How to pass DataSet: TDataSet as procedure parameter

Question

I am setting up a new procedure which will show a message after executing a query. I am using the "AfterOpen" Event where i have to pass the "DataSet: TDataSet" parameter.

procedure Tf_SeznamDluzniku.ShowInfoMessage(DataSet: TDataSet; info : string);
begin
  l_InfoMessage.Caption := info;
  img_success.Visible := True;
end;

query.AfterOpen := ShowInfoMessage(,'InfoMessage here')

Can somebody please explain me what is the DataSet variable and what i have to pass to the procedure as the first parameter ?

Answer 1

If it is attached to the event, it is the dataset that triggered the AfterOpen event. The dataset itself will call the procedure, and pass itself in that parameter.

But you added the Info parameter, which makes the procedure invalid as an event handler. Where do you want that info to come from? From the dataset?

Since it's an event handler, it's bad practise to call it yourself. You could do it, and just pass nil (or a specific dataset), since it's not used anyway. But you can get into weird situations, because it looks like the method is only going to be called after open, but then it turns out it's called on other occasions as well. So it's better to make a separate procedure to do what you want, and call that from the AfterOpen event handler. You can pass in info from the dataset, but you can also call that procedure from somewhere else, for instance to provide some initial caption until the dataset is opened:

// The procedure doesn't need the dataset, only the info to set.
procedure Tf_SeznamDluzniku.ShowInfoMessage(Info : string);
begin
  l_InfoMessage.Caption := info;
end;

// The actual event handler for YourDataset.OnAfterOpen (you have to attach them)
// This can set the info, and/or maybe set the success indicator right away..
procedure Tf_SeznamDluzniku.YourDataSetAfterOpen(DataSet: TDataSet);
begin
  ShowInfoMessage(DataSet.FieldByName('info').AsString);
  img_success.Visible := True;
end;

// For demonstration, another event handler for showing the form, to put in some initial caption.
procedure Tf_SeznamDluzniku.FormShow(const Sender: TObject);
begin
  ShowInfoMessage('Loading...');
end;