无法在活动

Question

我正在构建一个Xamarin.Android应用程序。我有一个工作服务，我正在尝试从该服务获取数据到我有的活动。我知道人们对发布的代码过多感到不安，但我不确定我的问题在哪里。我要发布我认为必要的内容：

服务

[Service]
public class DataService : Service
{

    public List<MyObjects> MyObjectList { get; private set; }
    public IBinder Binder { get; private set; }
    public override IBinder OnBind(Intent intent)
    {
        this.Binder = new DataServiceBinder(this);
        return this.Binder;
    }

    public override void OnCreate()
    {
        base.OnCreate();
        pendingIntent = PendingIntent.GetActivity(this, 0, new Intent(this, typeof(MainActivity)), 0);
        StartForeground(NotificationID, GetNotification("Started Sync"));
        //Do some work here to instantiate and populate the List
    }
}

服务活页夹

public class DataServiceBinder : Binder
{
    public DataServiceBinder(DataService service)
    {
        Service = service;
    }

    public DataService Service { get; private set; }
}

服务连接

public class DataServiceConnection : Object, IServiceConnection
    {
        public DataService Service { get; private set; }
        public DataServiceBinder dataServiceBinder;
        public bool IsConnected { get; private set; }

        public void OnServiceConnected(ComponentName name, IBinder service)
        {
            dataServiceBinder = service as DataServiceBinder;
            IsConnected = this.dataServiceBinder != null;

            Service = dataServiceBinder.Service;
            ServiceConnectionChanged?.Invoke(this, true);
        }

        public void OnServiceDisconnected(ComponentName name)
        {
            ServiceConnectionChanged?.Invoke(this, false);
            Service = null;
        }

        public event EventHandler<bool> ServiceConnectionChanged;
    }

活动

public class MyDataActivity : AppCompatActivity
    {
        private DataServiceConnection DataServiceConnection;

        protected override void OnCreate(Bundle savedInstanceState)
        {
            base.OnCreate(savedInstanceState);

            if (DataServiceConnection == null)
            {
                this.DataServiceConnection = new DataServiceConnection();
            }

            Intent serviceToStart = new Intent(this, typeof(DataService));
            BindService(serviceToStart, this.DataServiceConnection, Bind.AutoCreate);

            SetContentView(Resource.Layout.MyDataLayout);

            var adapter = new MyDataAdapter(DataServiceConnection.Service.MyObjectList);
        }

        protected override void OnResume()
        {
            var intent = new Intent(this, typeof(DataService));
            BindService(intent, DataServiceConnection, Bind.AutoCreate);
            base.OnResume();
        }

        protected override void OnPause()
        {
            UnbindService(DataServiceConnection);
            base.OnPause();
        }

在我的活动中，当我尝试从服务传递适配器列表时，＆＃34;服务＆＃34;是null，因为我得到一个空引用异常。为什么我的服务无效？我没有正确绑定我的服务吗？对于记录，它们的服务在应用程序上下文中启动。

Answer 1

好的，我找到了解决方案。由于连接到服务是异步的，我不能指望适配器能够像这样同步获取数据：

var adapter = new MyDataAdapter(DataServiceConnection.Service.MyObjectList);

相反，我修改了我的活动，如下所示：

public class MyDataActivity : AppCompatActivity
    {
        private DataServiceConnection DataServiceConnection;

        protected override void OnCreate(Bundle savedInstanceState)
        {
            base.OnCreate(savedInstanceState);

            if (DataServiceConnection == null)
            {
                this.DataServiceConnection = new DataServiceConnection();
            }

            Intent serviceToStart = new Intent(this, typeof(DataService));
            BindService(serviceToStart, this.DataServiceConnection, Bind.AutoCreate);
            DataServiceConnection.ServiceConnectionChanged += ServiceConnectionChanged;
            SetContentView(Resource.Layout.MyDataLayout);

        }
        private void ServiceConnectionChanged(object sender, bool isConnected)
        {
            if(DataServiceConnection.Service == null)
            {
                return;
            }
            if(isConnected)
            {
                var adapter = new MyDataAdapter(DataServiceConnection.Service.MyObjectList);
            }
        }
        protected override void OnResume()
        {
            var intent = new Intent(this, typeof(DataService));
            BindService(intent, DataServiceConnection, Bind.AutoCreate);
            base.OnResume();
        }

        protected override void OnPause()
        {
            UnbindService(DataServiceConnection);
            base.OnPause();
        }

我也修改了我的服务连接：

public class DataServiceConnection : Object, IServiceConnection
    {
        public DataService Service { get; private set; }
        public MyDataActivity DataActivity;
        public event EventHandler<bool> ServiceConnectionChanged;

        public DataServiceConnection(MyDataActivity myDataActivity)
        {
            DataActivity = myDataActivity;
        }

        public void OnServiceConnected(ComponentName name, IBinder service)
        {
            dataServiceBinder = service as DataServiceBinder;
            Service = dataServiceBinder.Service;
            ServiceConnectionChanged?.Invoke(this, true);
        }

        public void OnServiceDisconnected(ComponentName name)
        {
            ServiceConnectionChanged?.Invoke(this, false);
            Service = null;
        }
    }

现在我的解决方案只在连接服务时尝试将数据发送到适配器。这个解决方案对我有用，我希望它可以帮助其他任何人。