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Jason Smith's Xamarin Forms Performance Tips

Jason Smith, lead engineer of Xamarin Forms, gave a great talk at this year’s Evolve. He went through a long list of performance tips, as well as a Q&A session. I have not been able to find these tips in text format (slides, transcript, or blog post). In the interests of making this information more readily accessible to both me and others, I decided to summarize it here.

I’ve taken the liberty to rearrange and group related tips. But I want to be clear that this is entirely Jason’s content, not my own.

General

DO enable XAML compilation if you’re using XAML:

[assembly: XamlCompilation(XamlCompilationOptions.Compile)]

DO NOT bind when static assignment suffices.

DO NOT assign default values. Doing so has a cost even though you’re not modifying the value.

AVOID transparency. If you can achieve the same (or close enough) effect with full opacity, do so.

DO use async/await to avoid blocking the main thread.

CONSIDER inflating views off the main thread, but be sure to add it to the visual tree on the main thread. Failure to do so will not immediately crash your application, but will instead corrupt its state. Be particularly careful if you’re using MessagingCenter in your view’s constructor because it will not marshal the event to the correct thread for you.

Layout

DO NOT use a ContentView with Padding just to apply a margin to the child. Instead, use the Margin property on the child (as of Xamarin.Forms 2.2).

DO NOT use a StackLayout to host a single child.

DO NOT use a Grid when a StackLayout suffices.

DO NOT use multiple StackLayouts when a Grid suffices.

DO be aware of the Spacing (ColumnSpacing/RowSpacing for Grid) and Padding properties. Instead of this:

<StackLayout>
    <ContentView Padding="10,10,10,5">
        <Label Text="1"/>
    </ContentView>
    <ContentView Padding="10,0,10,5">
        <Label Text="2"/>
    </ContentView>
    <ContentView Padding="10,0,10,0">
        <Label Text="3"/>
    </ContentView>
</StackLayout>

Do this:

<StackLayout Padding="10" Spacing="5">
    <Label Text="1"/>
    <Label Text="2"/>
    <Label Text="3"/>
</StackLayout>

PREFER using LayoutOptions.Fill (or LayoutOptions.FillAndExpand). These are the defaults and shouldn’t be modified most of the time.

PREFER star-sized grid columns/rows rather than auto-sized.

DO NOT use multiple StackLayouts to simulate a Grid.

DO NOT use RelativeLayout if at all possible.

DO pack children into the view before parents.

Instead of this:

var page = new ContentPage();
var stackLayout = new StackLayout();
var image = new Image { Source = "person.png" };
var label = new Label { Text = "Name" };

page.Content = stackLayout;

navigationPage.Push(page);
stackLayout.Children.Add(image);
stackLayout.Children.Add(label);

Do this:

var page = new ContentPage();
var stackLayout = new StackLayout();
var image = new Image { Source = "person.png" };
var label = new Label { Text = "Name" };

stackLayout.Children.Add(image);
stackLayout.Children.Add(label);
page.Content = stackLayout;

navigationPage.Push(page);

Note that XAML automatically packs in the correct order.

DO pack views in your constructor rather than OnAppearing.

PREFER animating views with the TranslationX and TranslationY properties as this avoids the need for layout.

DO NOT call Layout() (and especially ForceLayout()).

CONSIDER creating a custom layout. This is likely the best choice if your layout is simple to describe in English but difficult to implement with stock layouts, if AbsoluteLayout almost does what you want, or if you just need raw speed.

DO NOT use expression-based constraints in RelativeLayout (and per above, try not to use RelativeLayout at all).

DO NOT use a StackLayout inside a ScrollView to simulate a ListView.

DO use a Grid to achieve layering.

Labels

DO NOT use multiple Labels when one will do (using spans with FormattedText if necessary).

DO disable Label wrapping if possible (LineBreakMode="NoWrap").

DO NOT change the default VerticalTextAlignment for Labels unless necessary. The default removes an entire measure cycle.

PREFER the VerticalTextAlignment and HorizontalTextAlignment properties of Label over VerticalOptions and HorizontalOptions.

AVOID sporadic updates to Labels. If the updates are to multiple Labels, update as a batch if possible.

ListViews

DO NOT put a ListView inside a ScrollView. Use the ListViews Header and Footer properties instead.

DO NOT use TableView where you can use a ListView. Today, the only real reason to use a TableView is for settings-style UI where pretty much every cell has unique content.

DO use ListViewCachingStrategy.RecycleElement when you can. If it’s not working, figure out why because it’s worth it. This is not the default.

DO use data template selectors to facilitate heterogeneous views within a single ListView. Don’t override OnBindingContextChanged to update achieve the same effect.

AVOID passing IEnumerable<T> as a data source to ListViews. Instead, try to use IList<T>.

DO NOT nest ListViews. Instead, use groups within a single ListView. Nesting is explicitly unsupported and will break your application.

DO use HasUnevenRows where your ListView has rows of differing sizes. If the content of the cell is modified dynamically (perhaps after loading it from the database), be sure to call ForceUpdateSize() on the cell.

DO await the PushAsync and PopAsync methods. Failure to do so is detrimental to both performance and correctness.

AVOID hiding/showing the navigation bar.

DO use the AppCompat backend for Android. This will improve both performance and the look of the application.

Images

BE AWARE that images on Android do not down-sample.

DO set Image.IsOpaque to true if possible.

DO load images from Content instead of Resources.

BindableProperty

DO NOT use the generic version of BindableProperty.Create (use the string-based version instead with C# 6’s nameof operator).

CarouselPage

DO NOT use CarouselPage.

DO use a CarouselView within a ContentPage.

MessagingCenter

PREFER to use something else (e.g. Prism).

DO pass MessagingCenter either a static or instance method, not a closure/lambda expression.

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