如何在后台执行计数请求,以下代码严重阻碍了我的UI。我试图实现NSAsynchronousFetchRequest
,但我无法将其整合在一起。任何帮助将不胜感激!
let NewMessage: NSFetchRequest<Mesages> = Mesages.fetchRequest()
NewMessage.predicate = NSPredicate(format: "timestamp = %@", date)
do {
//context.fetch blocking the main thread
let checkMessages = try context.fetch(NewMessage)
if checkMessages.count == 0 {
print('no duplicates')
}
} catch {
}
答案 0 :(得分:0)
将 PrivateQueueConcurrencyType 用于本地托管对象,并使用countForFetchRequest方法代替fetch:
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Collections.Specialized;
using System.Globalization;
using System.Windows.Data;
namespace TreeViewPOC
{
class ConvertingCollection<Tin, Tout> : ObservableCollection<Tout>
{
private IValueConverter _converter;
private bool _isObservableCollection;
private IEnumerable<Tin> _originalCollection;
private Dictionary<Tin, Tout> _mapping = new Dictionary<Tin, Tout>();
public ConvertingCollection(IValueConverter converter)
{
// save parameters
_converter = converter;
}
public ConvertingCollection(IEnumerable<Tin> originalCollection, IValueConverter converter)
{
// save parameters
_converter = converter;
OriginalCollection = originalCollection;
}
#region Properties
public IEnumerable<Tin> OriginalCollection
{
get
{
return _originalCollection;
}
set
{
if (!value.Equals(_originalCollection))
{
// manage older collection
if (_originalCollection != null && _isObservableCollection)
{
(_originalCollection as ObservableCollection<Tin>).CollectionChanged -= originalCollection_CollectionChanged;
this.Clear();
}
_originalCollection = value;
// setup original collection information.
_isObservableCollection = _originalCollection is INotifyCollectionChanged;
if (_originalCollection != null && _isObservableCollection)
{
(_originalCollection as INotifyCollectionChanged).CollectionChanged += originalCollection_CollectionChanged;
foreach (Tin item in _originalCollection)
{
AddConverted(item);
}
}
}
}
}
#endregion
/// <summary>
/// Indicates the time in milliseconds between two refreshes.
/// </summary>
/// <notes>
/// When the original collection isn't observable, it must be explored to reflect changes in the converted collection.
/// </notes>
// TODO
//public int RefreshRate { get; set; } = 1000;
/// <summary>
/// Flushes the collection.
/// </summary>
public new void Clear()
{
_mapping.Clear();
base.Clear();
}
#region Events management
private void originalCollection_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
{
switch (e.Action)
{
case NotifyCollectionChangedAction.Add:
foreach (Tin item in e.NewItems)
{
AddConverted(item);
}
break;
case NotifyCollectionChangedAction.Remove:
foreach (Tin item in e.OldItems)
{
RemoveConverted(item);
}
break;
}
}
#endregion
#region Helpers
/// <summary>
/// Converts an item and adds it to the collection.
/// </summary>
/// <param name="item">The original item.</param>
private void AddConverted(Tin item)
{
Tout converted = (Tout) _converter.Convert(item, typeof(Tout), null, CultureInfo.CurrentCulture);
_mapping.Add(item, converted);
this.Add(converted);
}
/// <summary>
/// Removes a converted itemfrom the collection based on its original value.
/// </summary>
/// <param name="item">The original item.</param>
private void RemoveConverted(Tin item)
{
this.Remove(_mapping[item]);
_mapping.Remove(item);
}
#endregion
}
}