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NextGrid & NextDBGrid

• Html Column Tutorial
• Upgrade existing projects for loading changes
• Edit events in NextGrid
• NextGrid Vista Style
• NextGrid How To
• Integrating Inplace Editors
• NextGrid Slide Style Tutorial
• InputLine Tutorial
• FieldChooser Tutorial
• Using Editors in runtime
• Grid Report Tutorial

NextGrid

• TreeColumn Tutorial
• NextGrid Quick Start
• Custom Draw in NextGrid
• Export to XML from NextGrid
• Optimize NextGrid
• NextGrid Custom Sorting
• VirtualColumn Tutorial
• Graphic Column Tutorial

NextDBGrid

• NextDBGrid Quick Start
• NextDBGrid Events
• NextDBGrid How To
• Sorting records in NextDBGrid
• LookupColumn Tutorial

NextInspector

• NextInspector Advanced tutorial
• ToolbarItem tutorial
• NextDBInspector Tutorial
• NextInspector Item Types
• Map VCL property to item
• NextInspector Quick Start

NextSheet

• NextSheet Sample Project
• NextSheet Quick Start
• Formating in NextSheet
• NextSheet Print Method

NextCollection

• NxAlertWindow Tutorial
• NxInfoPanel Tutorial
• Vista Styled Panels
• NxPathControl and NxNotebook tutorial
• NxPathControl Quick Start
• NxFlipPanel and NxHeaderPanel Quick Start
• NxOutlookBar Tutorial
• NxPageControl and NxNotebook Quick Start
• NxButton Tutorial

Misc

• Component Names Change
• NxComboBox styles
• Numeric FormatMask
• NxVirtualDataSet tutorial
• DateTime FormatMask
• NxPreview Quick Start
• 32bit Bitmaps Tutorial
• Quick update
• Enable typing unicode characters in InplaceEditors
• Using NxColorScheme
• NxProgress Tutorial
• NxAutoCompletion Tutorial

Labs

• NxReport Project

NextGrid .NET

• Working with tree
• Custom Drawing in NextGrid dot NET
• NextGrid Dot Net
• NextGrid Dot Net Events
• NextGrid dot NET Quick Start

Related Articles:

The TNxVirtualDataSet is a virtualized descendant of TDataSet. This implies that information about data it supplies and the data itself must be suplied just in time, i.e. at the moment Db components using the TNxVirtualDataSetneed it.

Because of this virtualization it is possible to use the TNxVirtualDatSet to turn almost anything rectangular (like a TCollection or a TStringList) into a DataSet. It is also possible to turn hardware into a database (like values from an usb thermometer or a joystick). Computed fields are also easy to implement (like the length of a line in a TStringList).

Virtualized are the following parts of the TDataSet:

Part	Event
Number of fields	OnFieldCount event
Field definitions	OnAddField event
Number of records	OnRecordCount event
Reading Field values	OnGetFieldData / OnGetBlobData events
Writing Field values	OnSetFieldData / OnSetFieldMode events

Request for data for these virtualized parts are peformed through a number of events.

Normally the first event handler called is the number of fields, after which TNxVirtualDataSet needs the definition of each field.

Shortly afterwards TnxVirtualDataSet will need the number of records. Then TnxVirtualDataSet starts making numerous requests for field values at a high pace. Efficient coding of this event is required as it is used when visual Db Controls are repainted.

Write support is experimental and a bit tricky. After an Append, Edit or Insert call to TNxVirtualDataSet the OnSetFieldData event is used to signal modification to field data. The program must cache this field data as it is not clear at that moment what operation will be performed on which record.

When the programm calls the Post, Cancel or Delete method, the OnSetFieldMode event is called with a set of UpdateModes. Normally it is a combination of umAppend, umEdit or umInsert and umPost. Depending on the presence of umAppend, umEdit or umInsert the programmer must add, change or insert a new record in the underlying storage. As parameters of the OnSetFieldMode event, TnxVirtualDataSet passes the recordnumber of the involved record.

Not combined with other values are the UpdateModes umDelete and umCancel. On encountering umDelete, the program must delete the involved. The last mode, umCancel simply means ignoring the previously changed data.

The demo application uses a TStringList to store lines of text. Each line represents a single record. The TNextDbGrid displays the followind data:

Field 0 the index of the line

Field 1 the text of the line

Field 2 the length of the line

Figure 1. Appending a record to the TNxVirtualDataSet.

function TForm1.NxVirtualDataSet1GetFieldData(Sender: TObject;
  const rowIndex, fieldIndex: Integer): OLEVariant;
begin
  case fieldIndex of
    0: Result := NxVirtualDataSet1.RecNo;
    1: if Assigned(fStorage) and (rowIndex >= 0) and (rowIndex < fStorage.Count)
       then
         Result := fStorage[rowIndex]
       else
         Result := '**Error**';
    2: if Assigned(fStorage) and (rowIndex >= 0) and (rowIndex < fStorage.Count)
       then
         Result := Length(fStorage[rowIndex])
       else
         Result := '-1';
       end;
  end;
end;