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Why am I unable to subclass an SWT control? [message #467624]

Wed, 01 February 2006 23:09

Jon Sorensen

Messages: 24
Registered: July 2009

Junior Member

To whom it may concern,

I wanted to subclass the Combo control so I could improve the API in regards
to associating "data" (an object) with an "item". Unfortunately when I do a
new on my derived class I get a Runtime exception (unable to subclass).

I find this restriction too limiting, especially when an API is not well
thought out and is difficult to use. I realize the SWT makes platform
dependent calls but the interface I added has nothing to do with the
underlying Windowing system.

Jon

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Re: Why am I unable to subclass an SWT control? [message #467627 is a reply to message #467624]

Wed, 01 February 2006 23:32

Alex Blewitt

Messages: 946
Registered: July 2009

Senior Member

There's documentation about why subclassing SWT controls is generally not allowed. Google for it, or look at the help pages regarding SWT and subclassing.

You *can* do it, if you assume the risks that it's entirely unsupported and that whenever a new version happens, you may have to change your code. But you'll need to override the checkSubclass() method to return true for your subclass in order to avoid the runtime exception being generated.

However, it sounds like you're working against the ethos of having separate model-view objects; the data shouldn't really be associated with the view widget.

Alex.

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Re: Why am I unable to subclass an SWT control? [message #467631 is a reply to message #467624]

Thu, 02 February 2006 04:35

Eclipse User

Originally posted by: sunil_kamath.nohotspammail.com

Jon <jon@tradescape.biz> wrote:
> To whom it may concern,
>
> I wanted to subclass the Combo control so I could improve the API in
> regards to associating "data" (an object) with an "item".
> Unfortunately when I do a new on my derived class I get a Runtime
> exception (unable to subclass).
>
> I find this restriction too limiting, especially when an API is not
> well thought out and is difficult to use. I realize the SWT makes
> platform dependent calls but the interface I added has nothing to do
> with the underlying Windowing system.
>
Use a ComboViewer instead of subclassing the Combo control.
Your model will contain the data and the Combo will contain the items.
The ComboViewer will manage the mapping between the two.

--
Sunil

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