Hi all, sending this mail also to Plasma to get more feedback.
*Background*:
As you might know the KWin effect library does not provide a stable ABI which
would be a
requirement to move most of the effects out of KWin or to get developers having
their own
effects on kde-look. KWin ships in my opinion too many effects and many are in
a bad or
proof of concept state. Nevertheless for obvious reasons I do not want to
delete them.
*How to fix the problem?*:
We need a stable ABI. So I have been thinking a lot on how to get there. The
Effects library
provides two pure virtual classes "EffectsHandler" and "EffectWindow" which are
both used
by the effects and implemented in core. The problem now is that extending for
example
EffectWindow by adding a new virtual method would break the ABI.
*Idea how to fix it*:
Now I want to outline how I think we can fix it. The general idea is to keep
the interface to the
effects stable by removing all virtuals from EffectsHandler and EffectWindow
and to delegate
to a pure virtual EffectBridge which is implemented in core. Whenever we need a
new
method we add a subclass to EffectBridge and add the virtual. The
EffectsHandler itself can
just be extended with the new method without breaking ABI.
Example code EfffectsHandler header:
class EffectsHandlerPrivate;
public class EffectsHandler
{
public:
void setNumberOfDesktops(int newDesktops);
private:
const EffectsHandlerPrivate *d;
};
Example code EffectsHandler cpp:
class EffectsHandlerPrivate
{
public:
EffectsBridge* bridge;
}
void EffectsHandler::setNumberOfDesktops(int newDesktops)
{
d->bridge->setNumberOfDesktops(2);
}
Example code for Bridge:
public class EffectsBridge
{
public:
virtual void setNumberOfDesktops = 0;
};
Example code for adding new virtual method:
public class EffectsBridge2 : public EffectsBridge
{
public:
virtual bool isWayland() const = 0;
};
class EffectsHandlerPrivate
{
public:
EffectsBridge2* bridge;
}
So I hope to get some feedback on whether this will work. I would target this
change to
happen just before or at Tokamak and hope to get some API review by Kevin.
Disclaimer:
Even after that change we cannot make the ABI stable in 4.7 as there is also
some work
needed on other parts like the GLUtils or WindowPaintData.
Cheers
Martin
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