Are keystrokes automatically bubbled up the parent hierarchy?

[vfclists]

When you enter a key which isn't handled by an object that receives key strokes, such as a button, a button or label's hotkey or a TEdit control, will all the items in the parent hierarchy receive the keystroke whether they respond to keys or not?

For instance if a TPanel contains a TEdit which doesn't respond to a key, will the TPanel react to it if the necessary event handlers are in place?

It is it also possible for a parent component to prevent a child component from receiving the event or vice versa?

PS. I just noticed this thread - https://forum.lazarus.freepascal.org/index.php/topic,66621.0.html

Does it have a bearing on this question?

[Thaddy]

It is the other way around: the parent window dispatches to the children. So, basically, you can intercept at the parent level and prevent it going to the children.

[vfclists]

It is the other way around: the parent window dispatches to the children. So, basically, you can intercept at the parent level and prevent it going to the children.

Does this mean that although a TPanel on its own doesn't respond to character entry strokes, it can block character entry strokes from getting to a TEdit contained in it?

It doesn't have any OnKeyXXXX events in the properties pane. Does it mean it would have to be cast to an ancestor component which deals with those events before being able to handle them?

[rvk]

It's probably easier to just set TForm.KeyPreview to true and use TForm.OnKeyDown.
Set the parameter Key to 0 if you want to block it from going to any other control on the form.

[Curt Carpenter]

I did not know that.  Good to know.

[Thaddy]

Rvk's answer is a practical way to implement my answer: You will see that the messages for the edit are first seen by the form.

[vfclists]

It is the other way around: the parent window dispatches to the children. So, basically, you can intercept at the parent level and prevent it going to the children.

I assume that the parent window must be the form itself. So if the form does not intercept the key does the event handling return to the active control that is responsive to key strokes, or does the key-handling go from the outermost parent inwards to the active control?

Eg an Tedit control is placed on the innermost of a set of containers nested four deep, will each container from outside inwards be able to receive the event and block it from processing by the TEdit?

Is there some other TPanel replacement that responds to key events, or can TPanel be rejigged to accept key events?

[rvk]

I assume that the parent window must be the form itself. So if the form does not intercept the key does the event handling return to the active control that is responsive to key strokes, or does the key-handling go from the outermost parent inwards to the active control?

The key goes to the actual component, not to the parent.
So if KeyPreview is true, it will go to the TForm first, otherwise it will arrive directly at the control.

Also see https://wiki.freepascal.org/LCL_Key_Handling

Is there some other TPanel replacement that responds to key events, or can TPanel be rejigged to accept key events?

Well, you can just expose OnKeyDown for the TPanel and use that.

Create a new project.
Put a TButton and TPanel on the form (in that order).
Double click the form (outside the TPanel) to create the TForm1.FormCreate() event.

Now paste in the following code (over the whole unit1.pas)

Code: Pascal  [Select][+][-]

1. unit Unit1;
2.  
3. {$mode objfpc}{$H+}
4.  
5. interface
6.  
7. uses
8.   Classes, SysUtils, Forms, Controls, Graphics, Dialogs, StdCtrls, ExtCtrls;
9.  
10. type
11.   TPanel = class(ExtCtrls.TPanel)
12.   published
13.     property OnKeyDown;
14.   end;
15.  
16. type
17.   { TForm1 }
18.   TForm1 = class(TForm)
19.     Button1: TButton;
20.     Panel1: TPanel;
21.     procedure FormCreate(Sender: TObject);
22.   private
23.  
24.   public
25.     procedure MyNewKeyDown(Sender: TObject; var Key: Word; Shift: TShiftState);
26.  
27.   end;
28.  
29. var
30.   Form1: TForm1;
31.  
32. implementation
33.  
34. {$R *.lfm}
35.  
36. { TForm1 }
37.  
38. procedure TForm1.FormCreate(Sender: TObject);
39. begin
40.   Panel1.TabStop := true;
41.   Panel1.OnKeyDown := @Self.MyNewKeyDown;
42. end;
43.  
44. procedure TForm1.MyNewKeyDown(Sender: TObject; var Key: Word; Shift: TShiftState);
45. begin
46.   Showmessage(chr(Key));
47. end;
48.  
49. end.

If you run this, the Button1 will have focus. If you press Tab then the TPanel has focus (we set the Panel1.TabStop to true in FormCreate.
If you now press a key, the MyNewKeyDown will be executed.

(Although it's not visible now that Panel1 has focus but that can be done too, of course.)

[vfclists]

I assume that the parent window must be the form itself. So if the form does not intercept the key does the event handling return to the active control that is responsive to key strokes, or does the key-handling go from the outermost parent inwards to the active control?

The key goes to the actual component, not to the parent.
So if KeyPreview is true, it will go to the TForm first, otherwise it will arrive directly at the control.

Also see https://wiki.freepascal.org/LCL_Key_Handling

Is there some other TPanel replacement that responds to key events, or can TPanel be rejigged to accept key events?

Well, you can just expose OnKeyDown for the TPanel and use that.

Create a new project.
Put a TButton and TPanel on the form (in that order).
Double click the form (outside the TPanel) to create the TForm1.FormCreate() event.

Now paste in the following code (over the whole unit1.pas)

Code: Pascal  [Select][+][-]

1. unit Unit1;
2.  
3. {$mode objfpc}{$H+}
4.  
5. interface
6.  
7. uses
8.   Classes, SysUtils, Forms, Controls, Graphics, Dialogs, StdCtrls, ExtCtrls;
9.  
10. type
11.   TPanel = class(ExtCtrls.TPanel)
12.   published
13.     property OnKeyDown;
14.   end;
15.  
16. type
17.   { TForm1 }
18.   TForm1 = class(TForm)
19.     Button1: TButton;
20.     Panel1: TPanel;
21.     procedure FormCreate(Sender: TObject);
22.   private
23.  
24.   public
25.     procedure MyNewKeyDown(Sender: TObject; var Key: Word; Shift: TShiftState);
26.  
27.   end;
28.  
29. var
30.   Form1: TForm1;
31.  
32. implementation
33.  
34. {$R *.lfm}
35.  
36. { TForm1 }
37.  
38. procedure TForm1.FormCreate(Sender: TObject);
39. begin
40.   Panel1.TabStop := true;
41.   Panel1.OnKeyDown := @Self.MyNewKeyDown;
42. end;
43.  
44. procedure TForm1.MyNewKeyDown(Sender: TObject; var Key: Word; Shift: TShiftState);
45. begin
46.   Showmessage(chr(Key));
47. end;
48.  
49. end.

If you run this, the Button1 will have focus. If you press Tab then the TPanel has focus (we set the Panel1.TabStop to true in FormCreate.
If you now press a key, the MyNewKeyDown will be executed.

(Although it's not visible now that Panel1 has focus but that can be done too, of course.)

Does this need to be done on every form which needs the functionality?

The only way I can see to avoid doing this in every form that needs it is to use an {$i keydownform.inc} in every unit that needs it.

In any case I subclassed the form component, but this method will be useful when I don't need it design in at runtime.

[rvk]

Does this need to be done on every form which needs the functionality?

No, you could put this in a unit, in a procedure, and call it from FormCreate;
( for instance HackMyPanel() )

But YIKES... I now see that OnkeyDown is not private for TPanel. It's just not exposed as property. But it is still an event 

So you can still do this without redeclaring the TPanel itself.

Code: Pascal  [Select][+][-]

1.   Panel1.OnKeyDown := @Self.MyNewKeyDown;

You can still create a procedure like HackMyPanel() but then you still need to call that in FormCreate.

So... either create a complete new visible component class from TPanel where you can use it in the Object Inspector... (like you did)
or just add Panel1.OnKeyDown := @Self.MyNewKeyDown; to your FormCreate where you need it.