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Cum se face paginare pe tabelele din Windows Azure

Daca lucram cu tabelele din Windows Azure o sa avem nevoie sa facem paginare. Default nu avem nici un mecanism de paginare, dar acesta este usor de implementat.
In postul anterior am descris modul prin care se pot obtine mai mult de 1000 de entitati dintr-un tabel. Pe baza mecanismului prezentat anterior putem sa parcurgem datele din tabel in format paginat.
Trebuie sa ne folosim de metoda Take din LINQ, care din fericire este suportata si de LINQ folosit pentru tabelele din Windows Azure( nu trebuie sa uitam ca doar o parte din LINQ este implementat pentru Windows Azure in acest moment). Apeland metoda Take putem sa obtinem doar o parte din entitati care ne sunt returnate de catre query.
Folosindu-ne de token-urile nextPartitionToken si nextRowToken putem sa luam urmatoarele x elemente incepand de pe o anumita pozitie.
Mai jos gasiti o implementare minimala a unui mecanism de paginare.
    public class Pagination<TItem>
where TItem : class
{
private const string NoValue = "novalue";

private string _nextPartitionToken;
private string _nextRowToken;
private DataServiceQuery<TItem> _dataServiceQuery;
private List<TItem> _items;
private int _pageSize;

/// <summary>
/// TRUE when another page exist.
/// </summary>
public bool HasNextPage { get { return _nextPartitionToken != NoValue && !string.IsNullOrEmpty(_nextPartitionToken); } }

/// <summary>
/// Gets all the items of the current page.
/// </summary>
public List<TItem> Items { get { return _items; } }

/// <summary>
/// The base contructor.
/// </summary>
/// <param name="dataServiceQuery">The query that is executed on Azure tables.</param>
/// <param name="pageSize">The size of the page.</param>
public Pagination(DataServiceQuery<TItem> dataServiceQuery, int pageSize)
{
_nextPartitionToken = NoValue;
_pageSize = pageSize;
_dataServiceQuery = dataServiceQuery.Take(_pageSize);
}

/// <summary>
/// Load the next page.
/// </summary>
public void NextPage()
{
if(string.IsNullOrEmpty(_nextPartitionToken))
{
return;
}

if (_nextPartitionToken!=NoValue)
{
_dataServiceQuery.AddQueryOption("NextPartitionKey", _nextPartitionToken);
_dataServiceQuery.AddQueryOption("NextRowKey", _nextRowToken);
}

//Get the result.
var result = _dataServiceQuery.Execute();
_items = result.ToList();

//Get tokens for the next page.
QueryOperationResponse queryOperationResponse = (QueryOperationResponse)result;
queryOperationResponse.Headers.TryGetValue("x-ms-continuation-NextPartitionKey", out _nextPartitionToken);
queryOperationResponse.Headers.TryGetValue("x-ms-continuation-NextRowKey", out _nextRowToken);
}
}
Ceea ce lipseste este calcularea numarului total de pagini. Pentru a putea face acest lucru este nevoie sa executam un query care sa returneze numarul total de entitati de pe un anumit tabel.
Pentru ca utilizatorul sa aiba optiunea de a putea accesa orice pagina este nevoie ca in constructor sa executam cate un query pentru fiecare pagina in parte si sa salvam token-urile.

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