value: {value}");

        }
    }

    ...

    private bool ShouldEnableRegisterButton()
    {
        var isValidUser = _userName.Validate();
        var isValidPass = _password.Validate();
        var isValidConfirmedPass = _confirmedPassword.Validate();

        var shouldEnable = isValidUser && isValidPass && isValidConfirmedPass;

        // Must set this or the PropertyChanged definitely won't fire!
        IsRegisterButtonEnabled = shouldEnable;

        _mvxLogger.Log(MvxLogLevel.Trace,
            () => $"RegisterViewModel : ShouldEnableRegisterButton() called. Returning: {shouldEnable}");

        return shouldEnable;
    }

    ...
}

显然这不是最优的。 MvvmCross 的 SetProperty() 应该触发 PropertyChanged 事件，但由于某种原因它没有。