Delphi TDBImage 多图形格式支持文件

Tip: (thanks to Josir Gomes for this) "To load the JPEG image from file, you have to use the LoadFromFile from the new fieldtype. If you use DbImage.Picture.LoadFromFile(), the image will not be loaded correctly." Removed Bugs: 11 July 2002: DBGraphicSupport.pas Problem with handling empty Blob-fields solved (TEnhBlobField.SaveToAnyImage) Graphic-Support This package enables standard TDBImage-components to display JPEG, EMF, WMF and ICO graphics, that are stored in Blob-fields in databases, and TQRDBImage-components (QuickReport) to print them as well. The advantage of GraphicSupport is, that you don't need an extra component in the palette, but just use the existing ones, as you are used to. GraphicSupport also shows a simple approach to add support for any type of graphic, you can get or write a TGraphic-compatible class for. Study the sources to understand why; especially the function GetGraphicClass. GraphicSupport is written for Delphi 6. If you dare a Delphi 5 or Delphi 4 Version, let me know. Problem Why doesn't a JPEG-stream, stored in a Blob-Field, get displayed in a TDBImage component? Simple: TBlobField (that handles the transfer of Blob-data to TBitmap and TPicture) doesn't know that there may be a suitable class for JPEGs, creates a TBitmap and passes the Blob-data to it. BANG....TBitmap spits out it's 'I don't know this format'- message and leaves the DBImage-canvas blank. Solution Everything we have to do is to define a descendant of TBlobField that "knows" additional graphic formats! But that's only half the way, because who tells the IDE or TDataset (or it's descendants), that it has to use the 'enhanced' Blob-Field instead of the standard one? To manage this situation it is necessary to understand, how TField-objects are managed by TDataset (and some other classes). If there's a need for a TField-object, any class that deals with database-stuff looks into the constant array "DefaultFieldClasses". This array stores a hardcoded list, that defines a TField-class for any TFieldType: (from unit DB.pas) type TFieldType = (ftUnknown, ftString, ftSmallint, ftInteger, ftWord, ftBoolean, ftFloat, ftCurrency, ftBCD, ftDate, ftTime, ftDateTime, ftBytes, ftVarBytes, ftAutoInc, ftBlob, ftMemo, ftGraphic, ftFmtMemo, ftParadoxOle, ftDBaseOle, ftTypedBinary, ftCursor, ftFixedChar, ftWideString, ftLargeint, ftADT, ftArray, ftReference, ftDataSet, ftOraBlob, ftOraClob, ftVariant, ftInterface, ftIDispatch, ftGuid); const DefaultFieldClasses: array[TFieldType] of TFieldClass = ( nil, { ftUnknown } <snip> ... <snip> TBlobField, { ftBlob } <<<<this one of special interest! TMemoField, { ftMemo } TGraphicField, { ftGraphic } <<<<this one too! <snip> ... <snip> TGuidField); { ftGuid } Now this constant array defines which TField-class has to be used, if a certain FieldType is given. What we know is, that constants aren't as constant as they seem. Especially the content of a constant array.So all we have to do, is replace the entries for ftBlob and ftGraphic at runtime, so that they contain a reference to TEnhBlobField and TEnhGraphicField before the IDE or any DB-class tries to create TField-objects. First try would be to include a unit that does the replacement in its initialization-section. But this will only work for TField-objects that are created at runtime. To let the Field-Editor create TEnhBlobField references at design-time, the replacement has to take place before the TField-objects are created by the IDE. For this purpose a package can do the replacement in its Register-procedure. If this package is installed into the IDE (and therefore gets loaded everytime the IDE starts up) anything will work just fine. The last remaining problem is a very hidden one. In IBDataset Borland redefines the constant array "DefaultFieldClasses", storing different values in it. This while using DB.pas itself and therefore knowing the original constant. So under some circumstances it has to be ensured, that both definitions are fixed, for what a secod variant of the GraphicSupport unit is needed: GraphicSupportIB. GraphicSupportIB does nothing else, than using IBCustomDataset and doing the replacement a second time. Addendum for Delphi 6: With Delphi 6 Borland compiled the DB-packages with $J- and overwriting the constants directly is not supported any longer. I've added a workaround to continue the functionality of my package: TFieldClass(Pointer(@DefaultFieldClasses[ftBlob])^) := TEnhBlobField; This line tricks the compiler, because - getting the address of a constant is allowed - @, - tracting it as a ointer is allowed too - Pointer(, - dereference pointers is okay as well - ^, - and typecasting works, sometimes even if it shouldn't - TFieldClass(. I'm very glad to present JPEGSupport for Delphi 6 - finally. :-) For QuickReport-Users: Remember: QRDBImage prints JPEGs too, if this package is installed! For questions or comments please contact Robert Kuhlmann email: robert.kuhlmann.dev@ewetel.net