MessagingCenter回调未被调用

Question

我正在使用Xamarin MessagingCenter实现设备定位检测器。我想要做的是从我的安卓项目中的MainActivity发送消息到我的.NET标准项目中的Singleton类实现。

如您所见，我重写了“OnConfigurationChanged(.)”方法在我的MainActivity内部和所有断点在我的IF语句中被击中时，我切换方向从景观到肖像。问题是，我较新地接收到了这些消息。在我的"OrientationHelper“中回调是较新的调用。

"OrientationHelper“在加载第一个页面时被实例化(对于那些会说我没有实例的人：)

MainActivity：

public override void OnConfigurationChanged(Android.Content.Res.Configuration newConfig)
{
    base.OnConfigurationChanged(newConfig);

    if (newConfig.Orientation == Android.Content.Res.Orientation.Landscape)
        MessagingCenter.Send(this, "OrientationContract"
            , new OrientationChangedEventArgs(Orientation.Landscape));

    else if (newConfig.Orientation == Android.Content.Res.Orientation.Portrait)
        MessagingCenter.Send(this, "OrientationContract"
            , new OrientationChangedEventArgs(Orientation.Portrait));
}

将从MainActivity接收消息的Singleton类：

public class OrientationHelper
{
    private OrientationHelper()
        => MessagingCenter.Subscribe<OrientationChangedEventArgs>(this, "OrientationContract"
            , s => DeviceOrientation = s.Orientation);

    private static OrientationHelper s_instace;
    public static OrientationHelper Instance
    {
        get
        {
            if (s_instace == null)
                s_instace = new OrientationHelper();
            return s_instace;
        }
    }

    private Orientation _deviceOrientation;
    public Orientation DeviceOrientation
    {
        get => _deviceOrientation;
        private set
        {
            if (_deviceOrientation == value)
                return;
            _deviceOrientation = value;
        }
    }
}

OrientationChangedEventArgs：

public class OrientationChangedEventArgs : EventArgs
{
    public Orientation Orientation { get; private set; }

    public OrientationChangedEventArgs(Orientation orientation)
        => Orientation = orientation;
}

Answer 1

订阅和发送方法的定义如下

• 订阅(对象订阅者，字符串消息，动作回调，TSender源= null)
• 发送(TSender发送者，字符串消息)发送(TSender发送方，字符串消息，TArgs args)

这两个调用中的第一个T参数应该匹配发送消息的类的类型。

MessagingCenter.Send<MyType, OrientationChangedEventArgs>(this, "OrientationContract"
        , new OrientationChangedEventArgs(Orientation.Landscape));

MessagingCenter.Subscribe<MyType, OrientationChangedEventArgs>(this, "OrientationContract"
        , s => DeviceOrientation = s.Orientation);