[翻译]创建ASP.NET WebApi RESTful 服务（7）

实现资源分页

本章我们将介绍几种不同的结果集分页方式，实现手工分页，然后将Response通过两个不同的方式进行格式化（通过Response的Envelop元数据或header）。

大家都知道一次查询返回几百条数据是很讨厌的事情，那么在WebApi中分页就更有必要。

手动分页和封装

接下来我们修改CoursesController来实现分页而不是返回所有数据。

   1:  public Object Get(int page = 0, int pageSize = 10)

   2:      {

   3:          IQueryable<Course> query;

4:

   5:          query = TheRepository.GetAllCourses().OrderBy(c => c.CourseSubject.Id);

   6:          var totalCount = query.Count();

   7:          var totalPages = (int)Math.Ceiling((double)totalCount / pageSize);

8:

   9:          var urlHelper = new UrlHelper(Request);

  10:          var prevLink = page > 0 ? urlHelper.Link("Courses", new { page = page - 1 }) : "";

  11:          var nextLink = page < totalPages - 1 ? urlHelper.Link("Courses", new { page = page + 1 }) : "";

12:

  13:          var results = query

  14:          .Skip(pageSize * page)

  15:          .Take(pageSize)

  16:          .ToList()

  17:          .Select(s => TheModelFactory.Create(s));

18:

  19:          return new

  20:          {

  21:              TotalCount = totalCount,

  22:              TotalPages = totalPages,

  23:              PrevPageLink = prevLink,

  24:              NextPageLink = nextLink,

  25:              Results = results

  26:          };

27:

  28:      }

.csharpcode, .csharpcode pre
{
font-size: small;
color: black;
font-family: consolas, "Courier New", courier, monospace;
background-color: #ffffff;
/*white-space: pre;*/
}
.csharpcode pre { margin: 0em; }
.csharpcode .rem { color: #008000; }
.csharpcode .kwrd { color: #0000ff; }
.csharpcode .str { color: #006080; }
.csharpcode .op { color: #0000c0; }
.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
.csharpcode .alt
{
background-color: #f4f4f4;
width: 100%;
margin: 0em;
}
.csharpcode .lnum { color: #606060; }

代码很简单，添加两个可选参数，它们最终会被格式化为QueryString。例如http://localhost:{your_port}/api/courses/?page=1代表你需要访问第一页的10条数据。不出意外，结果可能是这样的：

   1:  {

2:

   3:      "totalCount": 33,

   4:      "totalPages": 4,

   5:      "prevPageLink": "http://localhost:8323/api/courses?page=0&pageSize=10",

   6:      "nextPageLink": "http://localhost:8323/api/courses?page=2&pageSize=10",

   7:      "results": [ /* Array containts the results*/ ]

8:

   9:  }

结果是我们通过将数据封装为json通过Response的结果返回，这样记录总数也可以被返回。一个明显的缺点是这种类似的数据可能会污染我们期望的结果集，所以接下来我们通过在Response Header中添加一个X-Pagination参数来实现这一功能。

通过Response Header实现分页

实现代码如下：

   1:   public IEnumerable<StudentBaseModel> Get(int page = 0, int pageSize = 10)

   2:      {

   3:          IQueryable<Student> query;

4:

   5:          query = TheRepository.GetAllStudentsWithEnrollments().OrderBy(c => c.LastName);

6:

   7:          var totalCount = query.Count();

   8:          var totalPages = (int)Math.Ceiling((double)totalCount / pageSize);

9:

  10:          var urlHelper = new UrlHelper(Request);

  11:          var prevLink = page > 0 ? urlHelper.Link("Students", new { page = page - 1, pageSize = pageSize }) : "";

  12:          var nextLink = page < totalPages - 1 ? urlHelper.Link("Students", new { page = page + 1, pageSize = pageSize }) : "";

13:

  14:          var paginationHeader = new

  15:          {

  16:              TotalCount = totalCount,

  17:              TotalPages = totalPages,

  18:              PrevPageLink = prevLink,

  19:              NextPageLink = nextLink

  20:          };

21:

  22:          System.Web.HttpContext.Current.F.Add("X-Pagination",

  23:          Newtonsoft.Json.JsonConvert.SerializeObject(paginationHeader));

24:

  25:          var results = query

  26:          .Skip(pageSize * page)

  27:          .Take(pageSize)

  28:          .ToList()

  29:          .Select(s => TheModelFactory.CreateSummary(s));

30:

  31:          return results;

  32:      }

需要注意代码中，我们在Response.Headers中添加了一个通过json序列化的分页参数。这样返回结果及就可以直接被使用，无法再考虑额外的信息。

来源：http://bitoftech.net/2013/11/25/implement-resources-pagination-asp-net-web-api/