Xamarin.Forms Compiled Bindings

Compiled bindings are resolved more quickly than classic bindings, therefore improving data binding performance in Xamarin.Forms applications.

Data bindings have two main problems:

  1. There's no compile-time validation of binding expressions. Instead, bindings are resolved at runtime. Therefore, any invalid bindings aren't detected until runtime when the application doesn't behave as expected or error messages appear.
  2. They aren't cost efficient. Bindings are resolved at runtime using general-purpose object inspection (reflection), and the overhead of doing this varies from platform to platform.

Compiled bindings improve data binding performance in Xamarin.Forms applications by resolving binding expressions at compile-time rather than runtime. In addition, this compile-time validation of binding expressions enables a better developer troubleshooting experience because invalid bindings are reported as build errors.

The process for using compiled bindings is to:

  1. Enable XAML compilation. For more information about XAML compilation, see XAML Compilation.
  2. Set an x:DataType attribute on a VisualElement to the type of the object that the VisualElement and its children will bind to. Note that this attribute can be re-defined at any location in a view hierarchy.

Note

It's recommended to set the x:DataType attribute at the same level in the view hierarchy as the BindingContext is set.

At XAML compile time, any invalid binding expressions will be reported as build errors. However, the XAML compiler will only report a build error for the first invalid binding expression that it encounters. Any valid binding expressions that are defined on the VisualElement or its children will be compiled, regardless of whether the BindingContext is set in XAML or code. Compiling a binding expression generates compiled code that will get a value from a property on the source, and set it on the property on the target that's specified in the markup. In addition, depending on the binding expression, the generated code may observe changes in the value of the source property and refresh the target property, and may push changes from the target back to the source.

Important

Compiled bindings are currently disabled for any binding expressions that define the Source property. This is because the Source property is always set using the x:Reference markup extension, which can't be resolved at compile time.

Using compiled bindings

The Compiled Color Selector page demonstrates using compiled bindings between Xamarin.Forms views and ViewModel properties:

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:local="clr-namespace:DataBindingDemos"
             x:Class="DataBindingDemos.CompiledColorSelectorPage"
             Title="Compiled Color Selector">
    ...
    <StackLayout x:DataType="local:HslColorViewModel">
        <StackLayout.BindingContext>
            <local:HslColorViewModel Color="Sienna" />
        </StackLayout.BindingContext>
        <BoxView Color="{Binding Color}"
                 ... />
        <StackLayout Margin="10, 0">
            <Label Text="{Binding Name}" />
            <Slider Value="{Binding Hue}" />
            <Label Text="{Binding Hue, StringFormat='Hue = {0:F2}'}" />
            <Slider Value="{Binding Saturation}" />
            <Label Text="{Binding Saturation, StringFormat='Saturation = {0:F2}'}" />
            <Slider Value="{Binding Luminosity}" />
            <Label Text="{Binding Luminosity, StringFormat='Luminosity = {0:F2}'}" />
        </StackLayout>
    </StackLayout>    
</ContentPage>

The root StackLayout instantiates the HslColorViewModel and initializes the Color property within property element tags for the BindingContext property. This root StackLayout also defines the x:DataType attribute as the ViewModel type, indicating that any binding expressions in the root StackLayout view hierarchy will be compiled. This can be verified by changing any of the binding expressions to bind to a non-existent ViewModel property, which will result in a build error.

Important

The x:DataType attribute can be re-defined at any point in a view hierarchy.

The BoxView, Label elements, and Slider views inherit the binding context from the StackLayout. These views are all binding targets that reference source properties in the ViewModel. For the BoxView.Color property, and the Label.Text property, the data bindings are OneWay – the properties in the view are set from the properties in the ViewModel. However, the Slider.Value property uses a TwoWay binding. This allows each Slider to be set from the ViewModel, and also for the ViewModel to be set from each Slider.

When the application is first run, the BoxView, Label elements, and Slider elements are all set from the ViewModel based on the initial Color property set when the ViewModel was instantiated. This is shown in the following screenshots:

Compiled Color Selector

As the sliders are manipulated, the BoxView and Label elements are updated accordingly.

For more information about this color selector, see ViewModels and Property-Change Notifications.

Using compiled bindings in a DataTemplate

Bindings in a DataTemplate are interpreted in the context of the object being templated. Therefore, when using compiled bindings in a DataTemplate, the DataTemplate needs to declare the type of its data object using the x:DataType attribute.

The Compiled Color List page demonstrates using compiled bindings in a DataTemplate:

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:local="clr-namespace:DataBindingDemos"
             x:Class="DataBindingDemos.CompiledColorListPage"
             Title="Compiled Color List">
    <Grid>
        ...
        <ListView x:Name="colorListView"
                  ItemsSource="{x:Static local:NamedColor.All}"
                  ... >
            <ListView.ItemTemplate>
                <DataTemplate x:DataType="local:NamedColor">
                    <ViewCell>
                        <StackLayout Orientation="Horizontal">
                            <BoxView Color="{Binding Color}"
                                     ... />
                            <Label Text="{Binding FriendlyName}"
                                   ... />
                        </StackLayout>
                    </ViewCell>
                </DataTemplate>
            </ListView.ItemTemplate>
        </ListView>
        <!-- The BoxView doesn't use compiled bindings -->
        <BoxView Color="{Binding Source={x:Reference colorListView}, Path=SelectedItem.Color}"
                 ... />
    </Grid>
</ContentPage>

The ListView.ItemsSource property is set to the static NamedColor.All property. The NamedColor class uses .NET reflection to enumerate all the static public fields in the Color structure, and to store them with their names in a collection that is accessible from the static All property. Therefore, the ListView is filled with all of the NamedColor instances. For each item in the ListView, the binding context for the item is set to a NamedColor object. The BoxView and Label elements in the ViewCell are bound to NamedColor properties.

Note that the DataTemplate defines the x:DataType attribute to be the NamedColor type, indicating that any binding expressions in the DataTemplate view hierarchy will be compiled. This can be verified by changing any of the binding expressions to bind to a non-existent NamedColor property, which will result in a build error.

When the application is first run, the ListView is populated with NamedColor instances. When an item in the ListView is selected, the BoxView.Color property is set to the color of the selected item in the ListView:

Compiled Color List

Selecting other items in the ListView updates the color of the BoxView.

Combining compiled bindings with classic bindings

Binding expressions are only compiled for the view hierarchy that the x:DataType attribute is defined on. Conversely, any views in a hierarchy on which the x:DataType attribute is not defined will use classic bindings. It's therefore possible to combine compiled bindings and classic bindings on a page. For example, in the previous section the views within the DataTemplate use compiled bindings, while the BoxView that's set to the color selected in the ListView does not.

Careful structuring of x:DataType attributes can therefore lead to a page using compiled and classic bindings. Alternatively, the x:DataType attribute can be re-defined at any point in a view hierarchy to null using the x:Null markup extension. Doing this indicates that any binding expressions within the view hierarchy will use classic bindings. The Mixed Bindings page demonstrates this approach:

<StackLayout x:DataType="local:HslColorViewModel">
    <StackLayout.BindingContext>
        <local:HslColorViewModel Color="Sienna" />
    </StackLayout.BindingContext>
    <BoxView Color="{Binding Color}"
             VerticalOptions="FillAndExpand" />
    <StackLayout x:DataType="{x:Null}"
                 Margin="10, 0">
        <Label Text="{Binding Name}" />
        <Slider Value="{Binding Hue}" />
        <Label Text="{Binding Hue, StringFormat='Hue = {0:F2}'}" />
        <Slider Value="{Binding Saturation}" />
        <Label Text="{Binding Saturation, StringFormat='Saturation = {0:F2}'}" />
        <Slider Value="{Binding Luminosity}" />
        <Label Text="{Binding Luminosity, StringFormat='Luminosity = {0:F2}'}" />
    </StackLayout>
</StackLayout>   

The root StackLayout sets the x:DataType attribute to be the HslColorViewModel type, indicating that any binding expression in the root StackLayout view hierarchy will be compiled. However, the inner StackLayout redefines the x:DataType attribute to null with the x:Null markup expression. Therefore, the binding expressions within the inner StackLayout use classic bindings. Only the BoxView, within the root StackLayout view hierarchy, uses compiled bindings.

For more information about the x:Null markup expression, see x:Null Markup Extension.

Performance

Compiled bindings improve data binding performance, with the performance benefit varying. Unit testing reveals that:

  • A compiled binding that uses property-change notification (i.e. a OneWay, OneWayToSource, or TwoWay binding) is resolved approximately 8 times quicker than a classic binding.
  • A compiled binding that doesn't use property-change notification (i.e. a OneTime binding) is resolved approximately 20 times quicker than a classic binding.
  • Setting the BindingContext on a compiled binding that uses property change notification (i.e. a OneWay, OneWayToSource, or TwoWay binding) is approximately 5 times quicker than setting the BindingContext on a classic binding.
  • Setting the BindingContext on a compiled binding that doesn't use property change notification (i.e. a OneTime binding) is approximately 7 times quicker than setting the BindingContext on a classic binding.

These performance differences can be magnified on mobile devices, dependent upon the platform being used, the version of the operating system being used, and the device on which the application is running.