如果启用了异步任务，则异步任务的 UI 命令 *previous* 不起作用

Question

I must not grasp async stuff as good as I thought because this has had me scratching my head for a couple of hours now.

What I'm trying to accomplish is:

1. Switch UI to "Fetching data..."
2. Fetch data with an async call
3. Present final results based on async call

I have the following callback from my WPF button in my view

    private void Button_Click(object sender, RoutedEventArgs e)
    {
        GeneratorButton.IsEnabled = false;
        ResultMsg.Visibility = Visibility.Visible; //default text of "Fetching data..."
        string tickersString = Tickers.Text;

        if (!string.IsNullOrEmpty(tickersString))
        {
            tickersString = tickersString.ToUpper();
            string[] tickers = tickersString.Split(',', ' ');

            var stockGeneratorTask = Task.Factory.StartNew<bool>(() => GetStockDataAndGenerateCSV(tickers).Result);

            var allTasks = new Task[] { stockGeneratorTask };
            var tasksHandler = Task.Factory.ContinueWhenAll(allTasks, completedTasks =>
            {
                return stockGeneratorTask.Result;
            });

            if (tasksHandler.Result)
            {
                ResultMsg.Foreground = Brushes.Green;
                ResultMsg.Text = "Your report was successfully generated.";
            }
            else
            {
                ResultMsg.Text = "There was an error while generating your report. Please try again later.";
                ResultMsg.Foreground = Brushes.Red;
            }
        }

        GeneratorButton.IsEnabled = true;
    }

My expectations where that as soon as the user hits the button they'd see the default text of "fetching data..." due to the visibility property switch. What's happening, from what I understand, is that the program is going straight to the async task, and when it comes back it shows the success (or error) message, but it skips the "fetching data..." altogether UNLESS I comment out the code from the declaration of stockGeneratorTask down (no async call happening).

Now, if this visibility switch is happening before the async task gets triggered, shouldn't I see this change before it goes to actually make the async call? Why am I only seeing it after it comes back?

Answer 1

Event handlers are the one exception that allow for async void . Consider making the event handler async all the way and not mixing blocking calls like .Result which could lead to deadlocks. Assuming GetStockDataAndGenerateCSV returns Task<bool> then it should be awaited.

private async void Button_Click(object sender, RoutedEventArgs e) {
    GeneratorButton.IsEnabled = false;
    ResultMsg.Visibility = Visibility.Visible; //default text of "Fetching data..."
    var tickersString = Tickers.Text;

    if (!string.IsNullOrEmpty(tickersString)) {
        tickersString = tickersString.ToUpper();
        string[] tickers = tickersString.Split(',', ' ');
        if (await GetStockDataAndGenerateCSV(tickers)) {
            ResultMsg.Foreground = Brushes.Green;
            ResultMsg.Text = "Your report was successfully generated.";
        } else {
            ResultMsg.Foreground = Brushes.Red;
            ResultMsg.Text = "There was an error while generating your report. Please try again later.";
        }
    }

    GeneratorButton.IsEnabled = true;
}

Reference Async/Await - Best Practices in Asynchronous Programming