shenlong-tanwen
65 lines
2.7 KiB
Dart
65 lines
2.7 KiB
Dart
import 'dart:math' as math;
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import 'package:collection/collection.dart';
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import 'package:immich_mobile/domain/interfaces/asset.interface.dart';
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import 'package:immich_mobile/domain/models/asset.model.dart';
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import 'package:immich_mobile/domain/models/render_list_element.model.dart';
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import 'package:immich_mobile/service_locator.dart';
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import 'package:immich_mobile/utils/constants/globals.dart';
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class RenderList {
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final List<RenderListElement> elements;
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final int totalCount;
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/// offset of the assets from last section in [_buf]
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int _bufOffset = 0;
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/// assets cache loaded from DB with offset [_bufOffset]
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List<Asset> _buf = [];
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RenderList({required this.elements, required this.totalCount});
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/// Loads the requested assets from the database to an internal buffer if not cached
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/// and returns a slice of that buffer
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Future<List<Asset>> loadAssets(int offset, int count) async {
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assert(offset >= 0);
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assert(count > 0);
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assert(offset + count <= totalCount);
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// the requested slice (offset:offset+count) is not contained in the cache buffer `_buf`
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// thus, fill the buffer with a new batch of assets that at least contains the requested
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// assets and some more
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if (offset < _bufOffset || offset + count > _bufOffset + _buf.length) {
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final bool forward = _bufOffset < offset;
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// make sure to load a meaningful amount of data (and not only the requested slice)
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// otherwise, each call to [loadAssets] would result in DB call trashing performance
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// fills small requests to [batchSize], adds some legroom into the opposite scroll direction for large requests
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final len =
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math.max(kRenderListBatchSize, count + kRenderListOppositeBatchSize);
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// when scrolling forward, start shortly before the requested offset...
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// when scrolling backward, end shortly after the requested offset...
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// ... to guard against the user scrolling in the other direction
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// a tiny bit resulting in a another required load from the DB
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final start = math.max(
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0,
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forward
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? offset - kRenderListOppositeBatchSize
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: (len > kRenderListBatchSize ? offset : offset + count - len),
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);
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// load the calculated batch (start:start+len) from the DB and put it into the buffer
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_buf =
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await di<IAssetRepository>().fetchAssets(offset: start, limit: len);
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_bufOffset = start;
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assert(_bufOffset <= offset);
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assert(_bufOffset + _buf.length >= offset + count);
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}
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// return the requested slice from the buffer (we made sure before that the assets are loaded!)
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return _buf.slice(offset - _bufOffset, offset - _bufOffset + count);
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}
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}
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