Overriding Component Create Constructor in Separate Design-Time package - delphi

Question

Programming Environment: Delphi 6 and upwards
I am aware of the fact that since Delphi 6, custom components must have separate design- and run-time package. All run time features of the a component must therefore be in a separate unit and packaged separately to the component's design-time package.
My problem is the following: My component has code that needs to be run both when it is created on the form at run time and, additional code that needs to be run at design-time, when the component is placed on the form. I have managed to put the run time code into the separate run time unit, package it and deploy it successfully.
However, in the separate design-time module unit, how do I reference and add the design-time code that needs to be included into the component's create constructor during design-time, when the component gets placed onto the form?

Answer 1

You can separate design time behaviour from run time behaviour with
if [not] (csDesigning in ComponentState) then
But if your constructor code needs the DesignIDE design time package, e.g. from the units DesignEditors, DesignIntf, etc..., then I think you are stuck. Maybe some IOTA involvement can help. But since there does not seem to exist a notifier interface for the creation of components, that would require a custom IOTAFormEditor. Not that easy, if not impossible.

Answer 2

Why not use callbacks?
From your designtime package initialization code do this:
unit MyDsgnUnit;
interface
//TMyHook defined in AnImplUnit
TMyDesignHandlerObject = class
procedure MyMethod(Sender:TObject;ParentForm:TObject); { must match TMyHook }
end;
implementation
uses AnImplUnit, DesignUnitNamesHere;
procedure TMyDesignHandlerObject.MyMethod(Sender:TObject);
var
newObject:TMyComponent;
begin
newObject := TMyComponent(Sender);
DoSomethingThatneedsDesigntimeStuff(newObject);
end;
finalization
ADesignHandlerObject.Free;
initialization
ADesignHandlerObject := TMyDesignHandlerObject.Create;
AnImplUnit.AfterConstructionHook := TDesignHandlerObject.MyMethod;
and in your component do something like this:
unit AnImplUnit;
interface
type
TMyHook = procedure(Sender:TObject;ParentForm:TObject) of object;
var
AfterConstructionHook:TMyHook;
implementation
...
procedure TMyComponent.Create(AOwner:TComponent);
begin
inherited Create(AOwner);
DOMyStuff;
if Assigned(AfterConstructionHook)
AfterConstructionHook(Sender,Parent);
end;
Update Flushed out example more. There is no reason you can't add more parameters to the AfterConstructionHook, but since your reference to Sender is already of type TMyComponent, I don't see the point, when you can access everything public or protected, in TMyComponent(Sender) from within your hook function, and if you inherit locally (known as a protected-access class), you can access the protected stuff too.

Answer 3

Make sure your DPK defines a symbol of your choice, for example DESIGNTIME. Then you'll be able to use something like this to only include design-time units when needed:
uses Windows, Whatever, Something
{$IFDEF DESIGNTIME}
,DesignIntf
{$ENDIF}
;
Then to the same in your constructor code:
constructor TMyClass.Create(aOwner:TComponent);override;
begin
inherited;
{$IFDEF DESIGNTIME}
// I'm at design time.
{$ENDIF}
end;
When using this technique you should either use a separate DCU directory for your pacakge and your normal executable, or do a build each time you switch from the design time package to other projects. That's because Delphi will only re-build a DCU if the PAS has changed, when in this case the PAS doesn't tell the whole story, defined symbols also matter. If you have a DCU on disk that was compiled by Delphi while you were building your design time project, you might see a ToolsApi.DCU not found when trying to compile a normal project. Rebuilding re-compiles the DCU and makes the message go away. Just as well, if you re-compile (not re-build) the design time project after you built a normal project, your DCU might be stuck in it's non-DESIGNTIME state, leaving you without your special design-time behavior.

Answer 4

As long as your code does not require the IDE tools, like design interface and so on, then all you need to do is check the component flag and you can use it anywhere within the component, as follows...
procedure TNewEdit.Loaded;
begin
inherited;
if (csDesigning in ComponentState) then
ShowMessage('Designing')
else
ShowMessage('Running');
end;
However without really knowing what you are attempting to do, there are several doors left open... for example, if you want to change a property value at design time, and a different value at run time, then there are streaming issues you have to deal with.