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Ralph Kazemier
Guest
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 Posted: Wed Aug 13, 2003 2:45 pm Post subject: Re: How do I override Event of SubComponent |
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"Herwig Huber (TeamBeer)" <herwig.huber (AT) teambeer (DOT) net> wrote
| Quote: | Hi,
TDBGrid contains the following sub-components:
DBGrid->DataSource->DataSet
Is it possible to override the AfterCancel() event of TDataSet from
TDBGrid? |
Yes, just not favorable.
| Quote: | I tried the following, It compiles, but it doesn't work...
snip
__fastcall TMyDBGrid::TMyDBGrid(TComponent* Owner) : TDBGrid(Owner)
{
this->DataSource->DataSet->AfterCancel = Cancel;
}
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This is dangerous code. At this point (ie. inside the constructor) the
properties of your component do not have their values yet. These values are
defined in the dfm resource. The properties are not streamed into your
component yet, while the constructor executes. However, the VCL streaming
system calls TComponent::Loaded(), after the dfm resource is entirely read,
on each component created during the streaming of a dfm resource. That is
the moment you can be sure that the properties have their values. But you
blatantly assume that DataSource is set and that it has its DataSet property
set. This does not have to be the case and thus can result in a NULL
pointer crash. So, at least you should anticipate the fact these pointers
may be NULL.
Still, IMHO you need to find another way to be notified of the 'AfterCancel'
event. The end-users of TMyDBGrid will not be able to use the AfterCancel
once they link it to the grid. I would not expect that if I'd use a
TDBGrid-like component. Futhermore, what happens if the DataSet of the
DataSource is changed?
Internally TCustomDBGrid (ancestor for TDBGrid) uses a TGridDataLink
(derived from TDataLink) to respond to changes in the underlying dataset. I
guess you need a similar approach to be able to respond to the 'AfterCancel'
event in a proper way.
Ralph
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Herwig Huber (TeamBeer)
Guest
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| Posted: Wed Aug 13, 2003 8:27 pm Post subject: Re: How do I override Event of SubComponent |
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Dear Ralph,
First of all thank you very much for your explanations.
I made several tests with all your suggestions and now I realize it's
probably better to go another way :)
I also played around with TGridDataLink but I didn't get very far. I decided
to go a completly different way.
Thanks again. You preserved me of wasting more time on this.
Herwig
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