结果未出现在选择器Xamarin.Forms中

Question

I have a code scanner that after scans, it returns me a string value. Then I add this string value to an observable collection.

When the ScanCode() method is triggered, the camera opens up and it scans a code then adds the value to the list. Then it gets back to the page, but the picker remains empty. Kindly help to figure out the issue.

private ObservableCollection<string> _codes;
public ObservableCollection<string> Codes
{
    get { return _codes; }
    set
    {
        _codes = value;
        OnPropertyChanged();
    }
}


public async void ScanCode()
{

    codes= new ObservableCollection<string>();

    var cd = await CodeScanViewModel.CodePage1(); // returns the code in string

    if (cd != null)
    {

        _codes.Add(cd.ToString());             
    }
}

Then in my XAML, I have defined the picker as follows:

            <Picker
                Title="Codes" 
                ItemsSource="{Binding Codes}"
                VerticalOptions="Center" />

Answer 1

At this point, as you have set a new instance of collection to _codes but the didn't raise the property-changed notification. So the Picker control is still listening to the older instance for collection-changed events. In order to fix that, make sure to use:

public async void ScanCode()
{
    Codes = new ObservableCollection<string>();
...

Or,

public async void ScanCode()
{
    _codes = new ObservableCollection<string>();
     OnPropertyChanged(nameof(Codes));
...