setActiveSub function
Sets the active subscriber node and returns the previous one.
Used to establish a reactive context where dependencies are automatically tracked. The previous subscriber should be restored after the reactive computation completes.
Returns the previous active subscriber.
Implementation
@pragma('vm:prefer-inline')
@pragma('dart2js:tryInline')
@pragma('wasm:prefer-inline')
ReactiveNode? setActiveSub([ReactiveNode? sub]) {
final prevSub = activeSub;
activeSub = sub;
return prevSub;
}