How to save expression together with animation preset?

Question

I am trying to understand expressions and animation presets.
I have one single slider control, which controls the opacity of a layer, where the value is the slider value + slightly changed using an expression.
I can save the slider control as an animation preset, but the expression gets lost.
Yes, this is so basic that it can be, this is the way I like to learn. And as far as I know, expressions can be saved together with animation presets. I hope someone can tell me how!

Answer 1

The reason that the expression is not removed when you delete the slider is that the expression is only reading the slider value, the slider doesn't know that the expression is there. There's no reverse link.
You can build more professional effects but that involves editing the xml file that tells AE what effects are applied. This still would not generate a reverse link to an expression.
Enabling or disabling an effect only changes the way things are rendered. If you have keyframes on a Lens Flair Brightness and you disable the effect the Lens Flair will not render but the keyframe values will still be available to an expression. Again, there's no reverse link. Since an expression slider does not make a change in the pixel values of a layer turning the FX switch on or off will have no effect on an expression reading the value.
The "trick" in creating an Animation Preset that uses expressions and is removed with the "effect" is removed from the Effects Control Window is to only write expressions for properties that are available in the effects used. You could do this by applying the Transform Effect, or the Levels Individual Controls Effect, or the CC Composite Effect to the layer and writing the expression there instead of placing your expression in the Layer's opacity property. Take a look at this screenshot.