|
| 1 | +--- |
| 2 | +title: Debounce custom filter calls, implement min filter length |
| 3 | +description: how to debounce the custom server filter service calls and to implement min filter length |
| 4 | +type: how-to |
| 5 | +page_title: Debounce OnRead calls, Min Filter Length |
| 6 | +slug: combo-kb-debounce-onread |
| 7 | +position: |
| 8 | +tags: |
| 9 | +ticketid: 1460897 |
| 10 | +res_type: kb |
| 11 | +--- |
| 12 | + |
| 13 | +## Environment |
| 14 | +<table> |
| 15 | + <tbody> |
| 16 | + <tr> |
| 17 | + <td>Product</td> |
| 18 | + <td>ComboBox for Blazor</td> |
| 19 | + </tr> |
| 20 | + </tbody> |
| 21 | +</table> |
| 22 | + |
| 23 | + |
| 24 | +## Description |
| 25 | + |
| 26 | +I want to specify a debounce time for filtering. This way I can (for example) set the debounce time to 500(ms), and then only have the combo box service filter when the user stops typing. |
| 27 | + |
| 28 | +This can be useful for filtering with remote data - it invokes a filter on every keystroke and network/service delays can cause wrong data display or confusion, in addition to increased service load. |
| 29 | + |
| 30 | +I also want to implement a minimum filter length, if the input is below that length, the servive won't be called. |
| 31 | + |
| 32 | +## Solution |
| 33 | + |
| 34 | +Implement logic in the [OnRead event]({%slug components/combobox/events%}#onread) that will debounce the calls to the service with the desired timeout. For example, use a `CancellationTokenSource`. |
| 35 | + |
| 36 | +For min filter length, just add a check in the handler for the desired string length (in this example - 2 symbols). |
| 37 | + |
| 38 | +>caption Use a `CancellationTokenSource` to debounce OnRead filter calls in the combo box. Add Min Filter Length |
| 39 | +
|
| 40 | +````CSHTML |
| 41 | +@using System.Threading |
| 42 | +
|
| 43 | +@SelectedValue |
| 44 | +<br /> |
| 45 | +<TelerikComboBox Data="@Options" |
| 46 | + OnRead="@ReadItems" |
| 47 | + Filterable="true" |
| 48 | + Placeholder="Find what you seek by typing" |
| 49 | + @bind-Value="@SelectedValue"> |
| 50 | +</TelerikComboBox> |
| 51 | +
|
| 52 | +@code{ |
| 53 | + public string SelectedValue { get; set; } |
| 54 | + List<string> Options { get; set; } = new List<string>(); |
| 55 | + CancellationTokenSource tokenSource = new CancellationTokenSource(); // for debouncing the service calls |
| 56 | +
|
| 57 | + async Task RequestData(string userInput, string method) |
| 58 | + { |
| 59 | + // this method calls the actual service (in this case - a local method) |
| 60 | + Options = await GetOptions(userInput, method); |
| 61 | + } |
| 62 | +
|
| 63 | + async Task ReadItems(ComboBoxReadEventArgs args) |
| 64 | + { |
| 65 | + if (args.Request.Filters.Count > 0) // there is user filter input, skips providing data on initialization |
| 66 | + { |
| 67 | + Telerik.DataSource.FilterDescriptor filter = args.Request.Filters[0] as Telerik.DataSource.FilterDescriptor; |
| 68 | + string userInput = filter.Value.ToString(); |
| 69 | + string method = filter.Operator.ToString(); |
| 70 | +
|
| 71 | + if (userInput.Length > 1) // sample min filter length implementation |
| 72 | + { |
| 73 | + // debouncing |
| 74 | + tokenSource.Cancel(); |
| 75 | + tokenSource.Dispose(); |
| 76 | +
|
| 77 | + tokenSource = new CancellationTokenSource(); |
| 78 | + var token = tokenSource.Token; |
| 79 | +
|
| 80 | + await Task.Delay(300, token); // 300ms timeout for the debouncing |
| 81 | +
|
| 82 | + //new service request after debouncing |
| 83 | + await RequestData(userInput, method); |
| 84 | + } |
| 85 | + } |
| 86 | + else |
| 87 | + { |
| 88 | + if (Options?.Count < 1) |
| 89 | + { |
| 90 | + // when there is no user input you may still want to provide data |
| 91 | + // in this example we just hardcode a few items, you can either fetch all the data |
| 92 | + // or you can provide some subset of most common items, or something based on the business logic |
| 93 | + Options = new List<string>() { "one", "two", "three" }; |
| 94 | + } |
| 95 | + } |
| 96 | + } |
| 97 | +
|
| 98 | + async Task<List<string>> GetOptions(string userInput, string filterOperator) |
| 99 | + { |
| 100 | + Console.WriteLine("service called - debounced so there are fewer calls"); |
| 101 | + await Task.Delay(500); // simulate network delay, remove it for a real app |
| 102 | +
|
| 103 | + //sample logic for getting suggestions - here they are generated, you can call a remote service |
| 104 | + //for brevity, this example does not use the filter operator, but your actual service can |
| 105 | + List<string> optionsData = new List<string>(); |
| 106 | + for (int i = 0; i < 5; i++) |
| 107 | + { |
| 108 | + optionsData.Add($"option {i} for input {userInput}"); |
| 109 | + } |
| 110 | +
|
| 111 | + return optionsData; |
| 112 | + } |
| 113 | +```` |
| 114 | + |
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