实现虚拟模式的即时数据加载Windows窗体DataGridView控件

using System;using System.Data;using System.Data.SqlClient;using System.Drawing;using System.Windows.Forms;public class VirtualJustInTimeDemo : System.Windows.Forms.Form{    private DataGridView dataGridView1 = new DataGridView();    private Cache memoryCache;    // Specify a connection string. Replace the given value with a      // valid connection string for a Northwind SQL Server sample     // database accessible to your system.     private string connectionString =        "Initial Catalog=NorthWind;Data Source=localhost;" +        "Integrated Security=SSPI;Persist Security Info=False";    private string table = "Orders";    protected override void onl oad(EventArgs e)    {        // Initialize the form.         this.AutoSize = true;        this.Controls.Add(this.dataGridView1);        this.Text = "DataGridView virtual-mode just-in-time demo";        // Complete the initialization of the DataGridView.         this.dataGridView1.Size = new Size(800, 250);        this.dataGridView1.Dock = DockStyle.Fill;        this.dataGridView1.VirtualMode = true;        this.dataGridView1.ReadOnly = true;        this.dataGridView1.AllowUserToAddRows = false;        this.dataGridView1.AllowUserToOrderColumns = false;        this.dataGridView1.SelectionMode =            DataGridViewSelectionMode.FullRowSelect;        this.dataGridView1.CellValueNeeded += new            DataGridViewCellValueEventHandler(dataGridView1_CellValueNeeded);        // Create a DataRetriever and use it to create a Cache object         // and to initialize the DataGridView columns and rows.         try        {            DataRetriever retriever =                new DataRetriever(connectionString, table);            memoryCache = new Cache(retriever, 16);            foreach (DataColumn column in retriever.Columns)            {                dataGridView1.Columns.Add(                    column.ColumnName, column.ColumnName);            }            this.dataGridView1.RowCount = retriever.RowCount;        }        catch (SqlException)        {            MessageBox.Show("Connection could not be established. " +                "Verify that the connection string is valid.");            Application.Exit();        }        // Adjust the column widths based on the displayed values.         this.dataGridView1.AutoResizeColumns(            DataGridViewAutoSizeColumnsMode.DisplayedCells);        base.OnLoad(e);    }    private void dataGridView1_CellValueNeeded(object sender,        DataGridViewCellValueEventArgs e)    {        e.Value = memoryCache.RetrieveElement(e.RowIndex, e.ColumnIndex);    }    [STAThreadAttribute()]    public static void Main()    {        Application.Run(new VirtualJustInTimeDemo());    }}

public class Cache{    private static int RowsPerPage;    // Represents one page of data.       public struct DataPage    {        public DataTable table;        private int lowestIndexValue;        private int highestIndexValue;        public DataPage(DataTable table, int rowIndex)        {            this.table = table;            lowestIndexValue = MapToLowerBoundary(rowIndex);            highestIndexValue = MapToUpperBoundary(rowIndex);            System.Diagnostics.Debug.Assert(lowestIndexValue >= 0);            System.Diagnostics.Debug.Assert(highestIndexValue >= 0);        }        public int LowestIndex        {            get            {                return lowestIndexValue;            }        }        public int HighestIndex        {            get            {                return highestIndexValue;            }        }        public static int MapToLowerBoundary(int rowIndex)        {            // Return the lowest index of a page containing the given index.             return (rowIndex / RowsPerPage) * RowsPerPage;        }        private static int MapToUpperBoundary(int rowIndex)        {            // Return the highest index of a page containing the given index.             return MapToLowerBoundary(rowIndex) + RowsPerPage - 1;        }    }    private DataPage[] cachePages;    private IDataPageRetriever dataSupply;    public Cache(IDataPageRetriever dataSupplier, int rowsPerPage)    {        dataSupply = dataSupplier;        Cache.RowsPerPage = rowsPerPage;        LoadFirstTwoPages();    }    // Sets the value of the element parameter if the value is in the cache.     private bool IfPageCached_ThenSetElement(int rowIndex,        int columnIndex, ref string element)    {        if (IsRowCachedInPage(0, rowIndex))        {            element = cachePages[0].table                .Rows[rowIndex % RowsPerPage][columnIndex].ToString();            return true;        }        else if (IsRowCachedInPage(1, rowIndex))        {            element = cachePages[1].table                .Rows[rowIndex % RowsPerPage][columnIndex].ToString();            return true;        }        return false;    }    public string RetrieveElement(int rowIndex, int columnIndex)    {        string element = null;        if (IfPageCached_ThenSetElement(rowIndex, columnIndex, ref element))        {            return element;        }        else        {            return RetrieveData_CacheIt_ThenReturnElement(                rowIndex, columnIndex);        }    }    private void LoadFirstTwoPages()    {        cachePages = new DataPage[]{            new DataPage(dataSupply.SupplyPageOfData(                DataPage.MapToLowerBoundary(0), RowsPerPage), 0),             new DataPage(dataSupply.SupplyPageOfData(                DataPage.MapToLowerBoundary(RowsPerPage),                 RowsPerPage), RowsPerPage)};    }    private string RetrieveData_CacheIt_ThenReturnElement(        int rowIndex, int columnIndex)    {        // Retrieve a page worth of data containing the requested value.        DataTable table = dataSupply.SupplyPageOfData(            DataPage.MapToLowerBoundary(rowIndex), RowsPerPage);        // Replace the cached page furthest from the requested cell         // with a new page containing the newly retrieved data.        cachePages[GetIndexToUnusedPage(rowIndex)] = new DataPage(table, rowIndex);        return RetrieveElement(rowIndex, columnIndex);    }    // Returns the index of the cached page most distant from the given index     // and therefore least likely to be reused.     private int GetIndexToUnusedPage(int rowIndex)    {        if (rowIndex > cachePages[0].HighestIndex &&            rowIndex > cachePages[1].HighestIndex)        {            int offsetFromPage0 = rowIndex - cachePages[0].HighestIndex;            int offsetFromPage1 = rowIndex - cachePages[1].HighestIndex;            if (offsetFromPage0 < offsetFromPage1)            {                return 1;            }            return 0;        }        else        {            int offsetFromPage0 = cachePages[0].LowestIndex - rowIndex;            int offsetFromPage1 = cachePages[1].LowestIndex - rowIndex;            if (offsetFromPage0 < offsetFromPage1)            {                return 1;            }            return 0;        }    }    // Returns a value indicating whether the given row index is contained     // in the given DataPage.      private bool IsRowCachedInPage(int pageNumber, int rowIndex)    {        return rowIndex <= cachePages[pageNumber].HighestIndex &&            rowIndex >= cachePages[pageNumber].LowestIndex;    }}