MvvmCross iOS Messenger使用求助：跨ViewModel触发带参方法

Hey there! No worries at all—starting with MvvmCross can feel a bit overwhelming at first, but MvxMessenger is absolutely the right tool for this cross-viewmodel communication. Let me walk you through a complete, step-by-step example tailored to your needs.

Step 1: Define a Custom Message Class #

First, we need a message type to carry your string parameter. Create a class that inherits from MvxMessage—this will hold the data we want to pass between viewmodels:

public class SearchTriggerMessage : MvxMessage
{
    // The string parameter you want to pass
    public string SearchQuery { get; }

    public SearchTriggerMessage(object sender, string searchQuery) : base(sender)
    {
        SearchQuery = searchQuery;
    }
}

Step 2: Send the Message from FilterViewModel #

In your FilterViewModel, inject IMvxMessenger and use it to publish the message when your button is clicked. We'll use an MvxCommand to bind to the button's click event:

public class FilterViewModel : MvxViewModel
{
    private readonly IMvxMessenger _messenger;
    public ICommand TriggerSearchCommand { get; }

    // Inject IMvxMessenger via constructor (MvvmCross IoC handles this automatically)
    public FilterViewModel(IMvxMessenger messenger)
    {
        _messenger = messenger;
        TriggerSearchCommand = new MvxCommand(TriggerSearch);
    }

    private void TriggerSearch()
    {
        // Replace this with your actual string parameter (e.g., from a text field)
        var searchQuery = "Your desired search term here";
        
        // Publish the message with the sender (this ViewModel) and the query
        _messenger.Publish(new SearchTriggerMessage(this, searchQuery));
    }
}

Step 3: Subscribe to the Message in SearchHistoryViewModel #

Now, in SearchHistoryViewModel, we'll subscribe to the SearchTriggerMessage and call your target method when the message is received. Don't forget to manage the subscription token to avoid memory leaks:

public class SearchHistoryViewModel : MvxViewModel
{
    private readonly IMvxMessenger _messenger;
    private MvxSubscriptionToken _searchTriggerSubscription;

    public SearchHistoryViewModel(IMvxMessenger messenger)
    {
        _messenger = messenger;
        SubscribeToSearchTriggers();
    }

    private void SubscribeToSearchTriggers()
    {
        // Subscribe to the message and define the handler
        _searchTriggerSubscription = _messenger.Subscribe<SearchTriggerMessage>(message =>
        {
            // Call your method with the received string parameter
            HandleSearchTrigger(message.SearchQuery);
        });

        // Optional: If you need to run the handler on the UI thread (e.g., updating UI elements), use:
        // _searchTriggerSubscription = _messenger.Subscribe<SearchTriggerMessage>(message =>
        // {
        //     HandleSearchTrigger(message.SearchQuery);
        // }, MvxMainThreadTaskScheduler.Instance);
    }

    // Your target method with the string parameter
    private void HandleSearchTrigger(string searchQuery)
    {
        // Add your business logic here—e.g., load search history for the query
        Console.WriteLine($"Received search trigger with query: {searchQuery}");
    }

    // Clean up the subscription when the ViewModel is no longer needed
    public override void ViewDisappeared()
    {
        base.ViewDisappeared();
        _searchTriggerSubscription?.Dispose();
    }
}

Key Notes to Remember #

• Dependency Injection: MvvmCross's built-in IoC container will automatically resolve IMvxMessenger for you, so you don't need to manually instantiate it.
• Threading: If your target method updates UI elements, make sure to use MvxMainThreadTaskScheduler.Instance in the subscribe call to ensure the handler runs on the UI thread.
• Subscription Cleanup: Always dispose of the MvxSubscriptionToken when the ViewModel is destroyed (like in ViewDisappeared) to prevent memory leaks.

内容的提问来源于stack exchange，提问作者Sven0567