WPF vs. Xamarin.Forms App Lifecycle

Xamarin.Forms takes a lot of design guidance from the XAML-based frameworks that came before it, particularly WPF. However, in other ways it deviates significantly which can be a sticky point for people attempting to migrate over. This document attempts to identify some of those issues and provide guidance where possible to bridge WPF knowledge to Xamarin.Forms.

App Lifecycle

The application lifecycle between WPF and Xamarin.Forms is similar. Both start in external (platform) code and launch the UI through a method call. The difference is that Xamarin.Forms always starts in a platform-specific assembly which then initializes and creates the UI for the app.

WPF

  • Main method > App > MainWindow

Note

The Main method is, by default, auto generated and not visible in the code.

Xamarin.Forms

  • iOSMain method > AppDelegate > App > ContentPage
  • AndroidMainActivity > App > ContentPage
  • UWPMain method > App(UWP) > MainPage(UWP) > App > ContentPage

Application class

Both WPF and Xamarin.Forms have an Application class which is created as a singleton. In most cases, apps will derive from this class to provide a custom application, although this is not strictly required in WPF. Both expose an Application.Current property to locate the created singleton.

Global properties + persistence

Both WPF and Xamarin.Forms have a Application.Properties dictionary available where you can store global app-level objects that are accessible anywhere in the application. The key difference is that Xamarin.Forms will persist any primitive types stored in the collection when the app is suspended, and reload them when it is relaunched. WPF does not automatically support that behavior - instead, most developers relied on isolated storage, or utilized the built-in Settings support.

Defining pages and the Visual Tree

WPF uses the Window as the root element for any top-level visual element. This defines an HWND in the Windows world to display information. You can create and display as many windows simultaneously as you like in WPF.

In Xamarin.Forms, the top-level visual is always defined by the platform - for example on iOS, it's a UIWindow. Xamarin.Forms renders it's content into these native platform representations using a Page class. Each Page in Xamarin.Forms represents a unique "page" in the application, where only one is visible at a time.

Both WPFs Window and Xamarin.Forms Page include a Title property to influence the displayed title, and both have an Icon property to display a specific icon for the page (Note that the title and icon are not always visible in Xamarin.Forms). In addition, you can change common visual properties on both such as the background color or image.

It is technically possible to render to two separate platform views (e.g. define two UIWindow objects and have the second one render to an external display or AirPlay), it requires platform-specific code to do so and is not a directly supported feature of Xamarin.Forms itself.

Views

The visual hierarchy for both frameworks is similar. WPF is a bit deeper due to its support for WYSIWYG documents.

WPF

DependencyObject - base class for all bindable things
   Visual - rendering mechanics
      UIElement - common events + interactions
         FrameworkElement - adds layout
            Shape - 2D graphics
            Control - interactive controls

Xamarin.Forms

BindableObject - base class for all bindable things
   Element - basic parent/child support + resources + effects
      VisualElement - adds visual rendering properties (color, fonts, transforms, etc.)
         View - layout + gesture support

View Lifecycle

Xamarin.Forms is primarily oriented around mobile scenarios. As such, applications are activated, suspended, and reactivated as the user interacts with them. This is similar to clicking away from the Window in a WPF application and there are a set of methods and corresponding events you can override or hook into to monitor this behavior.

Purpose WPF Method Xamarin.Forms Method
Initial activation ctor + Window.OnLoaded ctor + Page.OnStart
Shown Window.IsVisibleChanged Page.Appearing
Hidden Window.IsVisibleChanged Page.Disappearing
Suspend/Lost focus Window.OnDeactivated Page.OnSleep
Activated/Got focus Window.OnActivated Page.OnResume
Closed Window.OnClosing + Window.OnClosed n/a

Both support hiding/showing child controls as well, in WPF it's a tri-state property IsVisible (visible, hidden, and collapsed). In Xamarin.Forms, it's just visible or hidden through the IsVisible property.

Layout

Page layout occurs in the same 2-pass (Measure/Arrange) that happens in WPF. You can hook into the page layout by overriding the following methods in the Xamarin.Forms Page class:

Method Purpose
OnChildMeasureInvalidated Preferred size of a child has changed.
OnSizeAllocated Page has been assigned a width/height.
LayoutChanged event Layout/size of the Page has changed.

There is no global layout event which is called today, nor is there a global CompositionTarget.Rendering event like found in WPF.

Common layout properties

WPF and Xamarin.Forms both support Margin to control spacing around an element, and Padding to control spacing inside an element. In addition, most of the Xamarin.Forms layout views have properties to control spacing (e.g. row or column).

In addition, most elements have properties to influence how they are positioned in the parent container:

WPF Xamarin.Forms Purpose
HorizontalAlignment HorizontalOptions Left/Center/Right/Stretch options
VerticalAlignment VerticalOptions Top/Center/Bottom/Stretch options

Note

The actual interpretation of these properties depends on the parent container.

Layout views

WPF and Xamarin.Forms both use layout controls to position child elements. In most cases, these are very close to each other in terms of functionality.

WPF Xamarin.Forms Layout Style
StackPanel StackLayout Left-to-right, or top-to-bottom infinite stacking
Grid Grid Tabular format (rows and columns)
DockPanel n/a Dock to edges of window
Canvas AbsoluteLayout Pixel/Coordinate positioning
WrapPanel n/a Wrapping stack
n/a RelativeLayout Relative rule-based positioning

Note

Xamarin.Forms does not support a GridSplitter.

Both platforms use attached properties to fine-tune children.

Rendering

The rendering mechanics for WPF and Xamarin.Forms are radically different. In WPF, the controls you create directly render content to pixels on the screen. WPF maintains two object graphs (trees) to represent this - the logical tree represents the controls as defined in code or XAML, and the visual tree represents the actual rendering that occurs on the screen which is performed either directly by the visual element (through a virtual draw method), or through a XAML-defined ControlTemplate which can be replaced or customized. Typically, the visual tree is more complex as it includes things such as borders around controls, labels for implicit content, etc. WPF includes a set of APIs (LogicalTreeHelper and VisualTreeHelper) to examine these two object graphs.

In Xamarin.Forms, the controls you define in a Page are really just simple data objects. They are similar to the logical tree representation, but never render content on their own. Instead, they are the data model which influences the rendering of elements. The actual rendering is done by a separate set of visual renderers which are mapped to each control type. These renderers are registered in each of the platform-specific projects by platform-specific Xamarin.Forms assemblies. You can see a list here. In addition to replacing or extending the renderer, Xamarin.Forms also has support for Effects which can be used to influence the native rendering on a per-platform basis.

The Logical/Visual Tree

There is no exposed API to walk the logical tree in Xamarin.Forms - but you can use Reflection to get the same information. For example, here's a method which can enumerate logical children with reflection.

Graphics

Xamarin.Forms does not include a graphics system for primitives beyond a simple rectangle (BoxView). You can include 3rd party libraries such as SkiaSharp to get cross-platform 2D drawing, or UrhoSharp for 3D.

Resources

WPF and Xamarin.Forms both have the concept of resources and resource dictionaries. You can place any object type into a ResourceDictionary with a key and then look it up with {StaticResource} for things which will not change, or {DynamicResource} for things which can change in the dictionary at runtime. The usage and mechanics are the same with one difference: Xamarin.Forms requires that you define the ResourceDictionary to assign to the Resources property whereas WPF pre-creates one and assigns it for you.

For example, see the definition below:

WPF

<Window.Resources>
   <Color x:Key="redColor">#ff0000</Color>
   ...
</Window.Resources>

Xamarin.Forms

<ContentPage.Resources>
   <ResourceDictionary>
      <Color x:Key="redColor">#ff0000</Color>
      ...
   </ResourceDictionary>
</ContentPage.Resources>

If you do not define the ResourceDictionary, a runtime error is generated.

Styles

Styles are also fully supported in Xamarin.Forms and can be used to theme the Xamarin.Forms elements that make up the UI. They support triggers (property, event and data), inheritance through BasedOn, and resource lookups for values. Styles are applied to elements either explicitly through the Style property, or implicitly by not supplying a resource key - just like WPF.

Device Styles

WPF has a set of predefined properties (stored as static values on a set of static classes such as SystemColors) which dictate system colors, fonts and metrics in the form of values and resource keys. Xamarin.Forms is similar, but defines a set of Device Styles to represent the same things. These styles are supplied by the framework and set to values based on the runtime environment (e.g. accessibility).

WPF

<Label Text="Title" Foreground="{DynamicResource {x:Static SystemColors.DesktopBrushKey}}" />

Xamarin.Forms

<Label Text="Title" Style="{DynamicResource TitleStyle}" />