Option to choose if current value is retained

[mokkabonna]

Consider having an option to choose if the current value of the 1st call is kept until the 2nd value has returned. Currently it is set to null.

Sometimes you don't want to clear the visible UI that contain the current values, but rather dynamically update it in-place when the new values arrive.

IIt could be that this should be the default. If so that would be a breaking change.