Delphi自定义控件：一个带有TLabel的TRichEdit

Question

I want to create an custom control (descendant of TRichEdit). I simply want add some text above the editfield.

I've created my own control and I override the constructor to create a TLabel for the caption. It works, but my problem: How is it possible to move the label above the richedit? When I set Top := -5 the label begins to disappaer.

Here's the code of the constructor:

constructor TDBRichEditExt.Create(AOwner: TComponent);
begin
  inherited;
  lblCaption := TLabel.Create(self);
  lblCaption.Parent := parent;
  lblCaption.Caption := 'Header';
  lblCaption.Top := -5;
end;

I think it's logic that the label disappaers since the richedit is the parent. I've tried

lblCaption.Parent := self.parent;

To make the form which owns the richedit the parent - but this dosn't work...

How could I achieve this? Thank you all!

Answer 1

I think it's logic that the label disappaers since the richedit is the parent

This is wrong. In your code, the parent of the TLabel is the parent of the TDBRichEditExt , as it should be. Notice that, in a method of TDBRichEditExt , Parent and Self.Parent is the same thing. If you would like the parent of the TLabel to be the TDBRichEditExt itself - which you do not - then you should set lblCaption.Parent := self; .

Now, if the parent of the TLabel is the parent of the TDBRichEditExt , then the Top property of the TLabel refers to the parent of TDBRichEditExt , not to the TDBRichEditExt itself. Hence, if the parent of the TDBRichEditExt is a TForm , then Top := -5 means that the TLabel will be positioned five pixels above the form's upper edge. You mean

lblCaption.Top := Self.Top - 5;

But -5 is a far too small number. What you really should use is

lblCaption.Top := Self.Top - lblCaption.Height - 5;

which in addition makes a 5 px space between the label and the Rich Edit.

Also, you would like

lblCaption.Left := Self.Left;

Another issue

But this will not work, because at the time the component is created, I do not think that the Parent has been set yet. So what you will need is to do the positioning of the label at a more appropriate time. In addition, this will move the label each time your component is moved, which is very important!

TDBRichEditExt = class(TRichEdit)
private
  FLabel: TLabel;
  FLabelCaption: string;
  procedure SetLabelCaption(LabelCaption: string);
public
  constructor Create(AOwner: TComponent); override;
  procedure SetBounds(ALeft: Integer; ATop: Integer; AWidth: Integer; AHeight: Integer); override;
published
  LabelCaption: string read FLabelCaption write SetLabelCaption;
end;

procedure TDBRichEditExt.SetBounds(ALeft, ATop, AWidth, AHeight: Integer);
begin
  inherited;
  if not assigned(Parent) then
    Exit;
  FLabel.Parent := self.Parent;
  FLabel.Top := self.Top - FLabel.Height - 5;
  FLabel.Left := self.Left;
end;

Details

In addition, when you hide the TDBRichEditExt , you want to hide the label as well. Thus you need

protected
  procedure CMVisiblechanged(var Message: TMessage); message CM_VISIBLECHANGED;

where

procedure TDBRichEditExt.CMVisiblechanged(var Message: TMessage);
begin
  inherited;
  if assigned(FLabel) then
    FLabel.Visible := Visible;
end;

And similarly for the Enabled property, and you also need to update the parent of the TLabel each time the parent of the TDBRichEditExt is changed:

protected
  procedure SetParent(AParent: TWinControl); override;

with

procedure TDBRichEditExt.SetParent(AParent: TWinControl);
begin
  inherited;
  if not assigned(FLabel) then Exit;
  FLabel.Parent := AParent;
end;