Try to improve infinite pagination

client/src/app/shared/video/abstract-video-list.html

@@ -7,7 +7,7 @@
   <div class="no-results" i18n *ngIf="pagination.totalItems === 0">No results.</div>
   <div
     myInfiniteScroller
-    [pageHeight]="pageHeight"
+    [pageHeight]="pageHeight" [firstLoadedPage]="firstLoadedPage"
     (nearOfTop)="onNearOfTop()" (nearOfBottom)="onNearOfBottom()" (pageChanged)="onPageChanged($event)"
     class="videos" #videosElement
   >

client/src/app/shared/video/abstract-video-list.ts

@@ -38,7 +38,7 @@ export abstract class AbstractVideoList implements OnInit, OnDestroy {
   ownerDisplayType: OwnerDisplayType = 'account'
 
   protected baseVideoWidth = 215
-  protected baseVideoHeight = 230
+  protected baseVideoHeight = 205
 
   protected abstract notificationsService: NotificationsService
   protected abstract authService: AuthService
@@ -55,6 +55,7 @@ export abstract class AbstractVideoList implements OnInit, OnDestroy {
   protected otherRouteParams = {}
 
   private resizeSubscription: Subscription
+  private firstLoadedPage: number
 
   abstract getVideosObservable (page: number): Observable<{ videos: Video[], totalVideos: number}>
   abstract generateSyndicationList ()
@@ -100,7 +101,11 @@ export abstract class AbstractVideoList implements OnInit, OnDestroy {
     this.loadMoreVideos(this.pagination.currentPage)
   }
 
-  loadMoreVideos (page: number) {
+  loadMoreVideos (page: number, loadOnTop = false) {
+    this.adjustVideoPageHeight()
+
+    const currentY = window.scrollY
+
     if (this.loadedPages[page] !== undefined) return
     if (this.loadingPage[page] === true) return
 
@@ -111,6 +116,8 @@ export abstract class AbstractVideoList implements OnInit, OnDestroy {
       ({ videos, totalVideos }) => {
         this.loadingPage[page] = false
 
+        if (this.firstLoadedPage === undefined || this.firstLoadedPage > page) this.firstLoadedPage = page
+
         // Paging is too high, return to the first one
         if (this.pagination.currentPage > 1 && totalVideos <= ((this.pagination.currentPage - 1) * this.pagination.itemsPerPage)) {
           this.pagination.currentPage = 1
@@ -125,8 +132,17 @@ export abstract class AbstractVideoList implements OnInit, OnDestroy {
         // Initialize infinite scroller now we loaded the first page
         if (Object.keys(this.loadedPages).length === 1) {
           // Wait elements creation
-          setTimeout(() => this.infiniteScroller.initialize(), 500)
+          setTimeout(() => {
+            this.infiniteScroller.initialize()
+
+            // At our first load, we did not load the first page
+            // Load the previous page so the user can move on the top (and browser previous pages)
+            if (this.pagination.currentPage > 1) this.loadMoreVideos(this.pagination.currentPage - 1, true)
+          }, 500)
         }
+
+        // Insert elements on the top but keep the scroll in the previous position
+        if (loadOnTop) setTimeout(() => { window.scrollTo(0, currentY + this.pageHeight) }, 0)
       },
       error => {
         this.loadingPage[page] = false
@@ -189,6 +205,13 @@ export abstract class AbstractVideoList implements OnInit, OnDestroy {
     this.videoPages = Object.values(this.loadedPages)
   }
 
+  protected adjustVideoPageHeight () {
+    const numberOfPagesLoaded = Object.keys(this.loadedPages).length
+    if (!numberOfPagesLoaded) return
+
+    this.pageHeight = this.videosElement.nativeElement.offsetHeight / numberOfPagesLoaded
+  }
+
   protected buildVideoHeight () {
     // Same ratios than base width/height
     return this.videosElement.nativeElement.offsetWidth * (this.baseVideoHeight / this.baseVideoWidth)

client/src/app/shared/video/infinite-scroller.directive.ts

@@ -6,10 +6,9 @@ import { fromEvent, Subscription } from 'rxjs'
   selector: '[myInfiniteScroller]'
 })
 export class InfiniteScrollerDirective implements OnInit, OnDestroy {
-  private static PAGE_VIEW_TOP_MARGIN = 500
-
   @Input() containerHeight: number
   @Input() pageHeight: number
+  @Input() firstLoadedPage = 1
   @Input() percentLimit = 70
   @Input() autoInit = false
 
@@ -23,6 +22,7 @@ export class InfiniteScrollerDirective implements OnInit, OnDestroy {
   private scrollDownSub: Subscription
   private scrollUpSub: Subscription
   private pageChangeSub: Subscription
+  private middleScreen: number
 
   constructor () {
     this.decimalLimit = this.percentLimit / 100
@@ -39,6 +39,8 @@ export class InfiniteScrollerDirective implements OnInit, OnDestroy {
   }
 
   initialize () {
+    this.middleScreen = window.innerHeight / 2
+
     // Emit the last value
     const throttleOptions = { leading: true, trailing: true }
 
@@ -92,6 +94,11 @@ export class InfiniteScrollerDirective implements OnInit, OnDestroy {
   }
 
   private calculateCurrentPage (current: number) {
-    return Math.max(1, Math.round((current + InfiniteScrollerDirective.PAGE_VIEW_TOP_MARGIN) / this.pageHeight))
+    const scrollY = current + this.middleScreen
+
+    const page = Math.max(1, Math.ceil(scrollY / this.pageHeight))
+
+    // Offset page
+    return page + (this.firstLoadedPage - 1)
   }
 }