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Dataview

Dataview: TdaQueryDataView

Example​

procedure ScriptEvent(var Value: variant);
begin
  Value := Null;
  if Dataview <> nil then
  begin
    if Dataview.FieldExists('OrderNumber') and Dataview.NotEmpty then
      Value := Dataview['OrderNumber'].Value;
  end;
end;

Usage​

Dataview exposes the active field or view map's live query data view, with its value type when known.

Additional Technical Info​

Dataview exposes the live query data view that owns the current map record. It is context, not a copied dataset: navigation, edits and field assignments operate on the same object that the map engine is processing.

Value

The generated fallback declaration is:

When the current view is known during compilation, Velox instead registers the most specific supported type:

Runtime viewScript type
SQLTdaSQLQueryDataView
XMLTdaXMLQueryDataView
JSONTdaJSONQueryDataView
client-dataset/fileTdaCDSQueryDataView
other or unavailableTdaQueryDataView

This changes which subtype properties compile. The common query-view methods and properties remain available through the base surface.

Binding and availability

Velox always declares the identifier so reusable script source can compile. It binds an instance only for a TvxCustomMapItem whose active item is:

  • a TvxFieldMap, where Dataview is FieldMap.ViewMap.View; or
  • a TvxViewMap, where Dataview is ViewMap.View.

Outside those contexts the name can be nil/unbound. Check it before dereferencing in shared scriptlets. A declared variable is not proof that a map record or data pipeline exists.

Behaviour and side effects

  • First, Next, Prior, Last, Locate and RecordPosition move the live map cursor. Moving it inside a map event can change which record later fields/details process.
  • Insert, Edit, Post, CancelPost, Delete and DeleteAll modify the active view/query. Manual state changes can interfere with the map engine's own insert/map/post/cancel sequence.
  • Fields['Alias'] resolves a configured data-view alias to the underlying field and raises when the alias is absent. FieldExists is the safe presence test.
  • Empty, NotEmpty and EOF ultimately use the current data pipeline. Record count/position and editing semantics are implemented by the concrete descendant, not by the abstract base alone.
  • For SQL views, the subtype also exposes table/link/flag metadata and its live Query. XML/JSON views are registered as their concrete hidden descendants but inherit the public CDS/query-view surface.

Edge cases and quirks

The base TdaQueryDataView contains no-op bodies for several virtual script methods because PascalScript cannot call abstract methods directly. Concrete SQL/CDS/XML/JSON descendants override the real operations. If only an unavailable base view were ever bound, methods such as Next, Edit, Insert, Post and FieldByName would have placeholder behaviour; normal map bindings use a concrete descendant.

The compile-time subtype is selected from the current MapItem before execution. A script compiled for one view family should not assume subtype-only members exist when reused under another family. The view and its fields are host-owned and can be reset or destroyed with the map/data definition; never free or retain them beyond the current execution.

Errors from missing fields, invalid dataset state, conversion or an underlying query can propagate or be caught by the enclosing map event, which then records them in the Velox log. Cursor operations can trigger linked/detail work depending on the concrete view. Repeated scans and FieldByName calls add work; database-backed or linked operations can be materially more expensive than reading the already-current field.

Related entries

External references

Created 2026-07-15