WPF MVVM - Update Dropdown When Clicked

Question

I have a dropdown (ComboBox) that displays all the com ports available on a machine. Now, ports come and go when you connect and disconnect devices.

For performance reasons I don't want to keep calling System.IO.Ports.SerialPort.GetPortNames(), but rather just call that when the user clicks on the Combobox? Is this possible? Is there an MVVM approach to this problem?

Answer 1

Use InvokeCommandAction.

xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"

DropDownOpenedCommand is an ICommand property on your ViewModel.

<ComboBox>
    <i:Interaction.Triggers>
        <i:EventTrigger EventName="DropDownOpened">
            <i:InvokeCommandAction Command="{Binding DropDownOpenedCommand}"/>
        </i:EventTrigger>
    </i:Interaction.Triggers>
</ComboBox>

Edit: obviously DropDownOpened not SelectionChanged, as Patrice commented.