使用 Xamarin.Forms(适用于 iOS)，我尝试实现在继续之前等待用户确认已设置 GeoLocation 权限的功能。

我尝试实现这一点的方法是让线程等待直到使用 AutoResetEvent 触发事件。 .

主要问题(我相信)位于以下代码中:

manager.AuthorizationChanged += (object sender, CLAuthorizationChangedEventArgs args) => {

    Console.WriteLine ("Authorization changed to: {0}", args.Status);

    if (UIDevice.CurrentDevice.CheckSystemVersion (8, 0)) {
        tcs.SetResult (args.Status == CLAuthorizationStatus.AuthorizedAlways || args.Status == CLAuthorizationStatus.AuthorizedWhenInUse);
    } else {
        tcs.SetResult (args.Status == CLAuthorizationStatus.Authorized);
    }

    _waitHandle.Set ();
};

manager.Failed += (object sender, Foundation.NSErrorEventArgs e) => {

    Console.WriteLine ("Authorization failed");

    tcs.SetResult (false);

    _waitHandle.Set ();
};

if (UIDevice.CurrentDevice.CheckSystemVersion (8, 0)) {
    manager.RequestWhenInUseAuthorization ();
}

_waitHandle.WaitOne ();

public class LocationManager : ILocationManager
{
    static EventWaitHandle _waitHandle = new AutoResetEvent (false);

    private TaskCompletionSource<bool> tcs = new TaskCompletionSource<bool>();

    public LocationManager ()
    {
    }

    public Task<bool> IsGeolocationEnabledAsync()
    {
        Console.WriteLine (String.Format("Avaible on device: {0}", CLLocationManager.LocationServicesEnabled));
        Console.WriteLine (String.Format("Permission on device: {0}", CLLocationManager.Status));

        if (!CLLocationManager.LocationServicesEnabled) {
            tcs.SetResult (false);
        } else if (CLLocationManager.Status == CLAuthorizationStatus.Denied || CLLocationManager.Status == CLAuthorizationStatus.Restricted) {
            tcs.SetResult (false);
        } else if (CLLocationManager.Status == CLAuthorizationStatus.NotDetermined) {

            Console.WriteLine ("Waiting for authorisation");

            CLLocationManager manager = new CLLocationManager ();

            manager.AuthorizationChanged += (object sender, CLAuthorizationChangedEventArgs args) => {

                Console.WriteLine ("Authorization changed to: {0}", args.Status);

                if (UIDevice.CurrentDevice.CheckSystemVersion (8, 0)) {
                    tcs.SetResult (args.Status == CLAuthorizationStatus.AuthorizedAlways || args.Status == CLAuthorizationStatus.AuthorizedWhenInUse);
                } else {
                    tcs.SetResult (args.Status == CLAuthorizationStatus.Authorized);
                }

                _waitHandle.Set ();
            };

            manager.Failed += (object sender, Foundation.NSErrorEventArgs e) => {

                Console.WriteLine ("Authorization failed");

                tcs.SetResult (false);

                _waitHandle.Set ();
            };

            if (UIDevice.CurrentDevice.CheckSystemVersion (8, 0)) {
                manager.RequestWhenInUseAuthorization ();
            }

            _waitHandle.WaitOne ();

            Console.WriteLine (String.Format ("Auth complete: {0}", tcs.Task.Result));

        } else {
            if (UIDevice.CurrentDevice.CheckSystemVersion (8, 0)) {
                tcs.SetResult (CLLocationManager.Status == CLAuthorizationStatus.AuthorizedAlways || CLLocationManager.Status == CLAuthorizationStatus.AuthorizedWhenInUse);
            } else {
                tcs.SetResult (CLLocationManager.Status == CLAuthorizationStatus.Authorized);
            }
        }

        return tcs.Task;
    }
}

最佳答案

无 a good, minimal, complete code example可靠地重现问题，不可能确切地知道问题是什么。但是你的代码肯定有一个明显的设计缺陷，我希望解决这个缺陷会解决你的问题。

有什么缺陷？你正在等待任何事情。你写了一个显然应该代表异步操作的方法——它的名称中有“Async”并返回一个 Task<bool>而不是 bool - 然后你以这样的方式编写方法，Task<bool>无论采用哪个代码路径，返回的内容都将始终完成。

为什么这如此糟糕？好吧，除了它完全无法利用您正在实现的接口(interface)的异步方面这一简单事实之外，很可能 CLLocationManager您正在使用的类希望能够在您的 IsGeolocationEnabledAsync() 所在的同一线程中运行方法被调用。由于此方法在引发事件之前不会返回，并且由于在方法返回之前无法引发事件，因此您会遇到死锁。

恕我直言，这就是您的类(class)应该如何实现:

public class LocationManager : ILocationManager
{
    public async Task<bool> IsGeolocationEnabledAsync()
    {
        bool result;

        Console.WriteLine (String.Format("Avaible on device: {0}", CLLocationManager.LocationServicesEnabled));
        Console.WriteLine (String.Format("Permission on device: {0}", CLLocationManager.Status));

        if (!CLLocationManager.LocationServicesEnabled) {
            result = false;
        } else if (CLLocationManager.Status == CLAuthorizationStatus.Denied || CLLocationManager.Status == CLAuthorizationStatus.Restricted) {
            result = false;
        } else if (CLLocationManager.Status == CLAuthorizationStatus.NotDetermined) {
            TaskCompletionSource<bool> tcs = new TaskCompletionSource<bool>();

            Console.WriteLine ("Waiting for authorisation");

            CLLocationManager manager = new CLLocationManager ();

            manager.AuthorizationChanged += (object sender, CLAuthorizationChangedEventArgs args) => {

                Console.WriteLine ("Authorization changed to: {0}", args.Status);

                if (UIDevice.CurrentDevice.CheckSystemVersion (8, 0)) {
                    tcs.SetResult (args.Status == CLAuthorizationStatus.AuthorizedAlways || args.Status == CLAuthorizationStatus.AuthorizedWhenInUse);
                } else {
                    tcs.SetResult (args.Status == CLAuthorizationStatus.Authorized);
                }
            };

            manager.Failed += (object sender, Foundation.NSErrorEventArgs e) => {

                Console.WriteLine ("Authorization failed");

                tcs.SetResult (false);
            };

            if (UIDevice.CurrentDevice.CheckSystemVersion (8, 0)) {
                manager.RequestWhenInUseAuthorization ();
                result = await tcs.Task;
            } else {
                result = false;
            }

            Console.WriteLine (String.Format ("Auth complete: {0}", tcs.Task.Result));

        } else {
            if (UIDevice.CurrentDevice.CheckSystemVersion (8, 0)) {
                result = CLLocationManager.Status == CLAuthorizationStatus.AuthorizedAlways || CLLocationManager.Status == CLAuthorizationStatus.AuthorizedWhenInUse;
            } else {
                result = CLLocationManager.Status == CLAuthorizationStatus.Authorized;
            }
        }

        return result;
    }
}

关于C# AutoResetEvent 不释放，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/33553640/