Modifying VCL Components

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Replacing a component class in delphi

Does anyone have any tips for modifying the VCL components and making them so they are relatively default within the IDE? This means not necessarily descending from them, but changing functionality in places that don't seem to allow such things in a descendant object. And if they can be used in place of the standard components, that's a bonus as well.

For example, one of the things I'd like to do is an alpha blended mouseover on a TButton. I notice no OwnerDraw event and the code to modify how the button was drawn seems to not be in a place that I can override. Any suggestions, or would I have to strip TButton and all its ancestors out of the VCL code and modify it directly?

I actually think that rather than being a duplicate, this question is just too scattered and fuzzy, and is thus unanswerable. — Warren P, Aug 16 '11 at 02:25
Not really a duplicate and the link really doesn't further answering the question. That said: What do you do when you get a "you can't override this" message, or you need to change the code in a private method ("method does not exist in base class" message)? Hopefully that is less "scattered and fuzzy". — Glenn1234, Aug 16 '11 at 07:19

score 3 · Answer 1 · edited May 23 '17 at 12:19

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You can choose one of the following methods:

Override ReadState of your form, as explained here
Use an interposer class, as explained here and here
Assign a new WindowProc, as explained here.

edited May 23 '17 at 12:19

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answered Aug 15 '11 at 18:43

NGLN

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And derive from TCustomButton, not from TButton. That gives you more control over the properties that are published. – Rudy Velthuis Aug 15 '11 at 18:49

score 1 · Answer 2 · answered Aug 15 '11 at 20:07

You are making several questions related to developing custom V.C.L. controls.

Since, its a very extended topic, you may want to browse or search the internet about it.

As a fast question, there are several ways to develop or extend a control.

Sometimes, the same control can be developed in different ways, other, it depends the kind of control, that you want to develop.

You also mention how to make a control, interact with the Delphi I.D.E.

Delphi controls work with packages, you can make a package with plain libraries without controls, you may want to start doing that, before making a control.

You can make a control, that doesn't interact at all, and only works by running the application, and later add interaction with the Delphi I.D.E.

In Delphi, when you develop a control and want to put it on form, while designing, internally, its like Delphi was your application, and already have your control.

score 1 · Answer 3 · answered Aug 16 '11 at 02:30

If you want to change the appearance of all TButtons in your application, you can do it without modifying the VCL at all. I see no necessity to modify or even subclass a VCL control yourself, when a few hundred button components exist out there. For example, for alpha-blended mouseovers, just check out the TMS controls.

However, for do-it-yourself techniques; Method one (recommended as simple) is to simply take all the TButtons out of your application and replace it with something else. VCL component replacement (globally) is easily done via gExperts. To see how to combine a standard control appearance (how a button looks) with some overlaid elements drawn in Delphi code, see TBitBtn as a sample. Rather than subclassing TButton or TCustomButton, I recommend you examine TBitBtn sources, and start from it.

Method two (not recommended as simple) is to start using a skinning system which can draw a completely different appearance over standard controls.

Modifying VCL Components

3 Answers3