在NSMutableDictionary中使用NSIndexPath作为键的问题？ [英] Issues using NSIndexPath as key in NSMutableDictionary?

问题描述

有没有任何特定的理由尝试使用 NSIndexPath 作为一个存储和检索 NSMutableDictionary 键可能会失败？

我原来试图这样做以存储 NSMutableDictionary UITableView的高度（ self.cellHeights ） 。每次你点击一个 UITableViewCell ，该单元格将基于存储在 NSMutableDictionary indexPath ：

   - （CGFloat）tableView： （UITableView *）tableView 
 heightForRowAtIndexPath：（NSIndexPath *）indexPath 
 {
 NSNumber * heightNSNumber = [self.cellHeights objectForKey：indexPath]; 
 if（！heightNSNumber）
 {
 heightNSNumber = [NSNumber numberWithFloat：100.0]; 
 [self.cellHeights setObject：heightNSNumber forKey：indexPath]; 
} 
 return [heightNSNumber floatValue]; 
} 
 
  - （void）tableView：（UITableView *）tableView 
 didSelectRowAtIndexPath：（NSIndexPath *）indexPath 
 {
 [tableView deselectRowAtIndexPath：indexPath animated ：是]; 
 NSNumber * heightNSNumber = [self.cellHeights objectForKey：indexPath]; 
 if（！heightNSNumber）
 {
 heightNSNumber = [NSNumber numberWithFloat：100.0]; 
 [self.cellHeights setObject：heightNSNumber forKey：indexPath]; 
} 
 
 if（[heightNSNumber floatValue] == 100.0）
 {
 [self.cellHeights setObject：[NSNumber numberWithFloat：50.0] 
 forKey： indexPath]; 
} else {
 [self.cellHeights setObject：[NSNumber numberWithFloat：100.0] 
 forKey：indexPath]; 
} 
 [tableView beginUpdates]; 
 [tableView endUpdates]; 
} 
  

由于我不知道的原因，将单元格高度设置为 tableView：didSelectRowAtIndexPath：通过 [self.cellHeights objectForKey：indexPath] 工作很好。但是，尝试通过 [self.cellHeights objectForKey：indexPath] 总是返回 tableView：heightForRowAtIndexPath： nil因为看起来用于存储高度的indexPath与用于获取单元格高度的indexPath不匹配，即使它们具有与 indexPath.section 相同的值，和 indexPath.row 。因为这样，相同索引路径的新对象被添加到 self.cellHeights （很明显，因为 self.cellHeights.count <

这种情况不会发生，当你存储单元格高度在NSMutableDictionary使用行c $ c> [NSNumber numberWithInteger：indexPath.row] ）作为键...所以这是我现在正在做，但我想明白为什么

解决方案

虽然我在讨论晚了，这是一个快速简单的解决方案，将允许您使用NSIndexPath实例作为字典键。

只需重新创建 indexPath添加以下行：

  indexPath = [NSIndexPath indexPathForRow：indexPath.row inSection：indexPath.section]; 
  

Voilà。 tableView：heightForRowAtIndexPath：在内部使用 NSMutableIndexPath 实例（如您将看到的断点）。不知怎的，在计算散列键时，这些实例似乎与 NSIndexPath 不一致。

通过将其转换回 NSIndexPath ，那么一切工作。

Is there any particular reason why attempting to store and retrieve a value in an NSMutableDictionary using an NSIndexPath as a key might fail?

I originally attempted to do this in order to store an NSMutableDictionary of UITableViewCell heights (self.cellHeights) for a UITableView. Each time you tapped a UITableViewCell, that cell would either expand or contract between two different heights based on the value stored in the NSMutableDictionary for that particular indexPath:

- (CGFloat)tableView:(UITableView *)tableView 
           heightForRowAtIndexPath:(NSIndexPath *)indexPath 
{
    NSNumber *heightNSNumber = [self.cellHeights objectForKey:indexPath];
    if (!heightNSNumber)
    {
        heightNSNumber = [NSNumber numberWithFloat:100.0];
        [self.cellHeights setObject:heightNSNumber forKey:indexPath];
    }
    return [heightNSNumber floatValue];
}

- (void)tableView:(UITableView *)tableView  
        didSelectRowAtIndexPath:(NSIndexPath *)indexPath
{
    [tableView deselectRowAtIndexPath:indexPath animated:YES];
    NSNumber *heightNSNumber = [self.cellHeights objectForKey:indexPath];
    if (!heightNSNumber)
    {
        heightNSNumber = [NSNumber numberWithFloat:100.0];
        [self.cellHeights setObject:heightNSNumber forKey:indexPath];
    }

    if ([heightNSNumber floatValue] == 100.0)
    {
        [self.cellHeights setObject:[NSNumber numberWithFloat:50.0] 
                             forKey:indexPath];
    } else {
        [self.cellHeights setObject:[NSNumber numberWithFloat:100.0] 
                             forKey:indexPath];
    }
    [tableView beginUpdates];
    [tableView endUpdates];
}

For reasons unknown to me, getting the cell height within tableView:didSelectRowAtIndexPath: via [self.cellHeights objectForKey:indexPath] works just fine. However, trying to get the cell height within tableView:heightForRowAtIndexPath: via [self.cellHeights objectForKey:indexPath] always returns nil because it seems that the indexPath used to store the height doesn't match the indexPath being used to fetch the cell height, even though they have the same values for indexPath.section and indexPath.row. Because of this, a new object for the "same" index path is added to self.cellHeights (as evident since self.cellHeights.count increases thereafter).

This does not happen when you store the cell heights in the NSMutableDictionary using the row ([NSNumber numberWithInteger:indexPath.row]) as the key...so that's what I'm doing for now, but I'd like to understand why indexPath isn't working as the key.

解决方案

Although I'm late in the discussion, here's a quick and simple solution that will allow you to use NSIndexPath instances as dictionary keys.

Just recreate the indexPath by adding the following line:

indexPath = [NSIndexPath indexPathForRow:indexPath.row inSection:indexPath.section];

Voilà. tableView:heightForRowAtIndexPath: uses NSMutableIndexPath instances internally (as you would see with a breakpoint). Somehow those instances seem uncooperative with NSIndexPath when calculating hash keys.

By converting it back to an NSIndexPath, then everything works.

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