Selectors

Auto UI supports fields in which the user can pick the value from some list of options. Selection of a single item as well as a selection of multiple items are supported.

Pure selection

Define selection type

First, you need to define a type of a selection. This type works as a "marker" type for the collection of options from which it is selected, and also specifies the type of the selected value (most commonly int, Guid, string or enum).

To do that, define a record which inherits from Selection<TValue> where TValue is the type of the selected value.

You can define the selection like this:

public record CountrySelection : Selection<Guid>;

The Selection<TValue> base class has the following properties:

• TValue Value - the selected value
• string DisplayName - the text representation of the option

Apply the Selector attribute

Mark the property in the model object with the [Selector(typeof(TSelectionType))] attribute, or set the selector by the convention.

If the property is of a primitive type (string, numbers, date & time values, enums, Guid) or any of its nullable variants, the user will be able to select a single value in a ComboBox control.

If the property is a collection of primitive types (for example List<int>, ...), Auto UI will generate a list of CheckBox controls.

If there is a lot of items, the default controls may not be working well. You may consider implementing your own controls or finding third-party controls with search capabilites.

Implement a service to provide the options

For each selection type, you have to implement ISelectionProvider<TSelectionType> which will provide the options whenever Auto UI will need them.

public class CountrySelectionProvider : ISelectionProvider<CountrySelection>
{
    private readonly AppDbContext dbContext;

    public CountrySelectionProvider(AppDbContext dbContext)
    {
        this.dbContext = dbContext;
    }

    public Task<List<CountrySelection>> GetSelectorItems()
    {
        return dbContext.Countries
            .OrderBy(c => c.Name)
            .Select(c => new CountrySelection()
            {
                Value = c.Id,
                DisplayName = c.Name
            })
            .ToListAsync();
    }
}

This class needs to be registered in the service collection:

services.AddScoped<ISelectionProvider<CountrySelection>, CountrySelectionProvider>();

Add the child viewmodel on the target page

The last step is to add a property of type SelectionViewModel<TSelectionType> in the viewmodel of the page where the property with Selection attribute is used. It can be in the root page viewmodel or anywhere the binding context hierarchy - it just has to be a public get & set property and it must be initialized. How the property is named is not important - Auto UI will be looking at the type of the property.

public SelectionViewModel<CountrySelection> Countries { get; set; } = new();

Now, DotVVM will be able to obtain the list of options, and it will store it automatically in this child viewmodel.

Selections with parameters

Sometimes, the list of options depend on other values. This often appears if a cascading drop-down list scenarios or other more complex situations.

There is a second version of the interface ISelectionProvider<TSelectionType, TParameter> with an additional type parameter representing the parameters which are needed to provide the options (it can be either a primitive type or an object).

In such case, you need to perform the following steps:

• Define the selection type as specified in the Define selection type chapter.

public record LocationByCountrySelection : Selection<int>;

• Implement the ISelectionProvider<TSelectionType, TParameter> interface:

public class LocationByCountrySelectionProvider : ISelectionProvider<LocationByCountrySelection, int?>
{
    private readonly AppDbContext dbContext;

    public LocationByCountrySelectionProvider(AppDbContext dbContext)
    {
        this.dbContext = dbContext;
    }

    public Task<List<LocationByCountrySelection>> GetSelectorItems(int? countryId)
    {
        if (countryId == null)
        {
            return Task.FromResult(new List<LocationByCountrySelection>());
        }

        return dbContext.Locations
            .Where(l => l.CountryId == countryId)
            .OrderBy(c => c.Name)
            .Select(l => new LocationByCountrySelection()
            {
                Value = l.Id,
                DisplayName = l.Name
            })
            .ToListAsync();
    }
}

• Register the service in the service collection:

services.AddScoped<ISelectionProvider<LocationByCountrySelection, int?>, LocationByCountrySelectionProvider>();

• Add the property in the viewmodel and initialize it in the constructor. Pass a lambda function which will provide the value of the parameter:

public SelectionViewModel<LocationByCountrySelection, int?> Locations { get; set; }

// constructor
public MyViewModel() 
{
    Locations = new SelectionViewModel<LocationByCountrySelection, int?>(() => this.CountryId);
    // ...
}

• If the parameter value (in our case it's the CountryId property) changes, you can easily request the list of options to be refreshed by calling the RequestRefresh() method:

<auto:Form DataContext="{value: Meetup}" 
           Validation.Enabled="false"
           Changed-CountryId="{command: _root.Locations.RequestRefresh()}" />

See also

• Auto UI Overview
• Metadata
• Extensibility
• Form control