Add New Item to MultiSelect

Description

How to allow the user to add new items to the Multiselect, and select them automatically?

Solution

The example below shows how to add new items from outside the MultiSelect. It follows this algorithm:

1. Add the new item to the Data and/or the remote data source.
2. Select the new item by adding it to the Value collection. Let's assume the Value collection is List<int> SelectedItems
3. Rebind() the MultiSelect.
4. (Data scenarios only) Refresh the SelectedItems instance, so that the component can detect the update.
5. (OnRead scenarios only) Raise a boolean flag in the OnRead handler.
6. (OnRead scenarios only) Check the boolean flag in OnAfterRenderAsync and refresh the SelectedItems instance

There is also a related example in GitHub. It shows how to add new items from inside the MultiSelect dropdown.

Add and select new MultiSelect items

@using Telerik.DataSource
@using Telerik.DataSource.Extensions

<h1>Add and select new MultiSelect items on the fly</h1>

<p>
    <TelerikTextBox @bind-Value="@NewText" Width="200px" PlaceHolder="type new item" />
    <TelerikButton OnClick="@AddItem">Add and Select Item</TelerikButton>
</p>

<h2>Bind MultiSelect with Data parameter</h2>

<TelerikMultiSelect @ref="@MultiSelect1"
                    Data="@MultiSelectData"
                    @bind-Value="@SelectedItems"
                    TextField="@nameof(SampleModel.Text)"
                    ValueField="@nameof(SampleModel.ID)"
                    Width="300px" />

<h2>Bind MultiSelect with OnRead event</h2>

<TelerikMultiSelect @ref="@MultiSelect2"
                    TItem="@SampleModel"
                    TValue="@int"
                    OnRead="@OnReadHandler2"
                    @bind-Value="@SelectedItems"
                    TextField="@nameof(SampleModel.Text)"
                    ValueField="@nameof(SampleModel.ID)"
                    Width="300px" />

@code{
    TelerikMultiSelect<SampleModel, int> MultiSelect1 { get; set; }
    TelerikMultiSelect<SampleModel, int> MultiSelect2 { get; set; }

    List<int> SelectedItems { get; set; } = new List<int>() { 1 };

    bool ShouldRaiseFlag2 { get; set; } = true;
    bool ShouldRefreshSelectedItems2 { get; set; }

    string NewText { get; set; }

    async Task AddItem()
    {
        // simulate network delay
        await Task.Delay(100);

        // create new item instance
        int newId = MultiSelectData.Count + 1;
        SampleModel newItem = new SampleModel() { ID = newId, Text = $"{NewText} {newId}" };

        // add new item to data
        MultiSelectData.Add(newItem);

        // add new item to selection
        SelectedItems.Add(newId);

        // rebind MultiSelects
        MultiSelect1.Rebind();
        MultiSelect2.Rebind();

        // refresh selection instance for MultiSelect 1
        SelectedItems = new List<int>(SelectedItems);

        // flag for refresh selection instance for MultiSelect 2
        ShouldRaiseFlag2 = true;
    }

    async Task OnReadHandler2(MultiSelectReadEventArgs args)
    {
        // simulate network delay
        await Task.Delay(100);

        DataSourceResult result = MultiSelectData.ToDataSourceResult(args.Request);

        args.Data = result.Data;
        args.Total = result.Total;

        if (ShouldRaiseFlag2)
        {
            ShouldRaiseFlag2 = false;
            // flag for refresh selection instance for MultiSelect 2
            ShouldRefreshSelectedItems2 = true;
        }
    }

    protected override async Task OnAfterRenderAsync(bool firstRender)
    {
        // (OnRead binding only)
        // refresh selection instance for MultiSelect 2
        // works also for selecting initial items
        if (ShouldRefreshSelectedItems2)
        {
            ShouldRefreshSelectedItems2 = false;
            SelectedItems = new List<int>(SelectedItems);
            StateHasChanged();
        }
        await base.OnAfterRenderAsync(firstRender);
    }

    List<SampleModel> MultiSelectData { get; set; } = new List<SampleModel> {
            new SampleModel { ID = 1, Text = "foo" },
            new SampleModel { ID = 2, Text = "bar" },
            new SampleModel { ID = 3, Text = "baz" }
        };

    public class SampleModel
    {
        public int ID { get; set; }
        public string Text { get; set; }
    }
}