未等待 Await Action()

Question

我正在使用 ZXing.Net.Mobile 库扫描二维码。我想将负责扫描的代码放在 class 中，这样我就可以从中调用一个方法，它会 return 结果。类似的东西：

var scanner = new ZXing.Mobile.MobileBarcodeScanner();

var result = await scanner.Scan();

if (result != null)
    Console.WriteLine("Scanned Barcode: " + result.Text);

这是来自官方存储库的示例，但它不起作用（至少对我而言）。所以我创建了一个 class 并将 DID 工作的代码放在别处。这是 class 代码：

public class QrHandler 
{
    public ZXingScannerPage scanPage { get; set; }
    public string Result { get; set; }
    public async Task<string> Scan()
    {
        var options = new ZXing.Mobile.MobileBarcodeScanningOptions
        {
            PossibleFormats = new List<ZXing.BarcodeFormat>
            {
                ZXing.BarcodeFormat.QR_CODE
            },
            TryHarder = false,
            AutoRotate = false,
            TryInverted = false,
        };

        scanPage = new ZXingScannerPage();
        scanPage.AutoFocus();

        scanPage.OnScanResult += (result) =>
        {
            scanPage.IsScanning = false;

            Device.BeginInvokeOnMainThread(async () =>
            {
                Application.Current.MainPage.Navigation.PopAsync();
                try
                {
                    Result = result.Text;    
                }
                catch (Exception ex)
                {
                    Result = ex.Message;
                }
            });
        };

        await Application.Current.MainPage.Navigation.PushAsync(scanPage);
        return Result;
    }
}

我正在通过如下方法调用它：

public async Task<string> Validate()
{
    string _Result = "OK";
    QrHandler q = new QrHandler();
    _Result = await q.Scan();
    return _Result;
}

问题是 Validate() returns _Result 在 q.Scan() returns 任何值之前。 q.Scan() 完成后不应该触发它吗？

Answer 1

在执行 OnScanResult 事件处理程序之前，您不能从 Scan() return。

不清楚此事件何时引发，但只要它在您预期的时候实际引发，您可以使用 SemaphoreSlim:

在 Scan() 方法中异步阻止

public async Task<string> Scan()
{
    var options = new ZXing.Mobile.MobileBarcodeScanningOptions
    {
        PossibleFormats = new List<ZXing.BarcodeFormat>
        {
            ZXing.BarcodeFormat.QR_CODE
        },
        TryHarder = false,
        AutoRotate = false,
        TryInverted = false,
    };

    scanPage = new ZXingScannerPage();
    scanPage.AutoFocus();

    using (SemaphoreSlim semaphoreSlim = new SemaphoreSlim(0, 1))
    {
        scanPage.OnScanResult += (result) =>
        {
            scanPage.IsScanning = false;

            Device.InvokeOnMainThread(async () =>
            {
                await Application.Current.MainPage.Navigation.PopAsync();
                try
                {
                    Result = result.Text;
                }
                catch (Exception ex)
                {
                    Result = ex.Message;
                }
                semaphore.Release();
            });
        };

        await Application.Current.MainPage.Navigation.PushAsync(scanPage);
        await semaphoreSlim.WaitAsync();
    }

    return Result;
}

未等待 Await Action()

Await Action() is not awaited

c#

xamarin.forms

zxing.net