UI Prefab Instantiation Position Issue

[ronjart]

Not sure if this is a bug, but I thought I'd share anyways. I'm instantiating a cursor prefab into my UIBlock2D OnHover/OnMove and I always get what seems to be a 1-frame flicker where the cursor object displays at the cursor prefab's default position. I've tried to instantiate with setting a initial position or set the proper position right after instantiation with no success. I've tried to disable the instantiated object and then re-enabling after a few frames for debugging purposes, but whenever I enable it, the cursor displays at the prefabs default position for one frame before it moves to the correct position. The cursor prefab is a single UIBlock2D that represents the UI cursor

  private void Hover(Gesture.OnHover evt) {
            if(evt.Interaction.ControlID == 1) { //left hand
                if(cursorLeftHand == null) {
                    cursorLeftHand = Instantiate(cursorPrefab, transform).GetComponent<UIBlock2D>();
                    cursorLeftHand.transform.position = evt.PointerWorldPosition;
                }
            }
        }

I've just changed the cursor prefabs default position to 1000 so the flicker isn't visible, which works fine. Just curious if I'm missing anything.

[adgray1]

While admittedly a nuance of the layout system, this is the expected and documented behavior. Long story short is the UIBlock.Position will always be used on the first frame a UIBlock GameObject is enabled, which means direct changes to transform.localPosition or transform.position will be overwritten by the calculated layout position when the Nova engine runs at the end of the frame.

The way to work around this is to use uiBlock.TrySetLocalPosition(newTransformLocalPosition) or uiBlock.TrySetWorldPosition(newTransformWorldPosition), which will set the respective transform position and update uiBlock.Position, ensuring the two are in sync before the engine update. The one requirement for both those methods to work is uiBlock.gameObject.activeInHierarchy must be true when they are called, otherwise they'll fail and return false.

In your case, that would like:

private void Hover(Gesture.OnHover evt) {
            if(evt.Interaction.ControlID == 1) { //left hand
                if(cursorLeftHand == null) {
                    cursorLeftHand = Instantiate(cursorPrefab, transform).GetComponent<UIBlock2D>();
                    cursorLeftHand.TrySetWorldPosition(evt.PointerWorldPosition);
                }
            }
        }