How to get notification if children have added/moved/removed?

Hello. How can i get a notifcation when a child have been placed on a gameobject?

When i derive from UIBehaviour, then i can use OnTransformParentChanged() when the gameobject have changed its own parent, but how do i get the notification if a child object have been added, moved or removed?

 

Simple answer is you can't through normal behaviour.
What you would need to do is attach a script to your parent GO which tracks the number of children (and/or their order), then validate that on update. If it changes, then do what you need to do.

Hope that helps.

Another way, would be to have the script on the Parent GO manage the adding / removal of children instead of just adding children directly. SO having a method which takes the new child and then adds it to the parent (plus anything else).

 

Yes, but how does the "Horizontal Layout Group" (unitys buildin) component do it? Every time i put a child on, then it automaticly updates so all children will have the same horizontal width and updated positions.

 

That is because every update it loops through it's children to validate their position, then sets the scene dirty if it changes.
You can view the Layout Groups source in the public repo to check if you like:
https://bitbucket.org/Unity-Technol...c87095b/UnityEngine.UI/UI/Core/Layout/?at=4.6

 

Ahh okay, thanks.
So the first solution you gave me was to check update every loop.
And the second solution to "know" when to update and do it manually.

So in conclusion, the first solution is easier to manage but a little slower because i have to "check" every loop in the update(), and the second solution is a little harder to manage because i have to "know" and manually udpate it.

Am i correct?

 

That's pretty much it. Unity doesn't have any "Child" style events or watches.

The second option is probably easier because you are managing the GO from itself, pretty much the same way as public property getters and setters, e.g.:

Code (CSharp):

1. public void AddChildToGO(gameobject GO)
2. {
3.     go.transform.parent = this;
4.     NotifyChildAdded();
5. }

*Note, the code above is just an example and from memory (so probably won't compile, but you should get the idea)

**Edit, if you are also adding UI elements as children, you'll see to use the SetParentAndAlign function (again from memory, the function to align UI which does more than just add a child)

 

Cool, thanks for your help

 

@SimonDarksideJ @Assassinbeast

In case either of you still care (or anyone else comes across this thread), there *is* actually a method that gets called when a transform's children change. Monobehaviours receive a message through the method OnTransformChildrenChanged() when the list of their children changes.

 

Thanks for the info @CoalCzar useful info. Knew about that function, but they added UI specific functions to make it easier.
Although now with 5.2, the goal posts have changed again to 3D meshes and base GO's, may have to dig in to it again

 

OnTransformChildreChanged is not work in editor mode.
Is it possible to get notification when a scene object' children have added/moved/removed?

 

OnTransformChildreChanged only works for your first level of child list. So when your child is adding an grandchild, you won't be notified.

 

Only really accurate way is to either create your own AddChild methods or have a script monitoring transform.count for all it's children. Either way it's a bit more overhead.

if using OnTransformChildrenChanged, you will need to filter the event up to parents from children (if you have grandchildren for example)

 

ExecuteAlways or ExecuteInEditMode attributes can be used

Code (CSharp):

1.  
2. using UnityEngine;
3. [ExecuteAlways]
4. public class RefreshChildrenPositions: MonoBehaviour
5. {
6.     private void OnTransformChildrenChanged()
7.     {
8.         print(0);
9.     }
10. }
11.  

 

Six years later that callback still doesn't appear to work correctly: it is invoked BEFORE the transform list is changed, rather than AFTER, so that if you need to react to the change ... you can't. You will find that the list of child-transforms is wrong: it hasn't changed (the opposite of the method name ).

Instead, the way that Unity's own team did it - manually polling! - appears to be the only correct solution.

 

Hello, you can delay execution through EditorApplication.delayCall https://docs.unity3d.com/ScriptReference/EditorApplication-delayCall.html

 

Bearing in mind that these are MonoBehaviour callbacks, i.e. this is outside the Editor. The method you cite is a good one for the Editor half of the solution, but MB needs code both in Editor and Rutnime.

We can fudge it at runtime, by delaying one frame before reacting, but that causes flickering visually so its not great.

 

If anyone is still here 7 years after the original post - assuming I get the event of a child being added/removed, is there a way to get a handle on that specific child?

 

No. This API was never properly planned, it's more of a hack IMHO.

However: update to my post from last year: the broken behaviour appears to be fixed in recent LTS releases. I don't know which version, I didn't see it in the release notes, but I noticed that my unit tests started passing in more recent UnityEditor versions, and seem to be stable/consistent.