Add ScrollToPositionDelegate and install in calls log fragment.

app/src/main/java/org/thoughtcrime/securesms/components/ScrollToPositionDelegate.kt

@@ -0,0 +1,116 @@
+package org.thoughtcrime.securesms.components
+
+import androidx.recyclerview.widget.LinearLayoutManager
+import androidx.recyclerview.widget.RecyclerView
+import io.reactivex.rxjava3.android.schedulers.AndroidSchedulers
+import io.reactivex.rxjava3.core.Observable
+import io.reactivex.rxjava3.disposables.CompositeDisposable
+import io.reactivex.rxjava3.disposables.Disposable
+import io.reactivex.rxjava3.kotlin.plusAssign
+import io.reactivex.rxjava3.kotlin.subscribeBy
+import io.reactivex.rxjava3.subjects.BehaviorSubject
+import org.signal.core.util.logging.Log
+import org.thoughtcrime.securesms.util.doAfterNextLayout
+import kotlin.math.max
+
+/**
+ * Delegate object to help manage scroll position requests.
+ *
+ * @param recyclerView The recycler view that will be scrolled
+ * @param canJumpToPosition Allows additional checks to see if we can scroll. For example, PagingMappingAdapter#isAvailableAround
+ * @param mapToTruePosition Allows additional offsets to be applied to the position.
+ */
+class ScrollToPositionDelegate private constructor(
+  private val recyclerView: RecyclerView,
+  canJumpToPosition: (Int) -> Boolean,
+  mapToTruePosition: (Int) -> Int,
+  disposables: CompositeDisposable
+) : Disposable by disposables {
+  companion object {
+    private val TAG = Log.tag(ScrollToPositionDelegate::class.java)
+    const val NO_POSITION = -1
+    private val EMPTY = ScrollToPositionRequest(NO_POSITION, true)
+    private const val SMOOTH_SCROLL_THRESHOLD = 25
+  }
+
+  private val listCommitted = BehaviorSubject.create<Unit>()
+  private val scrollPositionRequested = BehaviorSubject.createDefault(EMPTY)
+  private val scrollPositionRequests: Observable<ScrollToPositionRequest> = Observable.combineLatest(listCommitted, scrollPositionRequested) { _, b -> b }
+
+  constructor(
+    recyclerView: RecyclerView,
+    canJumpToPosition: (Int) -> Boolean = { true },
+    mapToTruePosition: (Int) -> Int = { it }
+  ) : this(recyclerView, canJumpToPosition, mapToTruePosition, CompositeDisposable())
+
+  init {
+    disposables += scrollPositionRequests
+      .observeOn(AndroidSchedulers.mainThread())
+      .filter { it.position >= 0 && canJumpToPosition(it.position) }
+      .map { it.copy(position = mapToTruePosition(it.position)) }
+      .subscribeBy(onNext = { position ->
+        recyclerView.doAfterNextLayout {
+          handleScrollPositionRequest(position, recyclerView)
+        }
+      })
+  }
+
+  private fun handleScrollPositionRequest(
+    request: ScrollToPositionRequest,
+    recyclerView: RecyclerView
+  ) {
+    requestScrollPosition(NO_POSITION, false)
+
+    val layoutManager = recyclerView.layoutManager as? LinearLayoutManager
+    if (layoutManager == null) {
+      Log.w(TAG, "Layout manager is not set or of an invalid type.")
+      return
+    }
+
+    if (recyclerView.scrollState == RecyclerView.SCROLL_STATE_DRAGGING) {
+      return
+    }
+
+    val position = max(0, request.position - 1)
+    val offset = when {
+      position == 0 -> 0
+      layoutManager.reverseLayout -> recyclerView.height
+      else -> 0
+    }
+
+    Log.d(TAG, "Scrolling to position $position with offset $offset.")
+
+    if (request.smooth && position == 0 && layoutManager.findFirstVisibleItemPosition() < SMOOTH_SCROLL_THRESHOLD) {
+      recyclerView.smoothScrollToPosition(position)
+    } else {
+      layoutManager.scrollToPositionWithOffset(position, offset)
+    }
+  }
+
+  /**
+   * Entry point for requesting a specific scroll position.
+   */
+  fun requestScrollPosition(position: Int, smooth: Boolean = true) {
+    scrollPositionRequested.onNext(ScrollToPositionRequest(position, smooth))
+  }
+
+  /**
+   * Reset the scroll position to 0
+   */
+  fun resetScrollPosition() {
+    requestScrollPosition(0, true)
+    recyclerView.requestLayout()
+  }
+
+  /**
+   * This should be called every time a list is submitted to the RecyclerView's adapter.
+   */
+  fun notifyListCommitted() {
+    listCommitted.onNext(Unit)
+  }
+
+  private data class ScrollToPositionRequest(
+    val position: Int,
+    val smooth: Boolean
+  )
+}