如何使用URL查询处理WCF-OData中的延续？

Question

我正在使用指向OData端点的WCF数据服务。如果我使用DataServiceQuery，则可以毫无困难地管理延续。

var collection = new DataServiceCollection<T>(); 
collection.LoadCompleted += (sender, e) => 
    { 
     if (e.Error != null) 
     { 
      callback(null, e.Error); 
      return; 
     } 

     var thisCollection = (DataServiceCollection<T>) sender; 
     if (thisCollection.Continuation != null) 
     { 
      thisCollection.LoadNextPartialSetAsync(); 
     } 
     else 
     { 
      var items = thisCollection.ToList(); 
      callback(items, e.Error); 
     } 
    }; 
collection.LoadAsync(query); 

不过，我看不出你如何能为DataServiceContext.BeginExecute做同样的（字符串的URL，...）方法。

_odataContext.BeginExecute<T>(new Uri(requestUrl), x => 
{ 
    var items = _odataContext.EndExecute<T>(x); 

    //not sure how to get the rest of the items with this method 
}); 

如何使用基于url的查询方法，但仍然得到延续支持？

Answer 1

的同步样本（使其更简单）：

var r = ctx.Execute<Product>(new Uri("http://services.odata.org/Northwind/Northwind.svc/Products")); 
QueryOperationResponse<Product> response = (QueryOperationResponse<Product>)r; 
response.Count(); 
Console.WriteLine(response.GetContinuation()); 

总之，Execute方法返回QueryOperationResponse，其实现IEnumerable的实例，但也暴露了延续。

Answer 2

为了完整起见，下面是完整的功能，它遵循URL查询的延续。

public void ExecuteFullQuery<T>(Uri requestUrl, Action<IEnumerable<T>> callback) 
{ 
    var list = new List<T>(); 
    ExecuteFullQueryImpl(requestUrl, list, callback); 
} 

private void ExecuteFullQueryImpl<T>(Uri requestUrl, List<T> items, Action<IEnumerable<T>> callback) 
{ 
    _odataContext.BeginExecute<T>(requestUrl, x => 
    { 
     var results = _odataContext.EndExecute<T>(x); 
     if (results != null) 
      items.AddRange(results.ToList()); 

     var response = (QueryOperationResponse<T>)results; 
     var continuation = response.GetContinuation(); 
     if (continuation != null) 
     { 
      ExecuteFullQueryImpl(continuation.NextLinkUri, items, callback); 
     } 
     else 
     { 
      callback(items); 
     } 
    }, 
    null); 
} 

Answer 3

使用一个懒惰的枚举

public IEnumerable<Product> GetProducts() 
{ 
    Uri request = new Uri("http://services.odata.org/Northwind/Northwind.svc/Products"); 

    var response = (QueryOperationResponse<Product>)ctx.Execute<Product>(request); 

    while (true) 
    { 
     foreach (Product p in response) 
     { 
      yield return p; 
     } 

     var continuation = response.GetContinuation(); 
     if (continuation == null) 
     { 
      yield break; 
     } 

     response = ctx.Execute(continuation); 
    } 
}