咨询：使用Reactive Subject触发DynamicData缓存更新的实现合理性

Hey there! Let's break down your implementation step by step—since you're new to Rx/RxUI/DynamicData, it's awesome you're experimenting with these tools, and there are a few tweaks and best practices to fix up your approach and avoid common pitfalls.

First off, your core idea is solid: load initial local data, then trigger server syncs on demand to update your cache. But let's dive into the details:

1. Subject Lifecycle & Management #

Your use of Subject<Unit> to trigger server calls works, but you're missing a key step: adding it to your cleanup disposable. Right now, when MyService is disposed, the subject won't be cleaned up, which could lead to memory leaks or lingering subscriptions. Fix this by adding _serverSubject to your CompositeDisposable:

_cleanup = new CompositeDisposable(AllData, data.Connect(), _serverSubject);

Also, while Subject gets the job done, if you're already using RxUI, consider swapping it for a ReactiveCommand (if the trigger comes from user interaction in a ViewModel) — it handles thread scheduling, error handling, and lifecycle management out of the box, eliminating the need for manual Subject usage entirely.

2. Async Handling in ObservableChangeSet.Create #

You’re using an async delegate inside ObservableChangeSet.Create, which is a problem. The Create method expects a Func<ISourceCache<MyData, Guid>, IDisposable>, but an async delegate returns a Task<IDisposable> instead. This can cause race conditions where your initial load runs after the cache is already exposed, or unexpected thread behavior.

Instead, wrap async operations with Observable.FromAsync to keep everything in Rx's reactive paradigm:

private IObservable<IChangeSet<MyData, Guid>> Initialise()
{
    return ObservableChangeSet.Create<MyData, Guid>(cache =>
    {
        // Wrap initial local load in Observable.FromAsync
        var initialLoad = Observable.FromAsync(() => LoadLocalData())
            .ObserveOn(RxApp.MainThreadScheduler) // Match Xamarin.Forms UI thread if needed
            .Subscribe(localData => cache.AddOrUpdate(localData),
                       ex => Console.WriteLine($"Initial load failed: {ex.Message}"));

        // Simplify server sync with SelectMany + FromAsync (no nested await!)
        var serverSync = _serverSubject
            .SelectMany(_ => Observable.FromAsync(() => GetDataFromServer()))
            .ObserveOn(RxApp.MainThreadScheduler)
            .Subscribe(serverData => cache.AddOrUpdate(serverData),
                       ex => Console.WriteLine($"Server sync failed: {ex.Message}"));

        return new CompositeDisposable(initialLoad, serverSync);
    }, d => d.Id);
}

This ensures async operations are properly integrated into the Rx pipeline, with clear error handling and thread scheduling (critical for Xamarin.Forms apps to avoid UI thread issues).

3. Thread Scheduling Best Practices #

Notice the ObserveOn(RxApp.MainThreadScheduler) calls above. DynamicData's ObservableCache isn't fully thread-safe across arbitrary threads, so using RxUI's built-in schedulers ensures cache updates happen on a consistent thread (usually the UI thread for Xamarin.Forms, since you'll likely bind this data to UI elements).

Also, in GetDataFromServer, stick with ConfigureAwait(false) — since this is a background operation, you don't need to return to the original thread, which improves performance.

4. Your Publish/Connect Usage is Correct! #

Great job using Publish() and Connect() on your initial data sequence. This ensures all subscribers to AllData share the same underlying sequence, so you don't re-run the initial load or re-subscribe to the server trigger multiple times. That's exactly the right approach for shared data caches.

Updated Full Code #

Here's the revised version of your service incorporating all these fixes:

using DynamicData;
using System;
using System.Reactive;
using System.Reactive.Disposables;
using System.Reactive.Linq;
using System.Reactive.Subjects;
using System.Threading.Tasks;
using ReactiveUI;

public class MyService : IDisposable
{
    private CompositeDisposable _cleanup;
    private Subject<Unit> _serverSubject = new Subject<Unit>();

    public MyService()
    {
        var data = Initialise().Publish();
        AllData = data.AsObservableCache();
        _cleanup = new CompositeDisposable(AllData, data.Connect(), _serverSubject);
    }

    public IObservableCache<MyData, Guid> AllData { get; }

    public void TriggerServer()
    {
        _serverSubject.OnNext(Unit.Default);
    }

    private IObservable<IChangeSet<MyData, Guid>> Initialise()
    {
        return ObservableChangeSet.Create<MyData, Guid>(cache =>
        {
            var initialLoad = Observable.FromAsync(() => LoadLocalData())
                .ObserveOn(RxApp.MainThreadScheduler)
                .Subscribe(localData => cache.AddOrUpdate(localData),
                           ex => Console.WriteLine($"Initial load error: {ex.Message}"));

            var serverSync = _serverSubject
                .SelectMany(_ => Observable.FromAsync(() => GetDataFromServer()))
                .ObserveOn(RxApp.MainThreadScheduler)
                .Subscribe(serverData => cache.AddOrUpdate(serverData),
                           ex => Console.WriteLine($"Server sync error: {ex.Message}"));

            return new CompositeDisposable(initialLoad, serverSync);
        }, d => d.Id);
    }

    private IObservable<MyData> LoadLocalData()
    {
        return Observable.Timer(TimeSpan.FromSeconds(3))
            .Select(_ => new MyData("localdata"));
    }

    private async Task<MyData> GetDataFromServer()
    {
        await Task.Delay(2000).ConfigureAwait(false);
        return new MyData("serverdata");
    }

    public void Dispose()
    {
        _cleanup?.Dispose();
    }
}

public class MyData
{
    public MyData(string value)
    {
        Value = value;
    }

    public Guid Id { get; } = Guid.NewGuid();
    public string Value { get; set; }
}

Final Takeaways #

• Your core logic is on the right track — using DynamicData for reactive data caching is perfect for Xamarin.Forms/RxUI apps.
• The biggest pitfalls you were approaching were improper async handling in the cache creation and missing Subject lifecycle management.
• As you get more comfortable, try replacing the Subject with ReactiveCommand tied to your ViewModel's user actions — it's more idiomatic for RxUI and reduces boilerplate.

内容的提问来源于stack exchange，提问作者Paul Charlton