Question

获取错误：*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[NSNull isEqualToString:]: unrecognized selector sent to instance' ...

SIGABRT来自else cell.textLabel.text = @"Blank";行：

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {UITableViewCell *cell = nil;
    static NSString *CellIdentifier = @"Cell";
    cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
    if (cell == nil) {
         cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleSubtitle reuseIdentifier:CellIdentifier] autorelease];
    }
     NSManagedObjectContext *context = [self managedObjectContext];
     NSEntityDescription *entity = [NSEntityDescription entityForName:@"Pattern" inManagedObjectContext:context];
 // Edit the sort method.
     NSSortDescriptor *sortDescriptor = [NSSortDescriptor sortDescriptorWithKey:@"patternName" ascending:YES selector:@selector(localizedCaseInsensitiveCompare:)];
      NSArray *sortDescriptors = [[[NSArray alloc] initWithObjects:sortDescriptor, nil] autorelease];

     NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
     [fetchRequest setSortDescriptors:sortDescriptors];
     [fetchRequest setEntity:entity];    
     _patterns = [context executeFetchRequest:fetchRequest error:nil];
     [fetchRequest release];
     NSArray *names = [_patterns valueForKey:@"patternName"];
     NSArray *urls = [_patterns valueForKey:@"patternUrl"];

     if (names!= nil) {
        cell.textLabel.text = [names objectAtIndex:indexPath.row];           
     }
     else cell.textLabel.text = @"Blank";
     cell.detailTextLabel.text = [urls objectAtIndex:indexPath.row];
     cell.accessoryType = UITableViewCellAccessoryDetailDisclosureButton;
     return cell;
 }

我可以添加已定义的记录（复制用户正在查看的页面的URL，将URL插入到url字段和name属性中）。编辑或创建新记录会弹出ModalViewController（显示现有记录）并捕获文本字段中的数据（用于新记录）。选择保存崩溃，只更改最后添加的已定义记录的记录，即使它已被编辑，并将其作为新记录添加，保留旧记录。我认为我跟踪细胞数量有些不对劲。

任何建议都会受到最高的赞赏。

Answer 1

您从executeFetchRequest获得的是对象数组而不是属性。您需要遍历对象数组并从每个对象中获取属性

此

_patterns = [context executeFetchRequest:fetchRequest error:nil];
     [fetchRequest release];
     NSArray *names = [_patterns valueForKey:@"patternName"];
     NSArray *urls = [_patterns valueForKey:@"patternUrl"];

     if (names!= nil) {
        cell.textLabel.text = [names objectAtIndex:indexPath.row];           
     }

应该成为

_patterns = [context executeFetchRequest:fetchRequest error:nil];
     [fetchRequest release];


     if (names!= nil) {
        cell.textLabel.text = [[_patterns objectAtIndex:indexPath.row] valueForKey:@"patternName"];           
     }

Answer 2

我猜这个问题实际上是在前一行：

cell.textLabel.text = [names objectAtIndex:indexPath.row];

我认为[name objectAtIndex：indexPath.row]返回的对象是NSNull。

尝试以下方法：

NSString *name = nil;
if (names) {
    name = [names objectAtIndex:indexPath.row];
    if (name && name == (id)[NSNull null]) { 
        name = nil;
    }
}
if (!name) {
    name = @"Blank";
}
cell.textLabel.text = name;

编辑：在最后的if语句中修正了我的逻辑。

当我尝试在UITableView中添加新记录时，cellForRowAtIndexPath会不断崩溃？

2 个答案: