Boost Performance with Compiled Bindings in Xamarin.Forms
Data binding is at the core of just about every Xamarin.Forms application. It enabled developers to easily bridge their user interface with their code behind form a simple markup. Data binding also simplifies user interactions and updates to the user interface automatigical! With all this awesome comes with some trade-offs. Xamarin.Forms has to analyze and resolve each binding, which can take precious CPU cycles. This is where compiled bindings come in and help boost performance when using data binding.
What are Compiled Bindings?
A compiled binding is a way of telling Xamarin.Forms at compile time of the type of data the binding is going to be using. This is means that Xamarin.Forms will not have to use any reflection at runtime to resolve the data binding. By using the special x:DataType
attribute on any VisualElement
you can enable a compiled binding.
Benefits
Using these bindings offer a few major benefits for you and your app:
- Compile-time validation for all binding expressions.
- Performance improvements as bindings are resolved at compile time.
Enabling XAML Compilation
A pre-requisite to use these bindings is that your app must turn on XAML Compilation. This is the default in all new apps and greatly improves performance in apps when using XAML. You can enable it by adding an assembly flag in your shared code:
[assembly:XamlCompilation(XamlCompilationOptions.Compile)]
Using Compiled Bindings
Let's use an example of a page that displays a list of Monkeys in a ListView
. This means we have a Monkey
with properties on it about the monkey, a MonkeysViewModel
that has data objects to bind to for the list, and a MonkeysPage
that is our user interface in XAML.
Here is what the user interface looks like to display a list of monkeys and the current count:
<ContentPage
xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
x:Class="Monkeys.Views.MonkeysPage"
Title="Monkeys">
<StackLayout>
<Label Margin="10,0" Text="{Binding MonkeyCount, StringFormat='{0} Monkeys'}"/>
<ListView ItemsSource="{Binding Monkeys}"
HasUnevenRows="true">
<ListView.ItemTemplate>
<DataTemplate>
<ViewCell>
<Grid Padding="10" RowSpacing="10" ColumnSpacing="10">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="Auto"/>
<ColumnDefinition Width="*"/>
</Grid.ColumnDefinitions>
<controls:CircleImage
BorderColor="Aqua"
BorderThickness="3"
HeightRequest="66"
WidthRequest="66"
HorizontalOptions="CenterAndExpand"
VerticalOptions="CenterAndExpand"
Aspect="AspectFill"
Source="{Binding Image}"/>
<StackLayout VerticalOptions="Center">
<Label Text="{Binding Name}"/>
<Label Text="{Binding Location}"/>
</StackLayout>
</Grid>
</ViewCell>
</DataTemplate>
</ListView.ItemTemplate>
</ListView>
</ContentPage>
Notice that we are binding from the MonkeysViewModel
to MonkeyCount
and Monkeys
and additionally to the Image
, Name
, and Location
of each monkey inside of the ItemTemplate
.
To enable these bindings for the MonkeysViewModels
we will bring in the namespace that it is in:
xmlns:viewmodels="clr-namespace:Monkeys.ViewModels"
Then on the ContentPage
node we can add the x:DataType
property:
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms" xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" x:Class="Monkeys.Views.MonkeysPage" xmlns:viewmodels="clr-namespace:Monkeys.ViewModels" x:DataType="viewmodels:MonkeysViewModel" Title="Monkeys"> </ContentPage>
That is all it takes to start! Now, if for some reason we have a spelling mistake in our bindings when we compile the library we will get an error as the bindings will now be validated.
If you are using any control that has a DataTemplate
you must also setup the compiled binding on it. In this app, we have a ListView that is data bound to anObservableCollection<Monkey>
. We can bring in the Monkey
's namespace just like we did for the MonkeysViewModel
.
xmlns:models="clr-namespace:Monkeys.Models"
Then on the DataTemplate
, we can specify the x:DataType
like we did before:
<ListView ItemsSource="{Binding Monkeys}" HasUnevenRows="true"> <ListView.ItemTemplate> <DataTemplate x:DataType="models:Monkey"> <ViewCell> </ViewCell> </DataTemplate> </ListView.ItemTemplate> </ListView>
And just like that we have enabled this on our entire page and it only took a few seconds.
Learn More
There are a lot more ways to use compiled bindings in your Xamarin.Forms apps, so be sure to read through the comprehensive documentation. You can find a sample used in this blog on GitHub.
18 comments
How do I handle Commands in a data template? I keep getting an error ‘ Command not found on the model'(specified in x:DataType of DataTemplate). The Commands are in the ViewModel
If you are doing an ancestor binding, set the x:DataType on the control that is referencing the parent viewmodel.
Your sample images are no longer displayed on this page, or is it broken on my end?
Thanks for the ping. We just made some changes to the syntax highlighting and that caused some issues on older blog posts. Fixed it up.
Good job – keep it up James 👍