Creating descendants of TComponent
lazarus / 31-LCL

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egsuh · 2021-01-19 01:44

I'd like to create my custom treeview like following, adding some pre-
defined actions so that I do not have to write the same codes (like
DragDrop event handlers, etc.) multiple times.  Following codes
compiles well, but I haven't applied yet (not executed yet).

Please comment on the following approach, and better ways, if any. 

Can I use TCustomTreeView instead?

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i type    {
TaqTreeView }    TaqTreeView = class(TTreeView)      procedure
DragDrop(Sender, Source: TObject; X, Y: Integer);      procedure
DragOver(Sender, Source: TObject; X, Y: Integer;          State:
TDragState; var Accept: Boolean);   private      function
MoveTreeNode(SNode: TTreeNode; TTV: TTreeView): TTreeNode;   public  
  constructor Create(AnOwner:TComponent); 
 end; implementation procedure TaqTreeView.DragOver(Sender, Source:
TObject; X, Y: Integer;  State: TDragState; var Accept:
Boolean);begin     // .....        Accept := True;end; procedure
TaqTreeView.DragDrop(Sender, Source: TObject; X, Y: Integer);begin   
 //......end; constructor TaqTreeView.Create(AnOwner:
TComponent);begin   inherited Create(AnOwner);    OnDragDrop:=
DragDrop;   OnDragOver:= DragOver;end;

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Blaazen · 2021-01-19 02:12

The way you do it is wrong. Ancestors already have (virtual) methods
DragDrop and DragOver. You implement them again and assign them to
events.
You'd better override those methods (mark them override;) and call
inherited; in their bodies.
Also, mark constructor with override; because it's virtual too.

Side note:
Assigning events in code should be (in {$mode objfpc}):
   OnDragDrop:=@DragDrop;
   OnDragOver:=@DragOver;
but do not use it here.

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egsuh · 2021-01-19 03:01

@Blaazen

Thank you Blaazen, I wanted this kind of comments. But it says that it
should be controlled by OnDragOver handler.

QuoteWhen an OnDragOver handler is installed, Accept is set to True
and can be changed by the handler.
at

https://lazarus-
ccr.sourceforge.io/docs//lcl/controls/tcontrol.dragover.html

Anyway I might have to override DragOver method.

Regarding @ before function name, I'm using {$mode Delphi} so I do not
need @.

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Blaazen · 2021-01-19 06:24

Some drag method of LCL calls DragOver and DragOver calls OnDragOver
(if there's any).
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i procedure
TControl.DragOver(Source: TObject; X,Y : Integer; State: TDragState; 
var Accept:Boolean);begin  {$IFDEF VerboseDrag} 
DebugLn('TControl.DragOver ',Name,':',ClassName,'
XY=',IntToStr(X),',',IntToStr(Y));  {$ENDIF}  Accept :=
Assigned(FOnDragOver);  if Accept then   
FOnDragOver(Self,Source,X,Y,State,Accept);end;
So you declare
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i procedure
DragOver(Sender, Source: TObject; X, Y: Integer;          State:
TDragState; var Accept: Boolean); override;
and now it's up to you. You probably don't need to call inherited. You
can manage calling of OnDragOver yourself or do not use it at all etc.
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i procedure
TaqTreeView.DragOver(Sender, Source: TObject; X, Y: Integer;  State:
TDragState; var Accept: Boolean);begin     // .....       Accept :=
True;end;

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egsuh · 2021-01-19 07:45

Thank you for sharing these codes. I looks like I don't have to assign
anything to OnDragOver handler, as you said. This is good for me for
convenience.

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PascalDragon · 2021-01-19 08:00

Quote from: egsuh on January 19, 2021, 02:44:07 amCan I use
TCustomTreeView instead?
The difference between TTreeView and TCustomTreeView is mainly that
the former provides various properties as published. So if you want to
control which properties are published and which are not (especially
if you don't want some published that TTreeView declares as such), you
better inherit from TCustomTreeView.

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egsuh · 2021-01-19 08:00

But I cannot override TTreeview.DragOver. It says there are no method
to override. There exists TControl.DragOver, but not TTreeview's one
is not defined. So, I'll first check writing DragOver without
assigning OnDragOver handler, and then will assign the event handler
if fail.

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Blaazen · 2021-01-19 10:25

I just test and it works:
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i {
TMyTreeView }   TMyTreeView = class(TCustomTreeView)   protected   
procedure DragOver(Source: TObject; X, Y: Integer; State: TDragState;
var Accept: Boolean); override;  end;The list of parameters must be
the same. Unit ComCtrls must be in uses. It works for TTreeView too.