TMemDataset¶

Base class for records-based data-access.

Declaration¶

Source position: memds.pp line 58

type

  TMemDataset = class(TDataSet)
  public
    constructor Create;
    destructor Destroy;
    function BookmarkValid : Boolean;
    function CompareBookmarks : LongInt;
    function CreateBlobStream : TStream;
    function Locate : Boolean;
    function Lookup : Variant;
    procedure CreateTable;
    function DataSize : Integer;
    procedure Clear(ClearDefs: Boolean);
    procedure Clear;
    procedure SaveToFile(const AFileName: string);
    procedure SaveToFile(const AFileName: string; SaveData: Boolean);
    procedure SaveToStream(F: TStream);
    procedure SaveToStream(F: TStream; SaveData: Boolean);
    procedure LoadFromStream;
    procedure LoadFromFile;
    procedure CopyFromDataset(DataSet: TDataSet);
    procedure CopyFromDataset(DataSet: TDataSet; CopyData: Boolean);
    property FileModified: Boolean; {r}
    property Filter;
  published
    property FileName: string; {rw}
    property Filtered;
    property Active;
    property FieldDefs;
    property BeforeOpen;
    property AfterOpen;
    property BeforeClose;
    property AfterClose;
    property BeforeInsert;
    property AfterInsert;
    property BeforeEdit;
    property AfterEdit;
    property BeforePost;
    property AfterPost;
    property BeforeCancel;
    property AfterCancel;
    property BeforeDelete;
    property AfterDelete;
    property BeforeScroll;
    property AfterScroll;
    property OnDeleteError;
    property OnEditError;
    property OnNewRecord;
    property OnPostError;
    property OnFilterRecord;
  end;

Inheritance¶

Description¶

TMemDataset is a TDataset descendant which implements an in-memory dataset. TMemDataset is a performant, single user dataset for non-mission critical use cases that do not require transactions. All record and field processing is done in memory; no data is read from or written to disk unless explicitly requested.

TMemDataset implements common facilities defined in the TDataset ancestor class. This includes using the FieldDefs property to define the structure for the dataset. Most (but not all) field types are supported in TMemDataset, including:

ftString
ftGuid
ftFixedChar
ftBoolean
ftCurrency
ftFloat
ftBCD
ftLargeInt
ftSmallInt
ftWord
ftInteger
ftAutoInc (behave like ftInteger i.e. no auto-increment functionality)
ftDateTime
ftDate
ftTime
ftFmtBCD
ftWideString
ftFixedWideChar
ftBytes
ftVarBytes
ftBlob
ftMemo
ftWideMemo

TMemDataset implements common data manipulation methods such as: Append, AppendRecord, Insert, InsertRecord, Delete, Clear, and Refresh. TMemDataset implements Bookmarks and common navigation methods like: First, Next, Prior, Last, Locate, BOF, and EOF. Methods are provided that allow loading and saving both structure and data from a file, a stream, or another TDataset descendent.

TMemDataset provides methods to filter records, but they are implemented in a different manner than in TDataset. The Filter property is ignored; use the OnFilterRecord method and the Filtered property for this functionality.

One notable missing feature is Indexes. Index definitions are not implemented in TMemDataset.

TMemDataset uses ideas taken from the THKMemTab component by Harri Kasulke. (Hamburg/Germany)

Properties¶

Member	Visibility	Description
Active read only	published	Is the dataset open or closed.
AfterCancel read only	published	Event triggered after a Cancel operation.
AfterClose read only	published	Event triggered after the dataset is closed.
AfterDelete read only	published	Event triggered after a successful Delete operation.
AfterEdit read only	published	Event triggered after the dataset is put in edit mode.
AfterInsert read only	published	Event triggered after the dataset is put in insert mode.
AfterOpen read only	published	Event triggered after the dataset is opened.
AfterPost read only	published	Event called after changes have been posted to the underlying database.
AfterScroll read only	published	Event triggered after the cursor has changed position.
BeforeCancel read only	published	Event triggered before a Cancel operation.
BeforeClose read only	published	Event triggered before the dataset is closed.
BeforeDelete read only	published	Event triggered before a Delete operation.
BeforeEdit read only	published	Event triggered before the dataset is put in edit mode.
BeforeInsert read only	published	Event triggered before the dataset is put in insert mode.
BeforeOpen read only	published	Event triggered before the dataset is opened.
BeforePost read only	published	Event called before changes are posted to the underlying database.
BeforeScroll read only	published	Event triggered before the cursor changes position.
FieldDefs read only	published	Definitions of available fields in the underlying database.
FileModified read only	public	Indicates if the in-memory dataset has been modified.
FileName	published	File name used to read or write field definitions and optional data.
Filter read only	public	Filter to apply to the data in memory.
Filtered read only	published	Is the filter active or not.

See all Properties

Methods¶

Member	Visibility	Description
BookmarkValid	public	Test whether ABookMark is a valid bookmark.
Clear	public	Clears the content in the in-memory dataset.
CompareBookmarks	public	Compare two bookmarks.
CopyFromDataset	public	Loads field definitions and optional data from the specified TDataset.
Create	public	Create a new TDataset instance.
CreateBlobStream	public	Create blob stream.
CreateTable	public	Creates the internal storage for records in the in-memory dataset.
DataSize	public	Size of the internal TMemoryStream used in the in-memory dataset.
Destroy	public	Free a TDataset instance.
LoadFromFile	public	Loads the content for the dataset from the specified file name.
LoadFromStream	public	Loads the content for the dataset from the specified stream.
Locate	public	Locate a record based on some key values.
Lookup	public	Search for a record and return matching values.
SaveToFile	public	Saves field definitions and optional record data to the specified file name.
SaveToStream	public	Saves field definitions and optional record data to the specified stream.

See all Methods

Events¶

Member	Visibility	Description
OnDeleteError read only	published	Event triggered when a delete operation fails.
OnEditError read only	published	Event triggered when an edit operation fails.
OnFilterRecord read only	published	Event triggered to filter records.
OnNewRecord read only	published	Event triggered when a new record is created.
OnPostError read only	published	Event triggered when a post operation fails.

See all Events