检索未查看的 UITableViewCells 的数据源

Posted 2023-03-11

【中文标题】检索未查看的 UITableViewCells 的数据源

【英文标题】：Retrieving Data Source of Unviewed UITableViewCells

【发布时间】：2020-04-30 12:59:19

【问题描述】：

我有一个 UITableView，它显示了以 xml 形式接收的元素，然后将其分类为部分和行。我使用 indexPathsForVisibleRows 跟踪屏幕上显示的元素并将其保存到 NSMutableSet 中：

@interface MainFeedTableViewController () <UIGestureRecognizerDelegate>

    NSMutableSet *viewedCellsIndexPaths;

@end

- (void)viewDidLoad 
    [super viewDidLoad];
    ...
    viewedCellsIndexPaths = [[NSMutableSet alloc] init];
    ...

- (void) trackVisibleCells 
// create array of currently displayed cells
NSArray *visibleCells = [_mainFeed indexPathsForVisibleRows];


// add cells to viewedCells set
[viewedCellsIndexPaths addObjectsFromArray:visibleCells];


// get unviewed cells count
NSUInteger cellsCount = 0;
NSInteger sections = [_mainFeed numberOfSections];
for (int i = 0; i < sections; i++) 
    cellsCount += [_mainFeed numberOfRowsInSection:i];

NSUInteger unViewedCellsCount = cellsCount - [viewedCellsIndexPaths count];


// pass unviewed cells count to cell
if (self.catchupCellDelegate) 
    [self.catchupCellDelegate updateUnreadCellsCount:unViewedCellsCount];

现在我想打开一个只显示尚未显示的项目的新视图。

基于this answer，我尝试从数据源创建另一个集合，然后[allData minusSet:viewedCellsIndexPaths]。问题是viewedCellsIndexPaths持有IndexPaths，而数据源持有，嗯，数据。

我尝试将整个表的 indexPaths 添加到 CellForRowAtIndexPath 内的另一个集合中，但这只给了我可见的索引路径，而不是整个表的索引路径。

然后我尝试使用 NSMutableOrderedSet 来跟踪可见的单元格，然后调用它的 LastObject 来找出最后查看的单元格并从该索引访问数据源：

NSIndexPath *lastIndex = (NSIndexPath *)viewedCellsIndexPaths.lastObject;

但是我得到了一个异常 [__NSSetM lastObject]: unrecognized selector sent to instance 0x2837acb2。

听起来像a bug within the language，我想不出办法解决。

我最后一次尝试based on this answer 是：

 NSMutableArray *unviewedCellsData = [NSMutableArray arrayWithArray:sectionOrderedFeed];
    for (NSIndexPath *indexPath in viewedCellsIndexPaths) 
        Items *item = [[sectionOrderedFeed objectAtIndex:indexPath.section] objectAtIndex:indexPath.row];
        [unviewedCellsData removeObject:item];
    

但迭代后 unViewedCellsData 与 sectionOrderedFeed 完全相同。

这里有什么帮助吗？

【问题讨论】：

你在哪里打电话trackVisibleCells？

测试你的代码，我得到Property 'lastObject' not found on object of type 'NSMutableSet *'，在NSIndexPath *lastIndex = (NSIndexPath *)viewedCellsIndexPaths.lastObject;这是正常的（这就是崩溃的意思）。

@Larme，正如我上面所说，在尝试这条线之前，我尝试将viewedCellsIndexPaths 类型更改为NSOrderedMutableSet，您可能已经错过了它。 @DonMag trackVisibleCells 在表格完成加载后立即调用

【参考方案1】：

这是我为快速测试所做的：

@interface ViewedCellsTableViewController ()

    NSMutableArray *myData;
    NSMutableSet *viewedCellsIndexPaths;
    NSMutableOrderedSet *orderedAllIndexPaths;


@end

@implementation ViewedCellsTableViewController

- (void)viewDidLoad 
    [super viewDidLoad];

    // just creating sample data...
    //  6 sections with a few rows in each section
    NSArray *rowsPerSection = @[@3, @5, @4, @8, @6, @7];

    myData = [NSMutableArray new];
    int iSection = 0;
    for (NSNumber *nRows in rowsPerSection) 
        NSMutableArray *aSec = [NSMutableArray new];
        for (int iRow = 0; iRow < [nRows intValue]; iRow++) 
            NSString *s = [NSString stringWithFormat:@"This is sec: %d row:%d", iSection, iRow];
            [aSec addObject:s];
        
        [myData addObject:aSec];
        ++iSection;
    

    // initialize ordered set of all index paths
    orderedAllIndexPaths = [NSMutableOrderedSet new];
    iSection = 0;
    for (NSArray *aSec in myData) 
        for (int iRow = 0; iRow < [aSec count]; iRow++) 
            NSIndexPath *p = [NSIndexPath indexPathForRow:iRow inSection:iSection];
            [orderedAllIndexPaths addObject:p];
        
        ++iSection;
    

    // init tracking set
    viewedCellsIndexPaths = [NSMutableSet new];

然后，当准备好继续前进时......

NSMutableOrderedSet *unViewedCellsIndexPaths = [orderedAllIndexPaths mutableCopy];
[unViewedCellsIndexPaths minusSet:viewedCellsIndexPaths];

NSLog(@"\nViewed:\n%@ \n..", viewedCellsIndexPaths);
NSLog(@"");

NSLog(@"\nUn-Viewed:\n%@ \n..", unViewedCellsIndexPaths);
NSLog(@"");