On-Demand Item Loading (Callback Mode)
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In contrast to the default loading mode, the ASPxComboBox also allows its items to be loaded from the server on demand, via callbacks. For instance, list items that are not currently displayed within the editor’s dropdown window can be dynamically loaded when an end-user scrolls the list. This item loading mode is available if the ASPxAutoCompleteBoxBase.EnableCallbackMode property is set to true. In this mode, the ASPxAutoCompleteBoxBase.CallbackPageSize property defines the number of items to be obtained from the server, each time it’s required.
This callback mode makes the first page load much faster, since only a few items need to be loaded initially.
Note
If the control ViewState is enabled, it gets all items on load, regardless of the EnableCallbackMode property value (exclude the multicolumn mode). Therefore, it is recommended that you disable the view state when you use callback mode.
Note however that in this mode, the ASPxComboBox editor still performs data processing operations (such as filtering) by itself, on the web application server level. For this purpose, the editor retrieves all data items of the bound data source from the database server to the web server, which increases the web server workload. This might result in a performance bottleneck when binding ASPxComboBox editors to large data sources (containing tens of thousands of records). To increase the ASPxComboBox‘s performance in this case, it’s recommended to dynamically populate the editor’s dropdown list by handling specific events.
Note
Setting the ASPxAutoCompleteBoxBase.EnableCallbackMode property affects the ASPxAutoCompleteBoxBase.EnableSynchronization property’s default value. See the ASPxAutoCompleteBoxBase.EnableSynchronization property’s description for more details.
Note
In the callback mode, the client-side combo box contains only items that are currently displayed within the editor’s drop-down window, so it is impossible to select an invisible item via the ASPxClientComboBox.SetText or ASPxClientEditBase.SetValue client method.