Carousel numViewsToShowOnScreen and config changes

Hey @elihart, I’ve been trying to set numViewsToShowOnScreen on a Carousel to a percentage of the display width (which gets updated upon config changes). To achieve this, what I’m doing is quite simple:

numViewsToShowOnScreen((17 * screenWidthPx / 100) / 100) // Where 17 is the percentage visible

This works well upon rendering the first time, but it breaks as soon as I rotate the device… and if I scroll through the carousel it seems to fix itself (probably due to recycling of the modes). I’m attaching a short GIF of what’s happening.

Any idea of what might be happening? I’ve tried invalidating the RecyclerView and the Carousel itself (during onBind), also assigning Random id’s to force Epoxy to rebuild the models, but none of this seem to solve the issue.

[andybdahl]

@EliHart We fixed it locally in our app already, I’ll be putting up a merge request when I get the time for it soon.

I fixed it by checking whether or not the carousel has been rendered dirty after a configuration change, so it’s rather simple.

On Thu, 11 Feb 2021, 17.10 Eli Hart, [e-mail] wrote:

Thanks for the investigation @andybdahl https://github.com/andybdahl , that seems to make sense. It does seem a little difficult to detect when the measured width/height is not accurate though. Some possibilities:

• figure out how to properly detect a configuration change
• add an option to the carousel to not use measured dimensions

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[andybdahl]

I deep dived into this a bit as I have experienced problem multiple times before. The culprit is the Carousel.getTotalWidthPx function, as this returns the wrong value after a configuration change. At first the totalWidthPx returns the measured width of the carousel as it hasn’t been laid out yet, which is the correct totalWidth for the carousel. After a configuration change, the carousel is already laid out before the change, and so the getTotalWidthPx returns the carousels actual width, which is “wrong” as it was the width before the configuration change, so this could be fixed by checking if the screen was rotated since the view was laid out, and thus returning a measured value instead, as the view hasn’t been laid out in the current configuration yet.