feat(ios): delete items in a range off screen in list component

ios/sdk/component/listview/HippyBaseListView.m

@@ -47,6 +47,7 @@ @implementation HippyBaseListView {
     HippyHeaderRefresh *_headerRefreshView;
     HippyFooterRefresh *_footerRefreshView;
     NSArray<HippyBaseListViewCell *> *_previousVisibleCells;
+    NSMutableDictionary<NSIndexPath *, NSNumber *> *_cachedItems;
 }
 
 @synthesize node = _node;
@@ -59,6 +60,10 @@ - (instancetype)initWithBridge:(HippyBridge *)bridge {
         _isInitialListReady = NO;
         _preNumberOfRows = 0;
         _preloadItemNumber = 1;
+        _cachedItems = [NSMutableDictionary dictionaryWithCapacity:64];
+        [[NSNotificationCenter defaultCenter] addObserver:self
+                                                 selector:@selector(didReceiveMemoryWarning)
+                                                     name:UIApplicationDidReceiveMemoryWarningNotification object:nil];
         [self initTableView];
     }
 
@@ -169,6 +174,13 @@ - (void)insertHippySubview:(UIView *)subview atIndex:(NSInteger)atIndex {
     }
 }
 
+- (void)removeHippySubview:(UIView *)subview {
+    [NSObject cancelPreviousPerformRequestsWithTarget:self
+                                             selector:@selector(purgeFurthestIndexPathsFromScreen)
+                                               object:nil];
+    [self purgeFurthestIndexPathsFromScreen];
+}
+
 #pragma mark -Scrollable
 
 - (void)setScrollEnabled:(BOOL)value {
@@ -310,6 +322,7 @@ - (void)tableView:(UITableView *)tableView didEndDisplayingCell:(UITableViewCell
     NSAssert([cell isKindOfClass:[HippyBaseListViewCell class]], @"cell must be subclass of HippyBaseListViewCell");
     if ([cell isKindOfClass:[HippyBaseListViewCell class]]) {
         HippyBaseListViewCell *hippyCell = (HippyBaseListViewCell *)cell;
+        [_cachedItems setObject:[hippyCell.cellView hippyTag] forKey:indexPath];
         hippyCell.node.cell = nil;
     }
 }
@@ -324,20 +337,15 @@ - (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(N
         cell = [[cls alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:identifier];
         cell.tableView = tableView;
     }
-    UIView *cellView = nil;
-    if (cell.node.cell) {
-        cellView = [_bridge.uiManager createViewFromNode:indexNode];
-    } else {
-        cellView = [_bridge.uiManager updateNode:cell.node withNode:indexNode];
-        if (nil == cellView) {
-            cellView = [_bridge.uiManager createViewFromNode:indexNode];
-        }
-    }
+    UIView *cellView = [_bridge.uiManager createViewFromNode:indexNode];
     HippyAssert([cellView conformsToProtocol:@protocol(ViewAppearStateProtocol)],
         @"subviews of HippyBaseListViewCell must conform to protocol ViewAppearStateProtocol");
     cell.cellView = (UIView<ViewAppearStateProtocol> *)cellView;
     cell.node = indexNode;
     cell.node.cell = cell;
+    if (cellView) {
+        [_cachedItems removeObjectForKey:indexPath];
+    }
     return cell;
 }
 
@@ -381,7 +389,8 @@ - (void)scrollViewDidScroll:(UIScrollView *)scrollView {
             [scrollViewListener scrollViewDidScroll:scrollView];
         }
     }
-
+    [NSObject cancelPreviousPerformRequestsWithTarget:self selector:@selector(purgeFurthestIndexPathsFromScreen) object:nil];
+    [self performSelector:@selector(purgeFurthestIndexPathsFromScreen) withObject:nil afterDelay:.5f];
     [_headerRefreshView scrollViewDidScroll];
     [_footerRefreshView scrollViewDidScroll];
 }
@@ -538,6 +547,13 @@ - (void)didMoveToSuperview {
     _rootView = nil;
 }
 
+- (void)didMoveToWindow {
+    if (!self.window) {
+        [NSObject cancelPreviousPerformRequestsWithTarget:self selector:@selector(purgeFurthestIndexPathsFromScreen) object:nil];
+        [self purgeFurthestIndexPathsFromScreen];
+    }
+}
+
 - (BOOL)isManualScrolling {
     return _manualScroll;
 }
@@ -558,7 +574,82 @@ - (BOOL)showScrollIndicator {
     return [_tableView showsVerticalScrollIndicator];
 }
 
+- (NSUInteger)maxCachedItemCount {
+    return NSUIntegerMax;
+}
+
+- (NSUInteger)differenceFromIndexPath:(NSIndexPath *)indexPath1 againstAnother:(NSIndexPath *)indexPath2 {
+    NSAssert([NSThread mainThread], @"must be in main thread");
+    long diffCount = 0;
+    for (NSUInteger index = MIN([indexPath1 section], [indexPath2 section]); index < MAX([indexPath1 section], [indexPath2 section]); index++) {
+        diffCount += [_tableView numberOfRowsInSection:index];
+    }
+    diffCount = diffCount + [indexPath1 row] - [indexPath2 row];
+    return labs(diffCount);
+}
+
+- (NSInteger)differenceFromIndexPath:(NSIndexPath *)indexPath
+            againstVisibleIndexPaths:(NSArray<NSIndexPath *> *)visibleIndexPaths {
+    NSIndexPath *firstIndexPath = [visibleIndexPaths firstObject];
+    NSIndexPath *lastIndexPath = [visibleIndexPaths lastObject];
+    NSUInteger diffFirst = [self differenceFromIndexPath:indexPath againstAnother:firstIndexPath];
+    NSUInteger diffLast = [self differenceFromIndexPath:indexPath againstAnother:lastIndexPath];
+    return MIN(diffFirst, diffLast);
+}
+
+- (NSArray<NSIndexPath *> *)findFurthestIndexPathsFromScreen {
+    NSUInteger visibleItemsCount = [[self.tableView visibleCells] count];
+    NSUInteger maxCachedItemCount = [self maxCachedItemCount] == NSUIntegerMax ? visibleItemsCount * 2 : [self maxCachedItemCount];
+    NSUInteger cachedCount = [_cachedItems count];
+    NSInteger cachedCountToRemove = cachedCount > maxCachedItemCount ? cachedCount - maxCachedItemCount : 0;
+    if (0 != cachedCountToRemove) {
+        NSArray<NSIndexPath *> *visibleIndexPaths = [_tableView indexPathsForVisibleRows];
+        NSArray<NSIndexPath *> *sortedCachedItemKey = [[_cachedItems allKeys] sortedArrayUsingComparator:^NSComparisonResult(id  _Nonnull obj1, id  _Nonnull obj2) {
+            NSIndexPath *ip1 = obj1;
+            NSIndexPath *ip2 = obj2;
+            NSUInteger ip1Diff = [self differenceFromIndexPath:ip1 againstVisibleIndexPaths:visibleIndexPaths];
+            NSUInteger ip2Diff = [self differenceFromIndexPath:ip2 againstVisibleIndexPaths:visibleIndexPaths];
+            if (ip1Diff > ip2Diff) {
+                return NSOrderedAscending;
+            }
+            else if (ip1Diff < ip2Diff) {
+                return NSOrderedDescending;
+            }
+            else {
+                return NSOrderedSame;
+            }
+        }];
+        NSArray<NSIndexPath *> *result = [sortedCachedItemKey subarrayWithRange:NSMakeRange(0, cachedCountToRemove)];
+        return result;
+    }
+    return nil;
+}
+
+- (void)purgeFurthestIndexPathsFromScreen {
+    NSArray<NSIndexPath *> *furthestIndexPaths = [self findFurthestIndexPathsFromScreen];
+    if (furthestIndexPaths) {
+        //purge view
+        NSArray<NSNumber *> *objects = [_cachedItems objectsForKeys:furthestIndexPaths notFoundMarker:@(-1)];
+        [_bridge.uiManager removeNativeViewFromTags:objects];
+        //purge cache
+        [_cachedItems removeObjectsForKeys:furthestIndexPaths];
+    }
+}
+
+
+- (void)didReceiveMemoryWarning {
+    [self cleanUpCachedItems];
+}
+
+- (void)cleanUpCachedItems {
+    //purge view
+    NSArray<NSNumber *> *objects = [_cachedItems allValues];
+    [_bridge.uiManager removeNativeViewFromTags:objects];
+    [_cachedItems removeAllObjects];
+}
+
 - (void)dealloc {
+    [[NSNotificationCenter defaultCenter] removeObserver:self];
     [_headerRefreshView unsetFromScrollView];
     [_footerRefreshView unsetFromScrollView];
 }