如何使用Prism/DryIoC解决Xamarin.Forms中IValueConverter中的依赖项

问题描述

我有一个Xamarin.Forms应用程序，该应用程序使用Prism和DryIoC作为容器.我有一个值转换器，需要使用通过IContainerRegistry注册的服务.

I have a Xamarin.Forms app that uses Prism and DryIoC as the container. I have a value converter where I need to make use of a service I have registered via IContainerRegistry.

containerRegistry.RegisterSingleton<IUserService, UserService>();

由于IValueConverter是通过XAML而不是DryIoC构造的，因此该如何解决这种依赖性而不必诉诸构造函数注入?我可以在Prism/DryIoC中使用服务定位器吗?如果可以，怎么办?

How do I resolve that dependency without having to resort to constructor injection since IValueConverter gets constructed by XAML and not by DryIoC? Can I use a Service Locator in Prism/DryIoC? And if so, how?

下面是值转换器代码:

public class MyValueConverter : IValueConverter
{
    private readonly IUserService _userService;

    public MyValueConverter()
    {
        // Ideally, I can use a service locator here to resolve IUserService
        //_userService = GetContainer().Resolve<IUserService>();
    }

    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        var isUserLoggedIn = _userService.IsLoggedIn;
        if (isUserLoggedIn)
            // Do some conversion
        else
            // Do some other conversion
        ...
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

推荐答案

我建议您更新到7.1预览版，因为它可以解决此确切问题.您的转换器只是这样:

I would encourage you to update to the 7.1 preview as it solves this exact issue. Your converter would simply be like:

public class MyValueConverter : IValueConverter
{
    private readonly IUserService _userService;

    public MyValueConverter(IUserService userService)
    {
        _userService = userService;
    }

    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        var isUserLoggedIn = _userService.IsLoggedIn;
        if (isUserLoggedIn)
            // Do some conversion
        else
            // Do some other conversion
        ...
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

您的XAML如下所示:

Your XAML then would look something like:

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms" 
            xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" 
            xmlns:converters="clr-namespace:DemoApp.Converters"
            xmlns:ioc="clr-namespace:Prism.Ioc;assembly=Prism.Forms"
            x:Class="DemoApp.Views.AwesomePage">
    <ContentPage.Resources>
        <ResourceDictionary>
            <ioc:ContainerProvider x:TypeArguments="converters:MyValueConverter"
                                   x:Key="myValueConverter" />
        </ResourceDictionary>
    </ContentPage.Resources>
</ContentPage>

请务必先查看发行说明尽管正在更新，因为该版本还包含一些重大更改.

Be sure to check out the release notes before updating though, as the release also contains some breaking changes.

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