Как загружать результаты, разбитые на страницы (службы WCF Data Services)
Службы Службы WCF Data Services позволяют службе данных ограничить количество сущностей, возвращаемых в отдельном канале ответа. Если это происходит, последняя запись в канале содержит ссылку на следующую страницу данных. Получить URI для следующей страницы можно, вызвав метод GetContinuation объекта QueryOperationResponse, возвращенного при выполнении запроса DataServiceQuery. URI, представленный этим объектом, можно затем использовать для загрузки следующей страницы результатов. Дополнительные сведения см. в разделе Загрузка отложенного содержимого (службы WCF Data Services).
Пример в этом разделе использует образец службы данных Northwind и автоматически сформированные клиентские классы службы данных. Эта служба и клиентские классы данных создаются после выполнения действий, описанных в разделе Краткое руководство по службам WCF Data Services.
Пример
Этот пример использует цикл do…while для загрузки сущностей Customers из разбитых на страницы результатов службы данных.
' Create the DataServiceContext using the service URI.
Dim context = New NorthwindEntities(svcUri)
Dim token As DataServiceQueryContinuation(Of Customer) = Nothing
Dim pageCount = 0
Try
' Execute the query for all customers and get the response object.
Dim response As QueryOperationResponse(Of Customer) = _
CType(context.Customers.Execute(), QueryOperationResponse(Of Customer))
' With a paged response from the service, use a do...while loop
' to enumerate the results before getting the next link.
Do
' Write the page number.
Console.WriteLine("Page {0}:", pageCount + 1)
' If nextLink is not null, then there is a new page to load.
If token IsNot Nothing Then
' Load the new page from the next link URI.
response = CType(context.Execute(Of Customer)(token), _
QueryOperationResponse(Of Customer))
End If
' Enumerate the customers in the response.
For Each customer As Customer In response
Console.WriteLine("\tCustomer Name: {0}", customer.CompanyName)
Next
' Get the next link, and continue while there is a next link.
token = response.GetContinuation()
Loop While token IsNot Nothing
Catch ex As DataServiceQueryException
Throw New ApplicationException( _
"An error occurred during query execution.", ex)
End Try
// Create the DataServiceContext using the service URI.
NorthwindEntities context = new NorthwindEntities(svcUri);
DataServiceQueryContinuation<Customer> token = null;
int pageCount = 0;
try
{
// Execute the query for all customers and get the response object.
QueryOperationResponse<Customer> response =
context.Customers.Execute() as QueryOperationResponse<Customer>;
// With a paged response from the service, use a do...while loop
// to enumerate the results before getting the next link.
do
{
// Write the page number.
Console.WriteLine("Page {0}:", pageCount++);
// If nextLink is not null, then there is a new page to load.
if (token != null)
{
// Load the new page from the next link URI.
response = context.Execute<Customer>(token)
as QueryOperationResponse<Customer>;
}
// Enumerate the customers in the response.
foreach (Customer customer in response)
{
Console.WriteLine("\tCustomer Name: {0}", customer.CompanyName);
}
}
// Get the next link, and continue while there is a next link.
while ((token = response.GetContinuation()) != null);
}
catch (DataServiceQueryException ex)
{
throw new ApplicationException(
"An error occurred during query execution.", ex);
}
Этот пример возвращает связанные сущности Orders вместе с каждой сущностью Customers и использует цикл do…while для загрузки страниц сущностей Customers, а также вложенный цикл while для загрузки страниц связанных сущностей Orders из службы данных.
' Create the DataServiceContext using the service URI.
Dim context = New NorthwindEntities(svcUri)
Dim nextLink As DataServiceQueryContinuation(Of Customer) = Nothing
Dim pageCount = 0
Dim innerPageCount = 0
Try
' Execute the query for all customers and related orders,
' and get the response object.
Dim response = _
CType(context.Customers.AddQueryOption("$expand", "Orders") _
.Execute(), QueryOperationResponse(Of Customer))
' With a paged response from the service, use a do...while loop
' to enumerate the results before getting the next link.
Do
' Write the page number.
Console.WriteLine("Customers Page {0}:", ++pageCount)
' If nextLink is not null, then there is a new page to load.
If nextLink IsNot Nothing Then
' Load the new page from the next link URI.
response = CType(context.Execute(Of Customer)(nextLink), _
QueryOperationResponse(Of Customer))
End If
' Enumerate the customers in the response.
For Each c As Customer In response
Console.WriteLine("\tCustomer Name: {0}", c.CompanyName)
Console.WriteLine("\tOrders Page {0}:", innerPageCount + 1)
' Get the next link for the collection of related Orders.
Dim nextOrdersLink As DataServiceQueryContinuation(Of Order) = _
response.GetContinuation(c.Orders)
While nextOrdersLink IsNot Nothing
For Each o As Order In c.Orders
' Print out the orders.
Console.WriteLine("\t\tOrderID: {0} - Freight: ${1}", _
o.OrderID, o.Freight)
Next
' Load the next page of Orders.
Dim ordersResponse = _
context.LoadProperty(c, "Orders", nextOrdersLink)
nextOrdersLink = ordersResponse.GetContinuation()
End While
Next
' Get the next link, and continue while there is a next link.
nextLink = response.GetContinuation()
Loop While nextLink IsNot Nothing
Catch ex As DataServiceQueryException
Throw New ApplicationException( _
"An error occurred during query execution.", ex)
End Try
// Create the DataServiceContext using the service URI.
NorthwindEntities context = new NorthwindEntities(svcUri);
DataServiceQueryContinuation<Customer> nextLink = null;
int pageCount = 0;
int innerPageCount = 0;
try
{
// Execute the query for all customers and related orders,
// and get the response object.
var response =
context.Customers.AddQueryOption("$expand", "Orders")
.Execute() as QueryOperationResponse<Customer>;
// With a paged response from the service, use a do...while loop
// to enumerate the results before getting the next link.
do
{
// Write the page number.
Console.WriteLine("Customers Page {0}:", ++pageCount);
// If nextLink is not null, then there is a new page to load.
if (nextLink != null)
{
// Load the new page from the next link URI.
response = context.Execute<Customer>(nextLink)
as QueryOperationResponse<Customer>;
}
// Enumerate the customers in the response.
foreach (Customer c in response)
{
Console.WriteLine("\tCustomer Name: {0}", c.CompanyName);
Console.WriteLine("\tOrders Page {0}:", ++innerPageCount);
// Get the next link for the collection of related Orders.
DataServiceQueryContinuation<Order> nextOrdersLink =
response.GetContinuation(c.Orders);
while (nextOrdersLink != null)
{
foreach (Order o in c.Orders)
{
// Print out the orders.
Console.WriteLine("\t\tOrderID: {0} - Freight: ${1}",
o.OrderID, o.Freight);
}
// Load the next page of Orders.
var ordersResponse = context.LoadProperty(c, "Orders", nextOrdersLink);
nextOrdersLink = ordersResponse.GetContinuation();
}
}
}
// Get the next link, and continue while there is a next link.
while ((nextLink = response.GetContinuation()) != null);
}
catch (DataServiceQueryException ex)
{
throw new ApplicationException(
"An error occurred during query execution.", ex);
}
См. также
Задачи
Как загрузить связанные сущности (службы WCF Data Services)