Question

我有一个Xamarin应用程序实现搜索功能，其中结果被分组。因此我使用了Grouped Listview。

private async void SearchRecipient()
{
    IList<Recipient> recipients = null;

    if (!string.IsNullOrWhiteSpace(RecipientSearched))
    {   
        recipients = await _service.GetRecipients(RecipientSearched.ToLower());

        FilteredRecipients.Clear();
        _userGroupedList.Clear();
        _officeGroupedList.Clear();

        if (recipients != null)
        {
            foreach (var r in recipients)
            {
                // Some logic to populate collections
                _userGroupedList.Add(selectable);
                _officeGroupedList.Add(selectable);
            }

            if (_userGroupedList.Count > 0)
                FilteredRecipients.Add(_userGroupedList);
            if (_officeGroupedList.Count > 0)
                FilteredRecipients.Add(_officeGroupedList);
        }
    }
}

FilteredRecipients是ObservableCollection，而_userGroupedList和_officeGroupedList是List。

public SearchRecipientPageModel()
{
    FilteredRecipients = new ObservableCollection<GroupedRecipientModel>();
    _userGroupedList = new GroupedRecipientModel("User");
    _officeGroupedList = new GroupedRecipientModel("Office");
}

搜索工作和分组也是如此。当我第二次重复搜索并且FilteredRecipients.Clear()抛出以下异常时会发生问题：

System.ArgumentOutOfRangeException: 'Index was out of range. Must be non-negative and less than the size of the collection. Parameter name: index'

更新只有在使用复选框选择了某些结果项时，才会出现问题。我认为是由于我的Checkbox Renderer实现，因为我用Switch替换了Checkbox，它似乎工作。我在使用TwoWay模式绑定时遇到了一些问题，但也许我没有正确修复它。

    public class CustomCheckBox : View
{
    public bool Checked
    {
        get => (bool)GetValue(CheckedProperty);
        set => SetValue(CheckedProperty, value);
    }

    public ICommand Command
    {
        get => (ICommand)GetValue(CommandProperty);
        set => SetValue(CommandProperty, value);
    }

    public object CommandParameter
    {
        get => GetValue(CommandParameterProperty);
        set => SetValue(CommandParameterProperty, value);
    }

    public static readonly BindableProperty CommandParameterProperty =
                                BindableProperty.Create("CommandParameter", typeof(object), typeof(CustomCheckBox), default(object));

    public static readonly BindableProperty CheckedProperty =
                                BindableProperty.Create("Checked", typeof(bool), typeof(CustomCheckBox), default(bool), propertyChanged: OnChecked);

    public static readonly BindableProperty CommandProperty =
                BindableProperty.Create("Command", typeof(ICommand), typeof(CustomCheckBox), default(ICommand));


    private static void OnChecked(BindableObject bindable, object oldValue, object newValue)
    {
        if (bindable is CustomCheckBox checkbox)
        {
            object parameter = checkbox.CommandParameter ?? newValue;

            if (checkbox.Command != null && checkbox.Command.CanExecute(parameter))
                checkbox.Command.Execute(parameter);
        }
    }
}

渲染

public class CustomCheckBoxRenderer : ViewRenderer<CustomCheckBox, CheckBox>
{
    protected override void OnElementChanged(ElementChangedEventArgs<CustomCheckBox> e)
    {
        base.OnElementChanged(e);
        if (Control == null && Element != null)
            SetNativeControl(new CheckBox());

        if (Control != null)
        {                
            Control.IsChecked = Element.Checked;
            Control.Checked += (s, r) => { Element.Checked = true; };
            Control.Unchecked += (s, r) => { Element.Checked = false; };
        }
    }

    protected override void OnElementPropertyChanged(object sender, PropertyChangedEventArgs e)
    {
        base.OnElementPropertyChanged(sender, e);

        if (e.PropertyName == nameof(Element.Checked))
            Control.IsChecked = Element.Checked;
    }
}

此外，由于Checked事件每次都会被提升两次，我仍在调查此渲染器的错误。

Answer 1

可怕的是，此错误在2年后的iOS ObservableCollection.Clear（）中仍然被观察到！（android对此没有问题。）很难重现，但我注意到它发生在某些异步方法和匿名函数中。（我经常在viewModels内部的命令上使用这些命令）

为我解决此问题的部分解决方案是包装在BeginInvokeOnMainThread（）中。

Device.BeginInvokeOnMainThread(() =>
{
    FilteredRecipients.Clear();
});

我没有创建自定义的ObservableCollection，而是创建了扩展方法：

public static void SafeClear<T>(this ObservableCollection<T> observableCollection)
{
    if(!MainThread.IsMainThread)
    {
        Device.BeginInvokeOnMainThread(() =>
        {
            while (observableCollection.Any())
            {
                 ObservableCollection.RemoveAt(0);
            }
        });
    }
    else
    {
        while (observableCollection.Any())
        {
             ObservableCollection.RemoveAt(0);
        }
    }
}

因此在OP的示例中，他将这样调用扩展方法：

FilteredRecipients.SafeClear();
_userGroupedList.SafeClear();
_officeGroupedList.SafeClear();

编辑-经过进一步测试后，clear（）仍然因索引不足错误而崩溃。最后，我们将@ПавелВоронцов的答案与扩展方法结合使用。

Answer 2

尝试以下代码而不是clear（）

FilteredRecipients = new ObservableCollection<YourModel>();

修改

Checked事件的解决方案每次都会引发两次。

if (e.OldElement != null) { Control.Checked -= (s, r) => { Element.Checked = true; }; Control.Unchecked -= (s, r) => { Element.Checked = false; }; } if (e.NewElement != null) { Control.Checked += (s, r) => { Element.Checked = true; }; Control.Unchecked += (s, r) => { Element.Checked = false; }; }

Answer 3

您可以使用此ObservableCollection来避免清除问题：

public class SafeObservableCollection<T> : ObservableCollection<T>
{
    /// <summary>
    /// Normal ObservableCollection fails if you are trying to clear ObservableCollection<ObservableCollection<T>> if there is data inside
    /// this is workaround till it won't be fixed in Xamarin Forms
    /// </summary>
    protected override void ClearItems()
    {
        while (this.Items.Any())
        {
            this.Items.RemoveAt(0);
        }
    }
}

Clear ObservableCollection抛出异常

3 个答案: