Tweak onboarding topics animation and entry.

Change-Id: Ic50329534bf8e60e2912aca1c2c4ad32daddd79a

Author: nickbutcher

app/src/main/java/com/materialstudies/owl/ui/onboarding/OnboardingFragment.kt

@@ -17,15 +17,16 @@
 package com.materialstudies.owl.ui.onboarding
 
 import android.os.Bundle
+import android.util.DisplayMetrics
 import android.view.LayoutInflater
 import android.view.View
 import android.view.ViewGroup
 import androidx.annotation.Px
 import androidx.core.view.forEach
-import androidx.core.view.postDelayed
 import androidx.dynamicanimation.animation.SpringAnimation
 import androidx.fragment.app.Fragment
 import androidx.navigation.fragment.findNavController
+import androidx.recyclerview.widget.LinearSmoothScroller
 import androidx.recyclerview.widget.RecyclerView
 import androidx.recyclerview.widget.RecyclerView.SCROLL_STATE_DRAGGING
 import com.materialstudies.owl.R
@@ -53,7 +54,7 @@ class OnboardingFragment : Fragment() {
                     // data ordered LTR, so reverse it before setting
                     submitList(topics.reversed())
                 }
-                smoothScrollToPosition(topics.size)
+                smoothScrollToPositionWithSpeed(topics.size)
                 addOnScrollListener(
                     OscillatingScrollListener(resources.getDimensionPixelSize(R.dimen.grid_2))
                 )
@@ -67,14 +68,18 @@ class OnboardingFragment : Fragment() {
  * Oscillates a [RecyclerView]'s children based on the horizontal scroll velocity.
  */
 private const val MAX_OSCILLATION_ANGLE = 6f // ±6º
+
 class OscillatingScrollListener(
     @Px private val scrollDistance: Int
 ) : RecyclerView.OnScrollListener() {
     override fun onScrolled(recyclerView: RecyclerView, dx: Int, dy: Int) {
         // Calculate a rotation to set from the horizontal scroll
-        val rotation =
-            (dx.coerceIn(-scrollDistance, scrollDistance) / scrollDistance) * MAX_OSCILLATION_ANGLE
+        val clampedDx = dx.coerceIn(-scrollDistance, scrollDistance)
+        val rotation = (clampedDx / scrollDistance) * MAX_OSCILLATION_ANGLE
         recyclerView.forEach {
+            // Alter the pivot point based on scroll direction to make motion look more natural
+            it.pivotX = it.width / 2f + clampedDx / 3f
+            it.pivotY = it.height / 3f
             it.spring(SpringAnimation.ROTATION).animateToFinalPosition(rotation)
         }
     }
@@ -87,3 +92,17 @@ class OscillatingScrollListener(
         }
     }
 }
+
+fun RecyclerView.smoothScrollToPositionWithSpeed(
+    position: Int,
+    millisPerInch: Float = 100f
+) {
+    val lm = requireNotNull(layoutManager)
+    val scroller = object : LinearSmoothScroller(context) {
+        override fun calculateSpeedPerPixel(displayMetrics: DisplayMetrics): Float {
+            return millisPerInch / displayMetrics.densityDpi
+        }
+    }
+    scroller.targetPosition = position
+    lm.startSmoothScroll(scroller)
+}