TMemDataset class

In unit MemDs

Implements an in-memory dataset.

Declaration

pascal

TMemDataset = class(TDataSet)

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

`FileModified`	public	Indicates if the in-memory dataset has been modified.
`Filter`	public	Filter for the dataset.
`FileName`	published	File name used to read or write field definitions and optional data.
`Filtered`	published	Indicates if records in the dataset are filtered using OnFilterRecord.
`Active`	published	Indicates if the in-memory dataset is Active.
`FieldDefs`	published	Field definitions for the in-memory dataset.
`BeforeOpen`	published	Event triggered before the dataset is opened.
`AfterOpen`	published	Event triggered after the dataset is opened.
`BeforeClose`	published	Event triggered before the dataset is closed.
`AfterClose`	published	Event triggered after the dataset is closed.
`BeforeInsert`	published	Event triggered before the dataset is put in insert mode.
`AfterInsert`	published	Event triggered after the dataset is put in insert mode.
`BeforeEdit`	published	Event triggered before the dataset is put in edit mode.
`AfterEdit`	published	Event triggered after the dataset is put in edit mode.
`BeforePost`	published	Event called before changes are posted to the underlying database.
`AfterPost`	published	Event called after changes have been posted to the underlying database.
`BeforeCancel`	published	Event triggered before a Cancel operation.
`AfterCancel`	published	Event triggered after a Cancel operation.
`BeforeDelete`	published	Event triggered before a Delete operation.
`AfterDelete`	published	Event triggered after a successful Delete operation.
`BeforeScroll`	published	Event triggered before the cursor changes position.
`AfterScroll`	published	Event triggered after the cursor has changed position.

Methods

`AllocRecordBuffer`	protected	Allocates memory needed for a record buffer in the in-memory dataset.
`FreeRecordBuffer`	protected	Frees memory allocated for the specified record buffer.
`GetBookmarkData`	protected	Gets Bookmark data from the specified record buffer.
`GetBookmarkFlag`	protected	Gets BookmarkFlag data from the specified record buffer.
`GetFieldData`	protected	Gets data for the specified field from the active record buffer.
`GetRecord`	protected	Gets a record buffer when navigating between rows in the in-memory dataset.
`GetRecordSize`	protected	Gets the value for the RecordSize property.
`InternalAddRecord`	protected	Performs actions needed to add a record in the in-memory dataset.
`InternalClose`	protected	Performs actions needed when closing the in-memory dataset.
`InternalDelete`	protected	Performs actions needed to delete a record in the in-memory dataset.
`InternalFirst`	protected	Performs actions needed to move to the first record in the in-memory dataset.
`InternalGotoBookmark`	protected	Performs actions need to find the record for the specified Bookmark.
`InternalInitFieldDefs`	protected	Performs actions needed to initialize field definitions for the in-memory dataset.
`InternalInitRecord`	protected	Performs actions needed to initialize a record in the in-memory dataset.
`ClearCalcFields`	protected	Clears calculated field values.
`InternalLast`	protected	Performs actions needed to move to the last record in the in-memory dataset.
`InternalOpen`	protected	Performs actions needed to open the in-memory dataset.
`InternalPost`	protected	Performs actions needed to post a record in the in-memory dataset.
`InternalSetToRecord`	protected	Performs actions needed to move the dataset to the specified record.
`IsCursorOpen`	protected	Indicates if the cursor for the dataset is open.
`SetBookmarkFlag`	protected	Sets the BookmarkFlag in the specified record buffer.
`SetBookmarkData`	protected	Sets the Bookmark data in the specified record buffer.
`SetFieldData`	protected	Sets the value for a field in the record buffer.
`GetRecordCount`	protected	Gets the value for the RecordCount property.
`SetRecNo`	protected	Sets the value for the RecNo property.
`GetRecNo`	protected	Gets the value for the RecNo property.
`SetFilterText`	protected	Silently drops value in the filter text.
`RaiseError`	protected	Raises an exception for an invalid operation in the in-memory dataset.
`CheckMarker`	protected	Checks for the specified marker in the internal stream for the in-memory dataset.
`WriteMarker`	protected	Writes the specified marker to the internal stream for the in-memory dataset.
`ReadFieldDefsFromStream`	protected	Reads field definitions from the specified stream.
`SaveFieldDefsToStream`	protected	Saves field definitions to the specified stream.
`LoadDataFromStream`	protected	Loads record data from the specified stream.
`SaveDataToStream`	protected	Saves record data to the specified stream.
`Create`	public	Constructor for the class instance.
`Destroy`	public	Destructor for the class instance.
`BookmarkValid`	public	Determines if the specified Bookmark is valid.
`CompareBookmarks`	public	Gets the relative order for the specified Bookmarks.
`CreateBlobStream`	public	Creates a stream used to read or write Blob field data in the in-memory dataset.
`Locate`	public	Locates a record with the specified values in the in-memory dataset.
`Lookup`	public	Searches for a record with the specified values, and returns a list of values.
`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.
`Clear`	public	Clears the content in the in-memory dataset.
`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.
`LoadFromStream`	public	Loads the content for the dataset from the specified stream.
`LoadFromFile`	public	Loads the content for the dataset from the specified file name.
`CopyFromDataset`	public	Loads field definitions and optional data from the specified TDataset.

Events

`OnDeleteError`	published	Event triggered when a delete operation fails.
`OnEditError`	published	Event triggered when an edit operation fails.
`OnNewRecord`	published	Event triggered when a new record is created.
`OnPostError`	published	Event triggered when a post operation fails.
`OnFilterRecord`	published	Event handler signalled to include or exclude records in the in-memory dataset.