如果启用了异步任务，则UI命令* previous * to async task不起作用

Question

我不应该像我想的那样抓住异步的东西，因为这让我头疼了几个小时。

我想要完成的是：

1. 将UI切换为“获取数据...”
2. 使用异步调用获取数据
3. 基于异步调用显示最终结果

在我看来，我的WPF按钮有以下回调

    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;
    }

我的期望是，只要用户点击按钮，他们就会看到由于可见性属性开关而导致的“获取数据...”的默认文本。根据我的理解，正在发生的事情是程序直接进入异步任务，当它返回时它会显示成功（或错误）消息，但它会完全跳过“获取数据......”，除非我发表评论从stockGeneratorTask声明中删除代码（没有异步调用发生）。

现在，如果在异步任务被触发之前发生了这种可见性切换，那么在实际进行异步调用之前，我不应该看到这种变化吗？为什么我回来后才看到它？

Answer 1

事件处理程序是允许async void的一个例外。考虑使事件处理程序始终异步，而不是混合阻塞调用，如.Result，这可能导致死锁。假设GetStockDataAndGenerateCSV返回Task<bool>然后它应该等待。

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;
}

参考Async/Await - Best Practices in Asynchronous Programming