I need to know the global position of a Composable in order to choose how to draw it, i.e. what `onG...

# compose

[Marcin Wisniowski]

I need to know the global position of a Composable in order to choose how to draw it, i.e. what onGloballyPositioned returns, but I need it before it's placed. I tried SubcomposeLayout, but it only allows me to measure the content itself on the spot, not where it would be drawn. How do I get the "current" global position in my Composable?

[Alex Vanyo]

PlacementScope.coordinates is probably the tool you want: it gives the ability to check the global coordinates of a layout as it is being placed.

The ordering of phases can be a bit tricky to get correct without introducing a composition loop or an incorrect frame. You can delay measuring a child until the placement step when you have the coordinates available, but then that means the child’s measurement can’t influence the size of the parent (but its intrinsic size can, and that might be enough)

[Marcin Wisniowski]

I want to display a tooltip anchored to the right if it's attached to something on the left half of the window, or to the left if it's attached to something on the right half. Since it's a floating tooltip, it doesn't influence any parent size anyway.

[Marcin Wisniowski]

Hmm, PlacementScope.coordinates does allow me to get the info before choosing where to display it, but I don't have the ability to change what is displayed. I need to display the tooltip's anchor point triangle on the left or right of it depending on the outcome.

[Marcin Wisniowski]

I guess I could pass both versions to Layout and then only display one of the two placeables. But that feels a bit hacky.

[Alex Vanyo]

If changing the side of the triangle is only a part of drawing, then since drawing happens after placement you should be able to decide what to do based on reading PlacementScope.coordinates and doing the drawing logic after that.

Otherwise, if it has to be choosing between two different @Composable s based on the side, then passing both versions and only placing one is probably your best bet.

[Marcin Wisniowski]

The fact the tooltip has to be a Popup threw a wrench into this, since I can't control the Popup position from a custom layout, it does it's own thing ignoring where I place(x, y) it.

[Alex Vanyo]

Ah for tooltips with Popup specifically there’s a PopupPositionProvider which is involved

[Marcin Wisniowski]

I can probably achieve the positioning behavior I want with the PopupPositionProvider alone, but then I'm back to being unable to control which anchor triangle image I display and where.

[Alex Vanyo]

I’m not sure what the ordering guarantees are for when the PopupPositionProvider.calculatePosition is called, maybe you could try a custom one to know where it will be put, and update some state based on that and see if that works?

[Marcin Wisniowski]

Thanks for the help, I've got it working with a custom PopupPositionProvider that in addition to the popup position also sets some state to control the anchor triangle type and position.

The issue with this approach is that I only have that state set on the second frame, and the triangle image shows up 1 frame delayed after the actual popup which is noticeable. I solved that by setting the alpha to 0 on the popup until I have the anchor info so they appear together on the second frame.