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Change DropDown UI Programmatically

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DropDownList
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Serge
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Serge asked on 03 Nov 2020, 04:48 PM
Is there a way to change the bound property value and have the dropdown reflect that change in the UI?

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Marin Bratanov
Telerik team
answered on 03 Nov 2020, 07:12 PM

Hello Serge,

Here's an example that explains the way this will operate and the framework behaviors that affect it:

 

@page "/"

@selectedValue

<br />
<button @onclick="@ChangeValue">1. change just value</button>

<button @onclick="@ChangeText">2. change just text - will not "work"</button>

<button @onclick="@ChangeEntireParameterRef">3. change entire parameter reference</button>

<br /><br />
<TelerikDropDownList Data="@myDdlData" TextField="MyTextField" ValueField="MyValueField" @bind-Value="@selectedValue">
</TelerikDropDownList>

@code {
    void ChangeValue()
    {
        //works because it changes the value of a primitive type parameter
        selectedValue = 4;
    }

    void ChangeText()
    {
        //does not work because you change the property of an item in a collection and that
        //will not propagate down the component tree - Blazor fires OnParametersSet for child
        //components only when the reference of a complex object changes
        //like the entire collection, not just a field in one of its items
        var index = myDdlData.FindIndex(itm => itm.MyValueField == selectedValue);
        if(index > -1)
        {
            myDdlData[index].MyTextField = DateTime.Now.ToString();
        }
    }

    void ChangeEntireParameterRef()
    {
        //change the data item - on its own does nothing like in the example above
        var index = myDdlData.FindIndex(itm => itm.MyValueField == selectedValue);
        if (index > -1)
        {
            myDdlData[index].MyTextField = DateTime.Now.ToString();
        }

        //create a new reference for the parameter - re-renders the component
        myDdlData = new List<MyDdlModel>(myDdlData);
    }


    public class MyDdlModel
    {
        public int MyValueField { get; set; }
        public string MyTextField { get; set; }
    }

    List<MyDdlModel> myDdlData = Enumerable.Range(1, 20).Select(x => new MyDdlModel { MyTextField = "item " + x, MyValueField = x }).ToList();

    int selectedValue { get; set; } = 3; //usually the current value should come from the view model data
}

 

 

Regards,
Marin Bratanov
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DropDownList
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Serge
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Marin Bratanov
Telerik team
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