DropDownList - Refresh Data
The most common reason you would use an ObservableCollection is to make a component (like a grid, treeview, treelist, dropdown) change or react when you change that collection.
When you want to refresh the component data source like that, there are two important framework behaviors you need to be aware of - when ObservableCollection instances fire events, and how to refresh the data of a component when it is not an observable collection.
In this article:
Rebind Method
You can refresh the data of the DropDownList by using the Rebind method exposed to the reference of the TelerikDropDownList. If you have manually defined the OnRead event the business logic defined in its event handler will be executed.
@* Clicking on the Rebind button will delete the first option from the dropdown and refresh the data *@
@using Telerik.DataSource.Extensions
<TelerikButton OnClick="@RebindDropDown">Rebind the DropDown</TelerikButton>
<TelerikDropDownList TItem="@String"
TValue="@String"
@ref="@DropDownRef"
OnRead="@ReadItems"
@bind-Value="@SelectedValue">
</TelerikDropDownList>
@code{
private TelerikDropDownList<string, string> DropDownRef { get; set; }
private void RebindDropDown()
{
if(Options.Count > 0)
{
Options.RemoveAt(0);
}
DropDownRef.Rebind();
}
public string SelectedValue { get; set; }
List<string> Options { get; set; } = new List<string>();
async Task ReadItems(DropDownListReadEventArgs args)
{
await Task.Delay(1000);
args.Data = Options.ToDataSourceResult(args.Request).Data;
}
protected override async Task OnInitializedAsync()
{
Options = new List<string>() { "one", "two", "three" };
}
}
As part of our
3.0.1release we introduced theRebindmethod to the component reference. This would make the rest of the approaches in this article obsolete.
Observable Data
Databound components can benefit from live updates - when the data source collection changes, the components should update to reflect that change. Most data-bound components in the Telerik UI for Blazor suite implement such functionality.
When the Data of the component is a collection that implements the INotifyCollectionChanged interface (such as ObservableCollection), the Telerik components subscribe to its CollectionChanged event to make live update. This means that adding items, removing items, or clearing the collection updates the components (its .Add(), .Remove() and .Clear() methods).
The Observable collections fire the CollectionChanged event only when their Add, Remove and Clear methods are called. They do not fire it when you change the value of a field of one of their elements.
Bind the DropDownList component to an ObservableCollection, so it can react to collection changes.
@* Add/remove an option to see how the DropDownList reacts to the change. *@
@using System.Collections.ObjectModel
<h4>Add option</h4>
<TelerikTextBox @bind-Value="@ValuetoAdd"></TelerikTextBox>
<TelerikButton OnClick="@AddOption">Add option</TelerikButton>
<br />
<h4>Remove the last option</h4>
<TelerikButton OnClick="@RemoveOption">Remove the last option</TelerikButton>
<br />
<h4>DropDownList options: @myDdlData.Count</h4>
<br />
<TelerikDropDownList Data="@myDdlData" TextField="MyTextField" ValueField="MyValueField" @bind-Value="@selectedValue">
</TelerikDropDownList>
@code {
string ValuetoAdd { get; set; }
int selectedValue { get; set; }
ObservableCollection<MyDdlModel> myDdlData = new ObservableCollection<MyDdlModel>(Enumerable.Range(1, 5).Select(x => new MyDdlModel { MyTextField = "item " + x, MyValueField = x }));
void AddOption()
{
if (!string.IsNullOrWhiteSpace(ValuetoAdd))
{
myDdlData.Add(
new MyDdlModel { MyTextField = ValuetoAdd, MyValueField = myDdlData.Count + 1 }
);
ValuetoAdd = string.Empty;
}
}
void RemoveOption()
{
if (myDdlData.Count > 0)
{
myDdlData.RemoveAt(myDdlData.Count - 1);
}
}
public class MyDdlModel
{
public int MyValueField { get; set; }
public string MyTextField { get; set; }
}
}
If you need to add/remove many items to/from the collection, consider creating a new collection and provide its reference to the data parameter. Thus, the component will re-render only once (when the data collection reference is changed) instead of re-rendering multiple times in response to the Add/Remove events.
New Collection Reference
In Blazor, the framework will fire the OnParametersSet event of a child component (which is how child components can react to outside changes) only when it can detect a change in the object it receives through the corresponding parameter (like Data for the data sources of Telerik components). This detection works as follows:
For strings and value types, this happens when their value changes.
-
For reference types (such as data collections like
List, or anyIEnumerable, and application-specific objects), this happens when the object reference changes.Thus, you would usually need to create a
newreference for the view-model field (such asTreeViewData = new List<MyTreeViewItem>(theUpdatedDataCollection);) when you want the component to update.
Create new collection reference to refresh the DropDownList data.
@* Add/remove an option to see how the DropDownList reacts to the change. *@
<h4>Add option</h4>
<TelerikTextBox @bind-Value="@ValuetoAdd"></TelerikTextBox>
<TelerikButton OnClick="@AddOption">Add option</TelerikButton>
<br />
<h4>Remove the last option</h4>
<TelerikButton OnClick="@RemoveOption">Remove the last option</TelerikButton>
<br />
<h4>Load new collection</h4>
<TelerikButton OnClick="@LoadNewData">Load data</TelerikButton>
<br />
<h4>DropDownList options: @myDdlData.Count</h4>
<br />
<TelerikDropDownList Data="@myDdlData" TextField="MyTextField" ValueField="MyValueField" @bind-Value="@selectedValue">
</TelerikDropDownList>
@code {
string ValuetoAdd { get; set; }
int selectedValue { get; set; }
List<MyDdlModel> myDdlData = Enumerable.Range(1, 5).Select(x => new MyDdlModel { MyTextField = "item " + x, MyValueField = x }).ToList();
void AddOption()
{
if (!string.IsNullOrWhiteSpace(ValuetoAdd))
{
myDdlData.Add(
new MyDdlModel { MyTextField = ValuetoAdd, MyValueField = myDdlData.Count + 1 }
);
myDdlData = new List<MyDdlModel>(myDdlData);
ValuetoAdd = string.Empty;
}
}
void RemoveOption()
{
if (myDdlData.Count > 0)
{
myDdlData.RemoveAt(myDdlData.Count - 1);
myDdlData = new List<MyDdlModel>(myDdlData);
}
}
void LoadNewData()
{
var newData = new List<MyDdlModel>(Enumerable.Range(6, 5).Select(x => new MyDdlModel { MyTextField = "item " + x, MyValueField = x }));
myDdlData = new List<MyDdlModel>(newData);
Console.WriteLine("New data collection loaded.");
}
public class MyDdlModel
{
public int MyValueField { get; set; }
public string MyTextField { get; set; }
}
}