问题
Question: I have two viewmodels that share a service with a list. My question is how to setup the notification so that both viewmodels know when this list is changed. Description below and code of where i'm at.
I found this example HERE that looks right for what I'm trying to do, but I have a question regarding how to be notified in my viewmodels of a change in my service. I'll add some code I've mocked up to see if I'm on the right track. I'm using WPF/MVVM Light.
First part is a service with a interface that will have a list of data, in this example I'm using a string list. I want a property in both viewmodels to have access to this list of data, and be notified when it's changed. I think what's throwing me is the interface IOC into my viewmodels. I'm understanding more and more why this is good, but I'm still wrapping my mind around it and I'm not sure how to setup the notification when the list is changed in the service. If my service was not injected I might have setup a event or property that my viewmodel property would access get/set but injecting my service does not expose my public fields, just methods. This is new to me so it's very likely i'm not understanding this correctly or missing something.
I used a List in my service instead of a ObservableCollection based on some reading I've done warning against using the ObservableCollection here. Thanks you for any help.
public class MyService : IMyService
{
private List<string> myList = new List<string>();
public List<string> getMyList()
{
return this.myList;
}
public void setMyList(List<string> value)
{
this.myList = value;
}
public void addValue(string value)
{
this.myList.Add(value);
}
public void insertValue(int index, string value)
{
this.myList.Insert(index, value);
}
}
public class MyViewModelOne : ViewModelBase
{
private readonly IMyService myService;
public MyViewModelOne(IMyService myService)
{
this.myService = myService;
}
public List<string> MyProperty // control item source will bind to this
{
get
{
return this.myService.getSource();
}
}
public void setSomeValue(value)
{
this.myService.addValue(value);
}
}
public class MyViewModelTwo : ViewModelBase
{
private readonly IMyService myService;
public MyViewModelTwo(IMyService myService)
{
this.myService = myService;
}
public List<string> MyProperty // control item source will bind to this
{
get
{
return this.myService.getSource();
}
}
public void setSomeValue(value)
{
this.myService.addValue(value);
}
}
回答1:
From what I understood about your problem, what you need essentially is that your INotifyPropertyChanged implementation at your service level and the list my list to be an ObservableCollection being injected from the service.
Now if there is a notification change it would be directly on the service and hence no explicit need of handling.
Your binding could look like "{Binding MyService.MyProperty}"
回答2:
I got this working two different ways, I went with the first example because I think it's easier to follow in the code.
This came up because I had a control in my mainview with related code that was growing and I realized I wanted the same control/behavior in a separate view that would use the same data/control for a different purpose. I did not want to duplicate this control/template/code in two places so I made it into a User Control. I then nest the user control in my views. The user control has it's own VM. The main view updates the service with new data, and the nested control listens on a event to know when there is new data.
Still very new to MVVM thinking so please feel free to point out in issues with either of these examples.
Example using a service with eventhandler.
public interface IMyInterface
{
event EventHandler OnSomeEvent;
void addSomeData(string value);
void getSomeData();
}
public class MyInterface: IMyInterface
{
public event EventHandler OnSomeEvent = delegate { };
public void addSomeData(string value)
{
// do stuff
OnSomeEvent();
}
public string getSomeData()
{
return "some data";
}
}
// Main ViewModel
public class ViewModelOne : ViewModelBase
{
IMyInterface myInterface;
public NotifyViewModel(IMyInterface myInterface)
{
this.myInterface = myInterface;
this.myInterface.OnItemSourceChanged += myInterface_OnSomeEvent;
}
void testEvent()
{
this.myInterface.addSomeData("test data");
}
}
// My nested user control
public class ViewModelTwo : ViewModelBase
{
IMyInterface myInterface;
public NotifyViewModel(IMyInterface myInterface)
{
this.myInterface = myInterface;
this.myInterface.OnItemSourceChanged += myInterface_OnSomeEvent;
}
void myInterface_OnSomeEvent(object sender, System.EventArgs e)
{
// do stuff
}
}
Example using MVVM Light Messaging
public class EventDataSource
{
public string MyItemSource { get; set; }
public EventDataSource()
{
MyItemSource = string.Empty;
}
}
// Message class
public class MyDataSourceMessage : MessageBase
{
public EventDataSource MyItemSource { get; set; }
public MyDataSourceMessage(EventDataSource myItemSource)
{
MyItemSource = myItemSource;
}
}
// Main ViewModel
public class ViewModelOne : ViewModelBase
{
public NotifyViewModel() {}
void testMessage()
{
EventDataSource msg = new EventDataSource() { MyItemSource = "magic message!"};
Messenger.Default.Send(new MyDataSourceMessage(msg as EventDataSource));
}
}
// My nested user control
public class ViewModelTwo : ViewModelBase
{
public NotifyViewModel()
{
Messenger.Default.Register<MyDataSourceMessage>(this, (action) => ReceiveMessage(action));
}
private ObservableCollection<string> myProperty = new ObservableCollection<string>();
public ObservableCollection<string> MyProperty
{
get { return myProperty; }
set
{
myProperty: = value;
RaisePropertyChanged(() => MyProperty);
}
}
void ReceiveMessage(MyDataSourceMessage action)
{
// do something with the data
MyProperty.Add(action.DGItemSource.ItemSource);
}
}
回答3:
Ok let me try to shed some light on this. First of all, change notification is not meant to pass information between view models, it is meant to notify the view itself that the a property of the view model has changed.
There are a few methods for view models to issue change notifications to views:
- INotifyPropertyChanged interface
- INotifyCollectionChanged interface
- A custom event with the name of the property suffixed with
Changed(e.g. an event calledMyPropChangedfor a property calledMyProp)
Having said all that, it is still possible for one view model to subscribe to the events generated by the above methods, and if you really need to, you may of course.
EDIT:
Check this link for a description on item number 3 above.
来源：https://stackoverflow.com/questions/29258494/multiple-viewmodels-sharing-a-service-with-notification