android - ViewPager2 with differing item heights and WRAP_CONTENT

Question

There are a few posts on getting ViewPager to work with varying height items that center around extending ViewPager itself to modify its onMeasure to support this.

However, given that ViewPager2 is marked as a final class, extending it isn't something that we can do.

Does anyone know if there's a way to make this work out?

---

E.g. let's say I have two views:

View1 = 200dp

View2 = 300dp

When the ViewPager2 (layout_height="wrap_content") loads -- looking at View1, its height will be 200dp.

But when I scroll over to View2, the height is still 200dp; the last 100dp of View2 is cut off.

Answer 1

The solution is to register a PageChangeCallback and adjust the LayoutParams of the ViewPager2 after asking the child to re-measure itself.

pager.registerOnPageChangeCallback(object : ViewPager2.OnPageChangeCallback() {
    override fun onPageSelected(position: Int) {
        super.onPageSelected(position)
        val view = // ... get the view
        view.post {
            val wMeasureSpec = MeasureSpec.makeMeasureSpec(view.width, MeasureSpec.EXACTLY)
            val hMeasureSpec = MeasureSpec.makeMeasureSpec(0, MeasureSpec.UNSPECIFIED)
            view.measure(wMeasureSpec, hMeasureSpec)

            if (pager.layoutParams.height != view.measuredHeight) {
                // ParentViewGroup is, for example, LinearLayout
                // ... or whatever the parent of the ViewPager2 is
                pager.layoutParams = (pager.layoutParams as ParentViewGroup.LayoutParams)
                    .also { lp -> lp.height = view.measuredHeight }
            }
        }
    }
})

Alternatively, if your view's height can change at some point due to e.g. asynchronous data load, then use a global layout listener instead:

pager.registerOnPageChangeCallback(object : ViewPager2.OnPageChangeCallback() {
    private val listener = ViewTreeObserver.OnGlobalLayoutListener {
        val view = // ... get the view
        updatePagerHeightForChild(view)
    }

    override fun onPageSelected(position: Int) {
        super.onPageSelected(position)
        val view = // ... get the view
        // ... IMPORTANT: remove the global layout listener from other views
        otherViews.forEach { it.viewTreeObserver.removeOnGlobalLayoutListener(layoutListener) }
        view.viewTreeObserver.addOnGlobalLayoutListener(layoutListener)
    }

    private fun updatePagerHeightForChild(view: View) {
        view.post {
            val wMeasureSpec = MeasureSpec.makeMeasureSpec(view.width, MeasureSpec.EXACTLY)
            val hMeasureSpec = MeasureSpec.makeMeasureSpec(0, MeasureSpec.UNSPECIFIED)
            view.measure(wMeasureSpec, hMeasureSpec)

            if (pager.layoutParams.height != view.measuredHeight) {
                // ParentViewGroup is, for example, LinearLayout
                // ... or whatever the parent of the ViewPager2 is
                pager.layoutParams = (pager.layoutParams as ParentViewGroup.LayoutParams)
                    .also { lp -> lp.height = view.measuredHeight }
            }
        }
    }
}

See discussion here:

https://issuetracker.google.com/u/0/issues/143095219