Xamarin Forms异步回调函数问题求助：FinishPurchase未触发

Hey there! Let's dig into why your FinishPurchase method isn't getting triggered and get that callback working properly.

First, let's look at the core problem with your current code snippet:

Task<bool> task = StockDataController.Instance.WasItemPurchased(CloudInfo.IAPMonthlySubscriptionProductID);
Task continuation = task.ContinueWith(t=>FinishPurchase());
continuation.W...

There are a few common reasons FinishPurchase might not be firing, and better ways to handle async callbacks in Xamarin.Forms that avoid these pitfalls.

1. The Best Practice: Use async/await (Simpler & More Reliable) #

ContinueWith can be tricky with thread contexts (especially for UI operations) and error handling. Xamarin.Forms works best with async/await, which automatically manages thread synchronization and makes your code easier to debug.

Here's how to rewrite your button click event with this approach:

private async void PurchaseButton_Clicked(object sender, EventArgs e)
{
    try
    {
        // Wait for the purchase check task to complete
        bool wasPurchased = await StockDataController.Instance.WasItemPurchased(CloudInfo.IAPMonthlySubscriptionProductID);
        
        // Optional: Use the wasPurchased result if you need to conditionally run FinishPurchase
        // if (wasPurchased)
        // {
            FinishPurchase();
        // }
    }
    catch (Exception ex)
    {
        // Handle any errors from the purchase check (e.g., network issues, API errors)
        Console.WriteLine($"Failed to check purchase status: {ex.Message}");
        // You could also show a user-friendly alert here
        // await DisplayAlert("Error", "Could not verify your purchase", "OK");
    }
}

This ensures:

• The task completes before calling FinishPurchase
• Any exceptions from WasItemPurchased are caught and handled
• UI operations in FinishPurchase run on the main thread (since await marries back to the original context)

2. If You Must Use ContinueWith (Fixing Your Original Code) #

If you need to stick with ContinueWith, you need to account for task states (success, failure, cancellation) and ensure UI operations run on the main thread:

private void PurchaseButton_Clicked(object sender, EventArgs e)
{
    Task<bool> purchaseCheckTask = StockDataController.Instance.WasItemPurchased(CloudInfo.IAPMonthlySubscriptionProductID);
    
    purchaseCheckTask.ContinueWith(t => 
    {
        if (t.IsCompletedSuccessfully)
        {
            // Make sure we run FinishPurchase on the main thread (critical for UI updates)
            Device.BeginInvokeOnMainThread(() => FinishPurchase());
        }
        else if (t.IsFaulted)
        {
            // Handle task failures (e.g., log errors, alert user)
            Device.BeginInvokeOnMainThread(() => 
            {
                Console.WriteLine($"Purchase check failed: {t.Exception?.InnerException?.Message}");
            });
        }
        else if (t.IsCanceled)
        {
            Device.BeginInvokeOnMainThread(() => 
            {
                Console.WriteLine("Purchase check was canceled");
            });
        }
    }, TaskScheduler.FromCurrentSynchronizationContext()); // Ensure continuation runs on UI thread
}

Why Your Original Code Might Have Failed #

• Task never completed: If WasItemPurchased is stuck waiting for a response (e.g., network timeout, unhandled async operation), the continuation won't fire.
• Uncaught exceptions: If WasItemPurchased threw an error, your original ContinueWith would still run, but if FinishPurchase relies on a successful task result, it might not execute as expected (or crash silently if it's on a non-UI thread).
• Thread context issues: Without specifying TaskScheduler.FromCurrentSynchronizationContext(), the continuation might run on a background thread, and if FinishPurchase updates UI elements, it could fail silently or throw an exception.

内容的提问来源于stack exchange，提问作者Jason Smith