ios - 滚动时，UILabels 从表格 View 单元格内容 View 中消失，但如果选择单元格，则会再次显示

Question

我有一个带有两个 uilabel 的自定义表格 View 单元格，一切正常启动，但是当我滚动到表格的另一部分并返回时，在任何地方都找不到标签。但是，如果我选择它们，它们会再次显示，直到它们出现为止。取消选择

- (id)initWithStyle:(UITableViewStyle)style
{
    self = [super initWithStyle:style];
    if (self) {
        // Custom initialization
    }
    return self;
}

- (void)viewDidLoad
{
    [super viewDidLoad];

    id appDelegate = [[UIApplication sharedApplication] delegate];
    _managedObjectContext = [appDelegate managedObjectContext];

    NSError *error;
    if (![[self fetchedResultsController] performFetch:&error]) {
        NSLog(@"%@ %@", error, [error userInfo]);
        abort();
    }

    _infoView.layer.cornerRadius = 5;
    _infoView.layer.masksToBounds = YES;
    _infoView.layer.shadowColor = [UIColor colorWithRed:205.0/255.0 green:205.0/255.0 blue:205.0/255.0 alpha:1.0].CGColor;
    _infoView.layer.shadowOpacity = 0.8;
    _infoView.layer.shadowRadius = 10.0;
    _infoView.layer.shadowOffset = CGSizeMake(2.0, 2.0);

    _addButton = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemAdd target:self action:@selector(add:)];
    self.navigationItem.rightBarButtonItem = _addButton;


    _searchBar.backgroundColor = [UIColor clearColor];
    _searchBar.clipsToBounds = YES;
    _searchBar.tintColor = [UIColor colorWithRed:200.0/255.0 green:200.0/255.0 blue:200.0/255.0 alpha:1.0];

    for (UIView *subview in _searchBar.subviews)
    {
        if ([subview isKindOfClass:NSClassFromString(@"UISearchBarBackground")])
        {
            [subview removeFromSuperview];
            break;
        }
    }

    _exercisesNumberLabel.text = [NSString stringWithFormat:@"%lu Exercises", (unsigned long)[_fetchedResultsController.fetchedObjects count]];
    [_exercisesNumberLabel sizeToFit];


}

- (void)viewDidAppear:(BOOL)animated
{
    [super viewDidAppear:animated];

    [self.tableView setContentOffset:CGPointMake(0, _headerView.bounds.size.height)];
}

- (void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

- (IBAction)add:(id)sender
{

}


#pragma mark - UITableViewDataSource

- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView
{
    return [[_fetchedResultsController sections] count];
}

- (NSString *)tableView:(UITableView *)tableView titleForHeaderInSection:(NSInteger)section
{
    return [[[_fetchedResultsController sections] objectAtIndex:section] name];
}

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
    id <NSFetchedResultsSectionInfo> sectionInfo = [[_fetchedResultsController sections] objectAtIndex:section];
    return [sectionInfo numberOfObjects];
}

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    NSString *cellIdentifier = nil;
    cellIdentifier = @"exerciseCell";


    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:cellIdentifier];

    if (!cell) {
        cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:cellIdentifier];
        [self stampCell:cell atIndexPath:indexPath];
    }


    [self configureCell:cell atIndexPath:indexPath];

    return cell;
}

- (void)stampCell:(UITableViewCell *)cell atIndexPath:(NSIndexPath *)indexPath
{
    UILabel *textLabel, *detailTextLabel;
    UIButton *playButton;
    //UIWebView *miniYouTubeView;

    textLabel = [[UILabel alloc] initWithFrame:CGRectMake(70, 2, 170, 22)];
    textLabel.tag = kExerciseCellTextLabel;
    textLabel.font = [UIFont systemFontOfSize:17.0];
    textLabel.textAlignment = NSTextAlignmentLeft;
    textLabel.textColor = [UIColor blackColor];
    [[cell contentView] addSubview:textLabel];

    detailTextLabel = [[UILabel alloc] initWithFrame:CGRectMake(70, 24, 170, 18)];
    detailTextLabel.tag = kExerciseCellDetailTextLabel;
    detailTextLabel.font = [UIFont systemFontOfSize:14.0];
    detailTextLabel.textAlignment = NSTextAlignmentLeft;
    detailTextLabel.textColor = [UIColor colorWithRed:128.0/255.0 green:128.0/255.0 blue:128.0/255.0 alpha:1.0];
    detailTextLabel.autoresizingMask = UIViewAutoresizingFlexibleRightMargin;
    [[cell contentView] addSubview:detailTextLabel];

    playButton = [[UIButton alloc] initWithFrame:CGRectMake(0, 0, 64, 44)];
    playButton.backgroundColor = [UIColor lightGrayColor];
    [playButton setImage:[UIImage imageNamed:@"121-landscape.png"] forState:UIControlStateNormal];
    [[cell contentView] addSubview:playButton];


    /*miniYouTubeView = [[UIWebView alloc] initWithFrame:CGRectMake(0, 0, 60, 44)];
    miniYouTubeView.tag = kExerciseCellMiniYouTubeView;
    miniYouTubeView.autoresizingMask = UIViewAutoresizingFlexibleRightMargin | UIViewAutoresizingFlexibleHeight;
    [[cell contentView] addSubview:miniYouTubeView];*/
}


#pragma mark - UITableViewDelegate

- (CGFloat)tableView:(UITableView *)tableView heightForRowAtIndexPath:(NSIndexPath *)indexPath
{
    return 44;
}

- (BOOL)tableView:(UITableView *)tableView canEditRowAtIndexPath:(NSIndexPath *)indexPath
{
    if ([[[_fetchedResultsController objectAtIndexPath:indexPath] isDefault] boolValue]) {
        return NO;
    }
    return YES;
}

- (void)tableView:(UITableView *)tableView commitEditingStyle:(UITableViewCellEditingStyle)editingStyle forRowAtIndexPath:(NSIndexPath *)indexPath
{
    if (editingStyle == UITableViewCellEditingStyleInsert) {

    }
    else if (editingStyle == UITableViewCellEditingStyleDelete) {
        Exercise *exercise = [_fetchedResultsController objectAtIndexPath:indexPath];
        [_managedObjectContext deleteObject:exercise];

        NSError *error;
        if (![_managedObjectContext save:&error]) {
            NSLog(@"%@ %@", error, [error userInfo]);
            abort();
        }
    }
}


#pragma mark - NSFetchedResultsControllerDelegate

- (NSFetchedResultsController *)fetchedResultsController
{
    if (_fetchedResultsController) {
        return _fetchedResultsController;
    }

    NSFetchRequest *request = [[NSFetchRequest alloc] init];
    NSEntityDescription *entity = [NSEntityDescription entityForName:kExercise inManagedObjectContext:_managedObjectContext];
    [request setEntity:entity];

    NSSortDescriptor *sortDescriptor = [NSSortDescriptor sortDescriptorWithKey:kExerciseName ascending:YES];
    NSArray *sortDescriptors = [NSArray arrayWithObject:sortDescriptor];
    [request setSortDescriptors:sortDescriptors];

    NSFetchedResultsController *fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:request managedObjectContext:_managedObjectContext sectionNameKeyPath:kExerciseFirstLetter cacheName:@"ExerciseList"];
    [self setFetchedResultsController:fetchedResultsController];
    [_fetchedResultsController setDelegate:self];

    return _fetchedResultsController;
}

- (void)controllerWillChangeContent:(NSFetchedResultsController *)controller
{
    [self.tableView beginUpdates];
}

- (void)controller:(NSFetchedResultsController *)controller didChangeSection:(id <NSFetchedResultsSectionInfo>)sectionInfo
           atIndex:(NSUInteger)sectionIndex forChangeType:(NSFetchedResultsChangeType)type
{
    switch(type) {
        case NSFetchedResultsChangeInsert:
            [self.tableView insertSections:[NSIndexSet indexSetWithIndex:sectionIndex] withRowAnimation:UITableViewRowAnimationFade];
            break;

        case NSFetchedResultsChangeDelete:
            [self.tableView deleteSections:[NSIndexSet indexSetWithIndex:sectionIndex] withRowAnimation:UITableViewRowAnimationFade];
            break;
    }
}

- (void)controller:(NSFetchedResultsController *)controller didChangeObject:(id)anObject
       atIndexPath:(NSIndexPath *)indexPath forChangeType:(NSFetchedResultsChangeType)type
      newIndexPath:(NSIndexPath *)newIndexPath
{
    UITableView *tableView = self.tableView;

    switch(type) {
        case NSFetchedResultsChangeInsert:
            [tableView insertRowsAtIndexPaths:@[newIndexPath] withRowAnimation:UITableViewRowAnimationFade];
            break;

        case NSFetchedResultsChangeDelete:
            [tableView deleteRowsAtIndexPaths:@[indexPath] withRowAnimation:UITableViewRowAnimationFade];
            break;

        case NSFetchedResultsChangeUpdate:
            [self configureCell:[tableView cellForRowAtIndexPath:indexPath] atIndexPath:indexPath];
            break;

        case NSFetchedResultsChangeMove:
            [tableView deleteRowsAtIndexPaths:@[indexPath] withRowAnimation:UITableViewRowAnimationFade];
            [tableView insertRowsAtIndexPaths:@[newIndexPath] withRowAnimation:UITableViewRowAnimationFade];
            break;
    }
}

- (void)controllerDidChangeContent:(NSFetchedResultsController *)controller
{
    [self.tableView endUpdates];
}

- (void)configureCell:(UITableViewCell *)cell atIndexPath:(NSIndexPath *)indexPath
{
    Exercise *exercise = [_fetchedResultsController objectAtIndexPath:indexPath];
    UILabel *textLabel = (UILabel *)[[cell contentView] viewWithTag:kExerciseCellTextLabel];
    UILabel *detailTextLabel = (UILabel *)[[cell contentView] viewWithTag:kExerciseCellDetailTextLabel];
    //UIWebView *miniYouTubeView = (UIWebView *)[[cell contentView] viewWithTag:kExerciseCellMiniYouTubeView];

    textLabel.text = [exercise valueForKey:kExerciseName];
    detailTextLabel.text = [NSString stringWithFormat:@"%@ - %@", [[exercise valueForKey:kExerciseEType] valueForKey:kExerciseTypeName], [[exercise valueForKey:kGear] valueForKey:kGearName]];

    //[textLabel sizeToFit];
    //[detailTextLabel sizeToFit];

    /*[miniYouTubeView loadRequest:[NSURLRequest requestWithURL:[NSURL URLWithString:[exercise valueForKey:kExerciseUrlAddress]]]];
    [miniYouTubeView setScalesPageToFit:YES];
    [miniYouTubeView setAutoresizesSubviews:YES];*/

    cell.accessoryType = UITableViewCellAccessoryDisclosureIndicator;

}

Answer 1

当表格滚动时，单元格会被重用。但是您的代码是以这样的方式编写的:每次重用单元格时，您都会不断添加越来越多的 subview 。

如果 cell 为 nil，则应仅调用 stampCell。

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath{
    NSString *cellIdentifier = @"exerciseCell";

    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:cellIdentifier];
    if (!cell) {
        cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:cellIdentifier];
        [self stampCell:cell atIndexPath:indexPath];
    }

    [self configureCell:cell atIndexPath:indexPath];

    return cell;
}

更好的解决方案是子类化 UITableViewCell 并将所有这些逻辑放入单元格而不是 View Controller 中。