feat: add support for nullable prefetchIndex

Signed-off-by: Sahil Kumar <xdsahil@gmail.com>

README.md

@@ -150,6 +150,9 @@ SuperPager(
     //
     // By default, the pager will start loading when the user scrolls
     // within 3 items of the end of the loaded content.
+    //
+    // If set to null, the pager will not start loading more content until
+    // they are specifically requested by the user.
     prefetchIndex: 3,
     
     // Defines the maximum number of items to keep in memory before

lib/src/paging_config.dart

@@ -8,7 +8,7 @@ class PagingConfig {
     this.maxSize,
   })  : initialLoadSize = initialLoadSize ?? pageSize * initialPageMultiplier,
         assert(
-          prefetchIndex <= pageSize / 2,
+          prefetchIndex == null || prefetchIndex <= pageSize / 2,
           'Prefetch index must be less than pageSize / 2',
         ),
         assert(
@@ -43,7 +43,11 @@ class PagingConfig {
   /// E.g., If this value is set to 3, a [SuperPager] will attempt to load the
   /// next page in advance when the user scrolls within 3 items of the end of
   /// currently loaded content.
-  final int prefetchIndex;
+  ///
+  /// A value of `null` indicates that no list items will be loaded until they
+  /// are specifically requested. This is generally not recommended, so that
+  /// users don't observe a end of list while scrolling.
+  final int? prefetchIndex;
 
   /// Defines requested load size for initial load from [PagingSource],
   /// typically larger than [pageSize], so on first load data there's a large

lib/src/widget/bidirectional_paging_list_view.dart

@@ -460,6 +460,10 @@ class _BidirectionalPagingListViewState<Key, Value>
 
     // Helper function to generate prepend and append load trigger notifications
     void generatePrependAppendLoadTriggerNotification(int index) {
+      // If there is no prefetch index, we don't need to generate any
+      // notifications.
+      if (fetchIndex == null) return;
+
       // Check if the index corresponds to near the top or bottom based on the
       // 'reverse' flag.
       final nearTop =

lib/src/widget/paging_sliver_list.dart

@@ -259,6 +259,10 @@ class _PagingSliverListState<Key, Value>
     final prefetchIndex = pager.config.prefetchIndex;
 
     void generatePrependAppendLoadTriggerNotification(int index) {
+      // If there is no prefetch index, we don't have to generate any
+      // notification.
+      if (prefetchIndex == null) return;
+
       // Generate prepend notification.
       if (index == prefetchIndex) {
         onBuildingPrependLoadTriggerItem?.call();