Triggers in .NET MAUI are not that different from triggers in Xamarin.Forms, but since this is not a frequently used feature, I thought I’d provide a quick deep-dive into their usage.
Triggers allow you to declare, in your XAML, how a control should appear based on data changes. You can combine state triggers with Visual State, a topic I’ll cover in a subsequent blog post.
This is the first of a series of short pieces based on material in my forthcoming book .NET MAUI For C# Developers (Packt* Publishing) which we are targeting for publication in early April.
For example, suppose we have a create account page, and we want the create button to be disabled if the user has not filled in a password. You can certainly do this in code (and .NET MAUI Community Toolkit will make this very easy with behaviors), but you can also do it declaratively in XAML using a DataTrigger.
<Button
Command="{Binding DoCreateAccountCommand}"
Grid.Column="1"
Grid.Row="2"
Style="{StaticResource LoginButton}"
Text="Create Account">
<Button.Triggers>
<DataTrigger
Binding="{Binding Source={x:Reference passwordEntry}, Path=Text.Length}"
TargetType="Button"
Value="0">
<Setter Property="IsEnabled" Value="False" />
</DataTrigger>
</Button.Triggers>
</Button>
Notice that the Button has a collection of Triggers. The type we want is a DataTrigger (as opposed, for example, to an event trigger). The trigger binds to the passwordEntry field’s Text.Length and looks for the value 0. If that is true, then the property IsEnabled is set to false. Once the Text.Length is not zero, the property is set to true.
That’s all it takes. You can see that triggers are often quite simple, but for some reason considered “advanced” and thus not used often; which is a shame because they can greatly simplify your code and make the logic easier to understand.
Note that the field you are checking (in this case password) must have its Text initialized to “” (in the ViewModel or code-behind, but of course the ViewModel is the right place). Otherwise it will be null, and the trigger may not act as expected.
In my next tutorial, I’ll tackle the more interesting and complex subject of Visual States.
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*This being my second book from Packt, I decided to ask how it is pronounced. Turns out it is one syllable, sounding more like packed than like packet. If you want to get it really right, say the word pack and sneak in a t sound at the end. (Just thought you’d want to know.)
