如何处理 await 方法中的事件引发以管理 Windows 商店应用程序中的 ProgressBar？

Question

我想在方法调用时显示进度条。我已经按照这张票中的建议处理了它：

它不等待在当前事件中引发的事件。即

public async void navigationHelper_LoadState(object sender, LoadStateEventArgs e)
{
    loading.Visibility = Windows.UI.Xaml.Visibility.Visible;
    await initializeThePage();
    loading.Visibility = Windows.UI.Xaml.Visibility.Collapsed;
}

//following method doesn't wait for SelectionChanged event. It should wait
private async Task initializeThePage()
{
    //few service call here
    cmdStore.SelectedIndex = 1;     //this should wait until cmdStore_SelectionChanged complete its call
}

private async void cmdStore_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
    //few service call here
    loading.Visibility = Windows.UI.Xaml.Visibility.Visible;
    await getOrderlistAccordingToStore(storeID);            //get store details 
    loading.Visibility = Windows.UI.Xaml.Visibility.Collapsed;
}

public async Task getOrderlistAccordingToStore(int storeID)
{
    //calls of get orderlist according to selected store
}

当cmdStore.SelectedIndex = 1;调用时，它调用cmdStore_SelectionChanged方法。在这种情况下 await initializeThePage(); 应该等到 cmdStore_SelectionChanged 完成。但它不会等待此事件，而是继续执行下一次。

有人可以建议我如何处理当前 await 调用中引发的事件吗？

Answer 1

cmdStore_SelectionChanged 绑定到事件处理程序并且是 async void。不能直接等待这个方法执行完成

您可以采取一些变通办法来解决这个问题。首先介绍show/hide加载器的辅助方法。

int waitCounter = 0;
void ShowLoader()
{
    waitCounter += 1;
    if (waitCounter > 1) // Already loader is visible 
        return;

    loading.Visibility = Windows.UI.Xaml.Visibility.Visible;
}

void HideLoader()
{
    waitCounter -= 1;

    if (waitCounter <= 0) // No more call to wait for 
       loading.Visibility = Windows.UI.Xaml.Visibility.Collapsed;
}

之后将您的实现修改为如下所示

public async void navigationHelper_LoadState(object sender, LoadStateEventArgs e)
{
    ShowLoader()
    await initializeThePage();
    HideLoader()
}

private async Task initializeThePage()
{
    //few service call here
    cmdStore.SelectedIndex = 1;
}

private async void cmdStore_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
    ShowLoader();
    //few service call here
    await getOrderlistAccordingToStore(storeID);            //get store details 
    HideLoader();
}

public async Task getOrderlistAccordingToStore(int storeID)
{
    //calls of get orderlist according to selected store
}

当页面首次初始化时，加载器将可见，并在 cmdStore_SelectionChanged 和 initializeThePage 完成其执行后消失。

如何处理 await 方法中的事件引发以管理 Windows 商店应用程序中的 ProgressBar？

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