Fix snapping after scroll actions in paging gallery

commit_hash:0bc9521306e4f743628b8bd796b4f15670d0e1b8

client/android/div/src/main/java/com/yandex/div/core/view2/items/DivViewWithItems.kt

@@ -3,6 +3,7 @@ package com.yandex.div.core.view2.items
 import android.util.DisplayMetrics
 import android.view.View
 import androidx.annotation.VisibleForTesting
+import androidx.core.view.doOnNextLayout
 import androidx.recyclerview.widget.LinearLayoutManager
 import androidx.recyclerview.widget.LinearSmoothScroller
 import androidx.recyclerview.widget.RecyclerView
@@ -10,6 +11,7 @@ import com.yandex.div.core.view2.divs.availableHeight
 import com.yandex.div.core.view2.divs.availableWidth
 import com.yandex.div.core.view2.divs.dpToPx
 import com.yandex.div.core.view2.divs.gallery.DivGalleryAdapter
+import com.yandex.div.core.view2.divs.gallery.PagerSnapStartHelper
 import com.yandex.div.core.view2.divs.pager.DivPagerAdapter
 import com.yandex.div.core.view2.divs.spToPx
 import com.yandex.div.core.view2.divs.widgets.DivPagerView
@@ -73,9 +75,20 @@ internal sealed class DivViewWithItems {
      */
     internal class PagingGallery(view: DivRecyclerView, direction: Direction) : Gallery(view, direction) {
 
-        override var currentItem: Int
-            get() = view.currentItem(direction)
-            set(value) = checkItem(value, itemCount) { view.smoothScrollToPosition(value) }
+        override fun createSmoothScroller(): RecyclerView.SmoothScroller {
+            return object : DivSmoothScroller(view) {
+                override fun calculateDtToFit(
+                    viewStart: Int, viewEnd: Int,
+                    boxStart: Int, boxEnd: Int,
+                    snapPreference: Int
+                ): Int {
+                    val itemSpacing = view.pagerSnapStartHelper?.itemSpacing ?: return 0
+                    val isHorizontal = (view.adapter as? DivGalleryAdapter)?.orientation == RecyclerView.HORIZONTAL
+                    val size = if (isHorizontal) view.width else view.height
+                    return (size - viewStart - viewEnd + itemSpacing) / 2
+                }
+            }
+        }
     }
 
     /**
@@ -91,19 +104,19 @@ internal sealed class DivViewWithItems {
             get() = view.currentItem(direction)
             set(value) {
                 checkItem(value, itemCount) {
-                    val smoothScroller = object : LinearSmoothScroller(view.context) {
-                        private val MILLISECONDS_PER_INCH = 50f // default is 25f, bigger - slower
-                        override fun getHorizontalSnapPreference() = SNAP_TO_START
-                        override fun getVerticalSnapPreference() = SNAP_TO_START
-                        override fun calculateSpeedPerPixel(displayMetrics: DisplayMetrics): Float {
-                            return MILLISECONDS_PER_INCH / displayMetrics.densityDpi
-                        }
-                    }
+                    val smoothScroller = createSmoothScroller()
                     smoothScroller.targetPosition = value
                     view.layoutManager?.startSmoothScroll(smoothScroller)
                 }
             }
 
+        protected open fun createSmoothScroller(): RecyclerView.SmoothScroller {
+            return object : DivSmoothScroller(view) {
+                override fun getHorizontalSnapPreference() = SNAP_TO_START
+                override fun getVerticalSnapPreference() = SNAP_TO_START
+            }
+        }
+
         override val itemCount: Int
             get() = view.itemCount
 
@@ -117,10 +130,27 @@ internal sealed class DivViewWithItems {
 
         override fun setCurrentItemNoAnimation(index: Int) {
             checkItem(index, itemCount) {
-                view.scrollToPosition(index)
+                view.pagerSnapStartHelper?.snapToPosition(index, waitForLayout = true)
+                    ?: view.scrollToPosition(index)
             }
         }
 
+        private fun PagerSnapStartHelper.snapToPosition(position: Int, waitForLayout: Boolean) {
+            val layoutManager = view.layoutManager ?: return
+            layoutManager.findViewByPosition(position)?.let { target ->
+                val offset = calculateDistanceToFinalSnap(layoutManager, target)
+                view.scrollBy(offset[0], offset[1])
+                return
+            }
+
+            if (!waitForLayout) return
+
+            view.doOnNextLayout {
+                snapToPosition(position, waitForLayout = false)
+            }
+            view.scrollToPosition(position)
+        }
+
         override fun getIndicesOfItemWithId(id: String): List<Int> {
             val adapter = view.adapter as? DivGalleryAdapter ?: return emptyList()
             return adapter.visibleItems.getIndicesWithId(id) { div }
@@ -205,6 +235,16 @@ internal sealed class DivViewWithItems {
             }
         }
     }
+
+    private open class DivSmoothScroller(view: RecyclerView): LinearSmoothScroller(view.context) {
+
+        override fun calculateSpeedPerPixel(displayMetrics: DisplayMetrics) =
+            MILLISECONDS_PER_INCH / displayMetrics.densityDpi
+
+        companion object {
+            private const val MILLISECONDS_PER_INCH = 50f // default is 25f, bigger - slower
+        }
+    }
 }
 
 /**

client/android/div/src/test/java/com/yandex/div/core/view2/items/GalleryItemsViewTest.kt

@@ -160,11 +160,13 @@ class GalleryItemsViewTest {
             on { resources } doReturn mock()
         }
         whenever(view.layoutManager).thenReturn(layoutManager)
+        val scrollCaptor = argumentCaptor<LinearSmoothScroller>()
         val underTest = createUnderTest(drv = view)
 
         underTest.currentItem = 5
 
-        verify(view).smoothScrollToPosition(5)
+        verify(layoutManager).startSmoothScroll(scrollCaptor.capture())
+        Assert.assertEquals(5, scrollCaptor.firstValue.targetPosition)
     }
 
     @Test

test_data/interactive_snapshot_test_data/div-gallery/paging-paddings.json

@@ -84,7 +84,7 @@
     {
       "div_actions": [
         {
-          "url": "div-action://set_next_item?id=gallery",
+          "url": "div-action://set_next_item?id=gallery&animated=false",
           "log_id": "scroll_forward"
         }
       ],
@@ -93,7 +93,7 @@
     {
       "div_actions": [
         {
-          "url": "div-action://scroll_to_end?id=gallery",
+          "url": "div-action://scroll_to_end?id=gallery&animated=false",
           "log_id": "scroll_to_end"
         }
       ],

test_data/interactive_snapshot_test_data/div-gallery/vertical-paging-paddings.json

@@ -97,7 +97,7 @@
     {
       "div_actions": [
         {
-          "url": "div-action://set_next_item?id=gallery",
+          "url": "div-action://set_next_item?id=gallery&animated=false",
           "log_id": "scroll_forward"
         }
       ],
@@ -106,7 +106,7 @@
     {
       "div_actions": [
         {
-          "url": "div-action://scroll_to_end?id=gallery",
+          "url": "div-action://scroll_to_end?id=gallery&animated=false",
           "log_id": "scroll_to_end"
         }
       ],