我的列表框没有更新

Question

I have a listbox to which I'm binding HistoryItems , where HistoryItems is a ObservableCollection of History . Here is the listbox declaration :

 <ListBox x:Name="RecentListBox" SelectionChanged="RecentListBox_SelectionChanged"   ItemsSource="{Binding HistoryItems,Converter={StaticResource HistoryValueConverter} }" ItemsPanel="{StaticResource ItemsPanelTemplate1_Wrap}" ItemTemplate="{StaticResource RecentViewModelTemplate}">

Here is the History class :

public class History : INotifyPropertyChanged
    {
        public History() { }

        int _id;
        string _date;
        string _url;
        string _name;

        public History(int id, string date,string url,string name)
        {
            this.id = id;
            this.date = date;
            this.url = url;
            this.name = name;
        }

        public int id
        {
            get
            {
                return _id;
            }
            set
            {
                if (value != _id)
                {
                    _id = value;
                    NotifyPropertyChanged("id");
                }
            }
        }
        public string date
        {
            get
            {
                return _date;
            }
            set
            {
                if (!value.Equals(_date))
                {
                    _date = value;
                    NotifyPropertyChanged("string");
                }
            }
        }

        public string url
        {
            get
            {
                return _url;
            }
            set
            {
                if (!value.Equals(_url))
                {
                    _url = value;
                    NotifyPropertyChanged("url");
                }
            }
        }

        public string name
        {
            get
            {
                return _name;
            }
            set
            {
                if (!value.Equals(_name))
                {
                    _name = value;
                    NotifyPropertyChanged("name");
                }
            }
        }

        public event PropertyChangedEventHandler PropertyChanged;
        private void NotifyPropertyChanged(String propertyName)
        {
            PropertyChangedEventHandler handler = PropertyChanged;
            if (null != handler)
            {
              //  App.viewModel.HistoryItems = (App.Current as App).dataHandler.retrieveHistory_DB();
                handler(this, new PropertyChangedEventArgs(propertyName));
            }
        }
    }

When I start the app the list gets populated, but after I do some modifs in some pivot children, and go back to the main panorama view, I try to update the HistoryItems in OnNavigatedTo :

 App.ViewModel.HistoryItems = (App.Current as App).dataHandler.retrieveHistory_DB();

but the listbox doesn't get updated (and the function returns the correct data). What could the problem be? History is INotifyPropertyChanged and the HistoryItems is a ObservableCollection<History> so there should be no problem.. What is causing the list to not update?

Answer 1

Since you are replacing HistoryItems when you refresh it doesn't matter that it's an ObservableCollection .

You can either clear the HistoryItems and then add the new items when you refresh. Or the ViewModel should implement INotifyPropertyChanged and the HistoryItems setter should raise the event.

Answer 2

Rewrite the setter for ViewModel.HistoryItems so that instead of doing this

_historyItems = value;

it does this

if (_historyItems == null) 
  _historyItems = new ObservableCollection<HistoryItem>();
_historyItems.Clear();
foreach (var hi in value) 
  _historyItems.Add(hi);

Answer 3

你需要NotifyPropertyChanged为App.ViewModel中的二传手HistoryItems