R – Passing parameters to a delphi TFrame

constructordelphitframe

To avoid singletons and global variables I'd like to be able to pass parameters to a TFrame component. However since a TFrame normally is included on form at design time it is only possible to use the default constructor.

The parent form can of course set some properties in the OnCreate callback after the TFrame has been created. However this does not ensure that a property is not forgotten, and the dependencies are not as clear as using a constructor.

A nice way would be if it was possible to register a factory for creating components while the dfm file is being read. Then the required parameters could be passed to the TFrame constructor when created by the factory. Is there a way of accomplishing this?

Or does anyone have a better solution on how to pass parameters to a TFrame?

Best Answer

All components, including descendants of TFrame, need to be able to be constructed using the constructor inherited from TComponent. Otherwise, they can't be used properly at design time. If the restriction of design-time use is acceptable to you, then you could override that constructor and raise an exception. That would prevent the component from being placed on a form at design time. Simply provide some other constructor that requires other parameters.

Because of the design-time requirement, all components need to be able to exist with some or all of their properties still at their default values. That doesn't mean the components have to do useful things while they're in that state, but they do need to be able to stay in that state indefinitely. It should be OK, for example, to place a component on a form, save the form, and close Delphi, with the intention of resuming the form-designing at a later time. The component should allow itself to be saved and restored, even if all its properties haven't been set up for final use yet.

My preferred option is to enforce the component's rules only at run time. Check that all the properties are set to sensible values before you allow them to be used. You can use assertions to enforce the proper use of your components. Consumers of your classes will learn very quickly if they haven't finished setting up your components on their forms.

Related Solutions

Delphi – Showing TFrame descendant’s additional properties on the object inspector

Which custom module class are you registering for your frame? Which version of Delphi are you using?

From my experiments with Delphi 2007, the custom module class which seems to work is TFrameModule. This class is contained in delphivclide100.bpl. Since there is no corresponding delphivclide.dcp you have to load it manually:

unit FrameTestReg;

interface

procedure Register;

implementation

uses
  Windows, DesignIntf,
  FrameTest;

procedure Register;
var
  delphivclide: THandle;
  TFrameModule: TCustomModuleClass;
begin
  delphivclide := GetModuleHandle('delphivclide100.bpl');
  if delphivclide <> 0 then
  begin
    TFrameModule := GetProcAddress(delphivclide, '@Vclformcontainer@TFrameModule@');
    if Assigned(TFrameModule) then
      RegisterCustomModule(TTestFrame, TFrameModule);
  end;
end;

end.

My FrameTest unit is very simple, it has no FrameTest.dfm, only the declaration of the new TFrame descendant:

unit FrameTest;

interface

uses
  Forms;

type
  TTestFrame = class(TFrame)
  private
    FHello: string;
  published
    property Hello: string read FHello write FHello;
  end;

implementation

end.

Using TFrameModule class, everything seems to work fine so far. I can create a new descendant of TTestFrame to include in the project and edit its published properties in the Object Inspector, put instances of this new descendant on a form in the IDE, edit their new published properties in the Object Inspector, write event handlers for their child components etc. In the .dfm resource I can see the expected "inline" directive for the instances. I haven't encountered any problem with it so far so perhaps this is the solution.

R – TFrame inheritance refactoring

In addition to changing base class of TMyFrameTreeView to TMyBaseFrame change the first word in the dfm file for TMyFrameTreeView from object to inherited.

Best Answer

Related Solutions

Delphi – Showing TFrame descendant’s additional properties on the object inspector

R – TFrame inheritance refactoring

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