Add EndlessRecyclerViewScrollListener.kt

More powerful. This class works if three types of RecyclerView.LayoutManager: LinearLayoutManager, GridLayoutManager and StaggeredGridLayoutManager.

app/src/main/java/chat/rocket/android/helper/EndlessRecyclerViewScrollListener.kt

+package chat.rocket.android.helper
+
+import android.support.v7.widget.GridLayoutManager
+import android.support.v7.widget.LinearLayoutManager
+import android.support.v7.widget.RecyclerView
+import android.support.v7.widget.StaggeredGridLayoutManager
+
+/**
+ * Created by Filipe de Lima Brito (filipedelimabrito@gmail.com) on 9/21/17.
+ * Source info: https://github.com/codepath/android_guides/wiki/Endless-Scrolling-with-AdapterViews-and-RecyclerView
+ */
+abstract class EndlessRecyclerViewScrollListener : RecyclerView.OnScrollListener {
+    private var visibleThreshold = 5 // The minimum amount of items to have below of the current scroll position before loading more.
+    private var currentPage = 0     // The current offset index of data loaded
+    private var previousTotalItemCount = 0     // The total number of items in the dataset after the last load
+    private var loading = true     // True if we are still waiting for the last set of data to load.
+    private val startingPageIndex = 0     // Sets the starting page index
+    private var layoutManager: RecyclerView.LayoutManager
+
+    constructor(layoutManager: LinearLayoutManager) {
+        this.layoutManager = layoutManager
+    }
+
+    constructor(layoutManager: GridLayoutManager) {
+        this.layoutManager = layoutManager
+        visibleThreshold *= layoutManager.spanCount
+    }
+
+    constructor(layoutManager: StaggeredGridLayoutManager) {
+        this.layoutManager = layoutManager
+        visibleThreshold *= layoutManager.spanCount
+    }
+
+    private fun getLastVisibleItem(lastVisibleItemPositions: IntArray): Int {
+        var maxSize = 0
+        for (i in lastVisibleItemPositions.indices) {
+            if (i == 0) {
+                maxSize = lastVisibleItemPositions[i]
+            } else if (lastVisibleItemPositions[i] > maxSize) {
+                maxSize = lastVisibleItemPositions[i]
+            }
+        }
+        return maxSize
+    }
+
+    // This happens many times a second during a scroll, so be wary of the code you place here.
+    // We are given a few useful parameters to help us work out if we need to load some more data,
+    // but first we check if we are waiting for the previous load to finish.
+    override fun onScrolled(view: RecyclerView?, dx: Int, dy: Int) {
+        var lastVisibleItemPosition = 0
+        val totalItemCount = layoutManager.itemCount
+
+        when (layoutManager) {
+            is StaggeredGridLayoutManager -> {
+                val lastVisibleItemPositions = (layoutManager as StaggeredGridLayoutManager).findLastVisibleItemPositions(null)
+                // get maximum element within the list
+                lastVisibleItemPosition = getLastVisibleItem(lastVisibleItemPositions)
+            }
+            is GridLayoutManager -> lastVisibleItemPosition = (layoutManager as GridLayoutManager).findLastVisibleItemPosition()
+            is LinearLayoutManager -> lastVisibleItemPosition = (layoutManager as LinearLayoutManager).findLastVisibleItemPosition()
+        }
+
+        // If the total item count is zero and the previous isn't, assume the
+        // list is invalidated and should be reset back to initial state
+        // If it’s still loading, we check to see if the dataset count has
+        // changed, if so we conclude it has finished loading and update the current page
+        // number and total item count.
+
+        // If it isn’t currently loading, we check to see if we have breached
+        // the visibleThreshold and need to reload more data.
+        // If we do need to reload some more data, we execute onLoadMore to fetch the data.
+        // threshold should reflect how many total columns there are too
+
+        // If the total item count is zero and the previous isn't, assume the
+        // list is invalidated and should be reset back to initial state
+        if (totalItemCount < previousTotalItemCount) {
+            this.currentPage = this.startingPageIndex
+            this.previousTotalItemCount = totalItemCount
+            if (totalItemCount == 0) {
+                this.loading = true
+            }
+        }
+        // If it’s still loading, we check to see if the dataset count has
+        // changed, if so we conclude it has finished loading and update the current page
+        // number and total item count.
+        if (loading && totalItemCount > previousTotalItemCount) {
+            loading = false
+            previousTotalItemCount = totalItemCount
+        }
+
+        // If it isn’t currently loading, we check to see if we have breached
+        // the visibleThreshold and need to reload more data.
+        // If we do need to reload some more data, we execute onLoadMore to fetch the data.
+        // threshold should reflect how many total columns there are too
+        if (!loading && lastVisibleItemPosition + visibleThreshold > totalItemCount) {
+            currentPage++
+            onLoadMore(currentPage, totalItemCount, view)
+            loading = true
+        }
+    }
+
+    // Call this method whenever performing new searches
+    fun resetState() {
+        this.currentPage = this.startingPageIndex
+        this.previousTotalItemCount = 0
+        this.loading = true
+    }
+
+    // Defines the process for actually loading more data based on page
+    abstract fun onLoadMore(page: Int, totalItemsCount: Int, view: RecyclerView?)
+}
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