re-render a computed value after set a observable object in callback

[NadavShaked]

I'm trying to render a mobx @computed value in UnifiedPicker component after I updated the object on the onSelectionChanged callback, the object is updated but the computed doesnt change.

this is the main code in the store:

 @observable
selectedMachineGroups: MachineGroup[];
...

 @computed
get getSelectedMachineGroupsCount(): number {
return this.selectedMachineGroups?.length ?? 0;
 }
 @action
setSelectedMachineGroups(machineGroups: MachineGroup[]) {
this.selectedMachineGroups = machineGroups;
 }

and this is the component code:

const onSelectionChanged = (selected: MachineGroup[]): void => {
    //TODO: insert all the machine groups? or only selected
    if (selected.length === 0 || (machineGroups && machineGroups.length === selected.length)) {
        machineGroupsAsyncData.setSelectedMachineGroups(machineGroups);
    } else {
        machineGroupsAsyncData.setSelectedMachineGroups(selected);
    }
    console.log(machineGroupsAsyncData.selectedMachineGroups);  //updated
    console.log(machineGroupsAsyncData.selectedMachineGroupsCount); //doesnt update
};

const unifiedPickerProps: IUnifiedPickerProps<MachineGroup> = {
    columnDefs: columnDefs,
    isSingleSelection: false,
...
    onSelectionChanged: onSelectionChanged,
    dataProvider: new DeviceGroupPicker(machineGroups),
    addButtonText: 'Apply',
    cancelButtonText: 'Cancel',
    pickerNameButtonProps: {
        type: PickerButtonType.Button,
    },
};

return (
		<React.Fragment>
			<RollingOutContextProvider value={{ UnifiedPicker: 'Beta' }}>
				<UnifiedPicker key={key} {...unifiedPickerProps} />
			</RollingOutContextProvider>
			<div
				style={{
					marginTop: '5px',
				}}
			>
				Selected:
			</div>
		</React.Fragment>
	);

Do you know what can be the problem?

Thanks

[mweststrate]

missing makeObservable? Typically it is very hard to provide help without a small reproduction.

[NadavShaked]

can you elaborate?

[mweststrate]

are you using mobx6? Did you forget to use makeObservable? If yes and no: check the migration guide. In all other cases or if that doesn't help, please create a sandbox reproduction, otherwise it is extremely hard and time consuming to help without getting a complete picture of the code.