setValue:forUndefinedKey this class is not key value coding-compliant for the key

下午开发过程中遇到一个错误，结果被的真惨，从上午 11 点查错一直查到下午 2 点才找到错误的原因，真的郁闷的不行。

关于查错这么久，主要的原因是：

 

1. 自己对 IOS 开发还不熟悉
2. 不知道怎么得到错误的详细信息, 连调出来 All Output 面板查看错误信息都不知道，真的差劲
3. 有错误信息后应该直接去 stackoverflow 找答案。应为 IOS 开发中文方面的资料和分享实在太少，出错就要直接上 stackoverflow.

 

下面说错误，错误的详细信息：

 

2013-07-12 15:49:43.476 MyPad[15944:f803] *** Terminating app due to uncaught exception 'NSUnknownKeyException', reason: '[<BenefitItemsTableCell 0x6d9bdd0> setValue:forUndefinedKey:]: this class is not key value coding-compliant for the key.'
*** First throw call stack:
(0x1490022 0x127fcd6 0x148fee1 0xd90022 0xd01f6b 0xd01edb 0x420c88 0xd1cd50 0x60471a 0x1491dea 0x13fb7f1 0x60326e 0x475230 0xd886e94 0x65ea1 0x47cc54 0x47d3ce 0x468cbd 0x4776f1 0x420d42 0x1491e42 0x23e679 0x248579 0x1cd4f7 0x1cf3f6 0x25c160 0x3e0e84 0x3e1767 0x3f0183 0x3f0c38 0x3e4634 0x2329ef5 0x1464195 0x13c8ff2 0x13c78da 0x13c6d84 0x13c6c9b 0x3e0c65 0x3e2626 0x1a0ad 0x2535)
terminate called throwing an exception(lldb)

其中出错的代码在：

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    static NSString *CellIdentifier = @"MyCustomerTableCell";

    // 这行代码运行出错
    MyTableCell *cell =(MyTableCell*) [tableView dequeueReusableCellWithIdentifier:CellIdentifier];

    ....
    return cell;
}

关于这个奇怪的错误，在 stackoverflow 找到了这样的解释：

There are a couple of options to resolve this - i'll let you decide which is the most appropriate.

The reason it's failing is because the owner is being passed as nil. You're binding the actionText outlet to the file's owner in IB, but then when loading the nib, the owner is nil. I'd guess that when loading with a nil owner behind the scenes an NSObject is used, which is why you're getting the key/value error.

My previous advice to pass the cell as the owner would also fail as I didn't know how the Nib is constructed; the cell is nil as you've yet to create it (and dequeue is passing nil back, so even pass cell as the owner is still essentially passing nil).

Two options:

Instantiate a new cell in your -cellForRowAtIndexPath:(NSIndexPath *)indexPath implementation, and pass that new cell as the owner (but i'd guess that this isn't the best solution for you)
Or, and I'd suggest this is the better solution, change the binding of actionText in your nib file to the Alert Cell and not the file's owner (You have File's Owner and an Alert Cell - bind the UILabel to the actionText outlet of the Alert Cell, and not the File's owner, which is what's being done at present) - I suspect this is what you want. With that in mind file's owner can become an NSObject again.
------- Original answer kept below as the file's owner class is also a common cause for this error -------

It suggests that you've 'instantiated' an AlertCell in InterfaceBuilder, and you're binding something to actiontext, but the class isn't set to AlertCell, it's still NSObject?

Take a look at the class text box on the identify tab of the tool palette for that object in Interface Builder. The class should be AlertCell, but i'd guess it's still set to NSObject.

As an aside, and feel free to ignore this advice, but there are a couple of extra things i'd encourage you to do, purely from an Objective C expectations/conventions point of view:

Name your files after your class (upper case the first character of the filename).
Prefix your class names; two uppercase characters, typically your initials (i'd name it DWAlertCell, for example).

我这英文，也只能勉强看个大概，最后知道了原因在那里：MyTableCell.h 中的 outlet 上面。

去代码中一查，果然原因在这里。这是因为我 从 StoryBoard 中连接好 outlet 后又手工的在 MyTableCell.h 中更改了outlet 的名字，之后重新连接了 outlet，本来以为这样万事大吉，可是却从 StoryBoard 中逐个检查 Controller 的 outlet  后，发现我重命名的 outlet 那个控件上面之前的 outlet 没有删掉，和新添加的  outlet同事存在，郁闷，这是 XCODE bug 吗。

删掉老的 outlet 后，万事大吉，郁闷，烦！

所以这样的错误，就去检查 StoryBoard 中的 outlet。