Expression: Snapshot of current value

Hi, how would you guys solve the following problem using expressions?:

I have custom Slider Control "xPos" , it is keyframed and its value changes constantly;

I have a custom Checkbox Control "Check". It is keyframed too. When Check is checked, i want to store xPos's Current Value in a Variable "a".

While xPos is changing further down the timeline, i want to be able to access "a", which should always hold the Value xPos had, when Check last changed to value 1.

When i uncheck Check, and check it again, i want a to update to the new current value of xPos.

If you want to see, what i tried so far, check this thread of mine : Expression: I am stuck

Problem is, that when i set a = xPos, it updates constantly, instead of just taking the current value and holding it...

Thanks and kind regards