collectionView SelectedChangedCommand executes multiple times – .NET MAUI: The Mystery Solved!

Are you tired of dealing with the infamous issue of `CollectionView` `SelectedChangedCommand` executing multiple times in your .NET MAUI application? Well, you’re not alone! This pesky problem has been plaguing developers for a while now, but fear not, dear reader, for today we’ll dive deep into the heart of the issue and provide you with a comprehensive solution.

What’s the fuss about?

The `CollectionView` in .NET MAUI is an incredibly powerful tool for displaying and managing collections of data. One of its most useful features is the `SelectedChangedCommand`, which allows you to execute a command whenever the selection in the collection changes. However, in some cases, this command can start firing off multiple times, leading to unexpected behavior and potential performance issues.

Why does it happen?

Before we dive into the solution, let’s quickly discuss why this issue occurs in the first place. There are several reasons why the `SelectedChangedCommand` might execute multiple times:

• Incorrect binding configuration: If the binding is not properly set up, the command can be executed multiple times due to the way the collection is refreshed.
• ListView grouping: When using grouping in a `ListView`, the selection change command can be triggered multiple times for each group change.
• Two-way binding: If the `SelectedItem` property is bound to a viewmodel property using two-way binding, the command can be executed multiple times when the selection changes.
• Custom behavior: In some cases, custom behaviors or renderers can interfere with the selection change event, causing the command to execute multiple times.

The solution: Debouncing the command execution

Now that we’ve covered the reasons behind the issue, let’s get to the good stuff! One of the most effective ways to solve this problem is by debouncing the command execution. Debouncing is a technique that prevents a function from being executed multiple times in a short period by delaying its execution until a certain amount of time has passed since the last invocation.

Using a DebounceBehavior

In .NET MAUI, we can create a custom `DebounceBehavior` to debounce the `SelectedChangedCommand` execution. Here’s an example implementation:

<local:DebounceBehavior 
    DefaultDelay="500" 
    DebounceCommand="{Binding SelectedChangedCommand}" />

In this example, the `DebounceBehavior` is applied to the `ListView` and is bound to the `SelectedChangedCommand`. The `DefaultDelay` property specifies the amount of time (in milliseconds) that the command should be delayed before execution.

Implementing the DebounceBehavior

Now, let’s implement the `DebounceBehavior` class:

public class DebounceBehavior : Behavior<BindableObject>
{
    public static readonly BindableProperty DebounceCommandProperty =
        BindableProperty.CreateAttached("DebounceCommand", typeof(ICommand), typeof(DebounceBehavior));

    public static readonly BindableProperty DefaultDelayProperty =
        BindableProperty.CreateAttached("DefaultDelay", typeof(int), typeof(DebounceBehavior), 500);

    public ICommand DebounceCommand
    {
        get { return (ICommand)GetValue(DebounceCommandProperty); }
        set { SetValue(DebounceCommandProperty, value); }
    }

    public int DefaultDelay
    {
        get { return (int)GetValue(DefaultDelayProperty); }
        set { SetValue(DefaultDelayProperty, value); }
    }

    private bool _isCommandExecuting;

    protected override void OnAttachedTo(BindableObject bindable)
    {
        base.OnAttachedTo(bindable);

        if (bindable is ListView listView)
        {
            listView.ItemSelected += OnItemSelected;
        }
    }

    protected override void OnDetachingFrom(BindableObject bindable)
    {
        base.OnDetachingFrom(bindable);

        if (bindable is ListView listView)
        {
            listView.ItemSelected -= OnItemSelected;
        }
    }

    private async void OnItemSelected(object sender, SelectedItemChangedEventArgs e)
    {
        if (_isCommandExecuting) return;

        _isCommandExecuting = true;

        await Task.Delay(DefaultDelay);

        if (DebounceCommand.CanExecute(null))
        {
            DebounceCommand.Execute(null);
        }

        _isCommandExecuting = false;
    }
}

In this implementation, we’ve created a `DebounceBehavior` that attaches to a `ListView` and listens for the `ItemSelected` event. When the event is triggered, the behavior delays the execution of the `DebounceCommand` by the specified `DefaultDelay` amount. This ensures that the command is executed only once, even if the selection changes rapidly.

Other solutions: Command throttling and rate limiting

While debouncing is an effective solution, there are other approaches you can take to solve this issue. Two other methods worth considering are command throttling and rate limiting.

Command throttling

Command throttling involves limiting the frequency of command execution within a certain time window. This approach is useful when you want to ensure that the command is executed at least once within a certain time frame.

public class ThrottleCommand : ICommand
{
    private readonly ICommand _command;
    private readonly int _throttleInterval;

    public ThrottleCommand(ICommand command, int throttleInterval)
    {
        _command = command;
        _throttleInterval = throttleInterval;
    }

    public bool CanExecute(object parameter)
    {
        return _command.CanExecute(parameter);
    }

    public void Execute(object parameter)
    {
        if (_lastExecution + _throttleInterval < DateTime.Now.Ticks)
        {
            _command.Execute(parameter);
            _lastExecution = DateTime.Now.Ticks;
        }
    }

    private DateTime _lastExecution;
}

In this example, we’ve created a `ThrottleCommand` class that takes an `ICommand` and a throttle interval (in milliseconds). The command is executed only if the last execution was more than the throttle interval ago.

Rate limiting

Rate limiting involves limiting the number of command executions within a certain time window. This approach is useful when you want to prevent excessive command executions.

public class RateLimitedCommand : ICommand
{
    private readonly ICommand _command;
    private readonly int _maxExecutions;
    private readonly int _executionWindow;

    public RateLimitedCommand(ICommand command, int maxExecutions, int executionWindow)
    {
        _command = command;
        _maxExecutions = maxExecutions;
        _executionWindow = executionWindow;
    }

    public bool CanExecute(object parameter)
    {
        return _command.CanExecute(parameter);
    }

    public void Execute(object parameter)
    {
        if (_executionCount < _maxExecutions && _lastExecution + _executionWindow <= DateTime.Now.Ticks)
        {
            _command.Execute(parameter);
            _executionCount++;
        }
    }

    private int _executionCount;
    private DateTime _lastExecution;
}

In this example, we’ve created a `RateLimitedCommand` class that takes an `ICommand`, a maximum number of executions, and an execution window (in milliseconds). The command is executed only if the number of executions is less than the maximum allowed and the last execution was more than the execution window ago.

Conclusion

In this article, we’ve explored the issue of `CollectionView` `SelectedChangedCommand` executing multiple times in .NET MAUI and presented several solutions to debounce the command execution. By using a debounce behavior, command throttling, or rate limiting, you can ensure that your command is executed only once, even in cases where the selection changes rapidly. Remember to choose the approach that best fits your specific use case and requirements.

Solution	Description
DebounceBehavior	Delays the command execution by a specified amount of time to prevent multiple executions.
Command Throttling	Limits the frequency of command execution within a certain time window.
Rate Limiting	Limits the number of command executions within a certain time window.

By applying these solutions, you’ll be able to tame the beast of multiple command executions and create a more stable and efficient .NET MAUI application.

Frequently Asked Question

Are you tired of dealing with the frustration of CollectionView’s SelectedChangedCommand firing multiple times in .NET MAUI? You’re not alone! Here are some commonly asked questions and answers to help you troubleshoot and solve this pesky problem: