code: function setReaction(vm) {
  	var observerData = vm._mobxObserver;
  
  	// Useful during development:
  	if (observerData.reaction)
  		throw Error("Reaction already set.");
  
  	observerData.stale = true;
  	observerData.reaction = new mobx.Reaction(observerData.name, function() {
  		observerData.stale = true;
  		if (vm.hooks.becameStale)
  			vm.hooks.becameStale(vm, vm.data);
  	});
  
  	// The reaction should be started right after creation. (See: https://github.com/mobxjs/mobx/blob/5.6.0/src/core/reaction.ts#L35)
  	// But it doesn't seem to be mandatory... ?
  	// Not doing it, as that would trigger becameStale() and a vm.redraw() right now !
  	// In case we need it, see convoluted implementation of fireImmediately in MobX autorun(): https://github.com/mobxjs/mobx/blob/5.6.0/src/api/autorun.ts#L146
  	//observerData.reaction.schedule();
  }
docs: Creates the observer Reaction: