Attached Behaviors have been around for quite a while, and though I personally always liked them, they have a fundamental flaw: they can lead to huge memory leaks!
I’ve seen quite a few fixes for them (like this one from MVP Joost Van Schaik), though none proved to be truly “universal” and final!
To demonstrate the problem, let’s take a practical example:
- create an app with two pages
- page 1 will have a button to navigate to page 2
- page 2 will have a button that when clicked, will navigate back to page 1 after a pause of 2 seconds
Here’s the view model for page 2, Page2ViewModel:
public class Page2ViewModel
{
public event EventHandler GoBack;
public ICommand DelayedGoBackCommand { get; private set; }
public MainViewModel()
{
DelayedGoBackCommand = new CustomCommand(OnDelayedGoBackCommand);
}
private async void OnDelayedGoBackCommand()
{
await Task.Delay(2000);
GoBack?.Invoke(this, EventArgs.Empty);
}
}
The code is really simple: when the DelayedGoBackCommand gets invoked, we will make a 2 seconds pause, and then raise the GoBack event.
Now lets say that all view models have been registered as singletons in the App class, something like this:
sealed partial class App : Application
{
public static Page2ViewModel Page2ViewModel { get; } = new Page2ViewModel();
// remaining code
}
This is the code behind for page 2, where we will set the page view model:
public class Page2
{
public Page2()
{
InitializeComponent();
DataContext = App.Page2ViewModel;
}
}
And this is the view we will be using for page 2:
<Page x:Class="MyApp.Page2"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:core="using:Microsoft.Xaml.Interactions.Core"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:i="using:Microsoft.Xaml.Interactivity"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
x:Name="PageRoot"
mc:Ignorable="d">
<StackPanel Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
<i:Interaction.Behaviors>
<core:EventTriggerBehavior EventName="GoBack"
SourceObject="{Binding}">
<core:CallMethodAction MethodName="GoBack"
TargetObject="{Binding Frame,
ElementName=PageRoot}" />
</core:EventTriggerBehavior>
</i:Interaction.Behaviors>
<Button Command="{Binding DelayedGoBackCommand,
Mode=OneTime}"
Content="Go Back" />
</StackPanel>
</Page>
As you can see above, the button is binded to the Page2ViewModel.DelayedGoBackCommand. Also, we will be using an EventTriggerBehavior to monitor the Page2ViewModel.GoBack event, and when it gets raised, we will use the CallMethodAction to invoke the Page2.Frame.GoBack method.
Now here’s the catch: while behaviors have a way of being notified when they are to be detached, that never happens!! Not when you leave the page, not when the page gets unloaded, and not even when garbage collection runs.
As such, when we navigate back from page 2, Page2ViewModel.GoBack event will still hold a reference to the EventTriggerBehavior, leading to a memory leak!
But things get even worse in this example: everytime we navigate to page 2, we will subscribe over and over again the GoBack event, so multiple invocations will eventually occur - definitely not what we wanted!
Introducing the MonitoredInteraction class
The Cimbalino Toolkit now has the MonitoredInteraction class to solve this issue!
The MonitoredInteraction was built as a direct replacement of the Microsoft.Xaml.Interactivity.Interaction, and will monitor the attached object Loaded and Unloaded events and call for the attachment and detachment of all behaviors it contains!
Here’s how page 2 view would look if put it to use:
<Page x:Class="MyApp.Page2"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:core="using:Microsoft.Xaml.Interactions.Core"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:behaviors="using:Cimbalino.Toolkit.Behaviors"
x:Name="PageRoot"
mc:Ignorable="d">
<StackPanel Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
<behaviors:MonitoredInteraction.Behaviors>
<core:EventTriggerBehavior EventName="GoBack"
SourceObject="{Binding}">
<core:CallMethodAction MethodName="GoBack"
TargetObject="{Binding Frame,
ElementName=PageRoot}" />
</core:EventTriggerBehavior>
</behaviors:MonitoredInteraction.Behaviors>
<Button Command="{Binding DelayedGoBackCommand,
Mode=OneTime}"
Content="Go Back" />
</StackPanel>
</Page>
Really, really simple, yet it will finally ensure that behaviors do get detached when one navigates away of a page, and re-attached when navigating in!
Hopefully someone from the Behaviors SDK team will read this, and take some of the suggestions here to fix this well known issue that’s been around for quite a while! ;)
