Custom Drawn Interface

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LCL-CustomDrawn-Android has the following features:

  • Backends for X11, Android and Windows and a partially working one for Cocoa, more can be added in the future
  • Painting is done completely inside Lazarus without any interference from the native libraries except for text drawing. This assures a complete perfection of the executed drawings in all platforms and a uniform level of supported features
  • Only 1 native window is utilized for each form, in the Android backend at the moment this is 1 native window for the entire application
  • Utilizes the Lazarus Custom Drawn Controls for implementing the LCL standard controls
  • Utilizes as it's painting engine the lcl parts: TLazIntfImage, TRawImage, lazcanvas and lazregions


Diagram of the Custom Drawn Interface

customdrawn diagram.png


  • Blue - New elements implemented as part of the LCL-CustomDrawn project
  • Yellow - Pre-existing LCL elements
  • Gray - System APIs

The LCL-CustomDrawn Backends

LCL-CustomDrawn needs backends to implement the most basic parts of the widgetset. Each backend should implement the following minimal parts:

  • TWidgetSet.Run, ProcessMessages, etc
  • TForm
  • TWinControl with all events
  • TTrayIcon


This backend is working. See this page Custom Drawn Interface/Android

And also Android Programming


This backend is working. See Custom Drawn Interface/X11


This backend is working.


This backend is working.


This is planned, but not yet started.


TCanvas will be fully non-native in this widgetset and based on LazCanvas. All drawings to visual controls and on the OnPaint event of controls of the form are naturally double-buffered because the entire drawing of the form is first performed on an off-screen buffer and then copied in 1 operation to the form native canvas. In reality there is only 1 TLazCanvas for the entire form, but sub-controls will think they are on a separate canvas because the property BaseWindowOrg sets an internal start position of the canvas and also because all drawings will be clipped to reflect the size and shape of the sub-control.

Getting the TLazCanvas object inside TCanvas

This works only in the LCL-CustomDrawn interface:

uses lazcanvas, lclintf;

  MyLazCanvas: TLazCanvas:
  if nctLazCanvas in LCLIntf.GetAvailableNativeCanvasTypes(MyCanvas.Handle) then
    MyLazCanvas := TLazCanvas(LCLIntf.GetNativeCanvas(MyCanvas.Handle, nctLazCanvas));
    // do something here with TLazCanvas

Drawing optimization roadmap

Optimizations already done

Optimizations to be done

  • Buffer the native bitmap of the form instead of generating it on each operation
  • Reuse the buffered native bitmap contents if they don't change
  • Support invalidating only a rectangle instead of the entire control

Font rendering

When the define CD_UseNativeText is activated, LCL-CustomDrawn will use the native text rendering of the platform as provided by the backend. If it isn't activated, then it will use LazFreeType to render the text. The define is automatically activated for some backends if convenient when using them.

Message and Common Dialogs

Message and Common Dialogs might be native if this is considered very convenient for the backend. If not, they will be non-native. At the moment the Android backend has native message boxes.

Windowed visual controls

All Windowed visual controls (TButton, TPageControl, etc) will be based in the Lazarus Custom Drawn Controls

Conditional defines accepted by LCL-CustomDrawn

One easy way to set a conditional define for the LCL-CustomDrawn is to define it in the file lcl/interfaces/customdrawn/

Here are defines which affect the functionality offered by this interface:

  • CD_UseNativeText - Activates using native text instead of PasFreeType. This define will be automatically set if convenient for a particular backend, don't set it manually unless you know what you are doing.

And here debug information defines:

  • VerboseCDPaintProfiler - Adds profiling information to indicate how fast the paint event is processed
  • VerboseCDWinAPI - Extended verbose information for LCLIntf calls, except those which are covered by one of these defines instead:
    • VerboseCDText - Verbose info for text winapi calls
    • VerboseCDDrawing - Verbose info for Canvas and drawing operations
    • VerboseCDBitmap - Verbose info for Bitmap and rawimage creation and handling
  • VerboseCDForms - Extended verbose information for TWSCustomForm methods and about the non-native form from customdrawnproc (when utilized)
  • VerboseCDEvents - Extended verbose information for native events (for example mouse click, key input, etc). This excludes the paint event
  • VerboseCDPaintEvent - Debug info for the paint event
  • VerboseCDApplication - Verbose info for App routines from the Widgetset object
  • VerboseCDMessages - Verbose info for messages from the operating system (Paint, keyboard, mouse)


Lazclock customdrawn.png

lcl android 30 mar.png vpr-android.png

See Also