问题描述
询问是否可以对 CKRecord 使用下标在Swift中。虽然我已经知道如何执行发问者想要的操作，但是每次排列都会给我带来堆栈溢出：
This question asks whether one can use subscripting with CKRecord in Swift. While I already knew how to do what the questioner wanted, every permutation of it gives me a stack overflow:
subscript(key: String) -> CKRecordValue? {
get {
return objectForKey(key) as CKRecordValue?
}
set {
setObject(newValue, forKey: key)
}
}
在getter中发生堆栈溢出。 （我从未尝试过setter，所以它也可能在那里发生。）我尝试使用 objectForKey：， objectForKeyedSubscript：和 valueForKey：。都产生相同的结果：堆栈溢出。
The stack overflow occurs in the getter. (I've never tried the setter, so it may occur there, too.) I've tried implementing with objectForKey:, objectForKeyedSubscript:, and valueForKey:. All produce the same result: a stack overflow.
这很奇怪，因为 CKRecord 肯定是用Objective写的-C。为什么要递归调用Swift的下标方法？这个不成立。内特·库克（Nate Cook）在回答提问者时，想知道为什么Swift不会自动桥接 objectForKeyedSubscript：。好吧，也许执行此操作的代码尚未完全完成，但是会导致此问题。我将不得不尝试另一个具有 objectForKeyedSubscript：的类。
This is very strange, since CKRecord is certainly written in Objective-C. Why would it recursively call Swift's subscript method? It makes no sense. Nate Cook, in his answer to the questioner, wonders why Swift doesn't bridge objectForKeyedSubscript: automatically. Well, maybe the code to do that is not fully baked, but is causing this problem. I will have to try it with another class that has objectForKeyedSubscript:.
通常看来， objectForKeyedSubscript：是桥接的。我使用适当的方法在Objective-C中创建了一个类，将其添加到桥接头文件中，索引器在那里并没有问题地被编译。甚至更好，它可以正常工作而不会引起堆栈溢出。
It appears that objectForKeyedSubscript: is ordinarily bridged. I created a class in Objective-C with the appropriate methods, added it to the bridging header, and the indexers were there and compiled without issue. Even better, it worked without a stack overflow.
这意味着 CKRecord 发生了非常不寻常的事情。
This means that something very unusual is going on with CKRecord.
如果在Swift中创建一个继承自 NSObject 并使用 String 作为键在其上实现下标方法，它变为 objectForKeyedSubscript：。 （对于纯Swift类，我怀疑不是这种情况。）您可以通过将Swift类导入Objective-C并验证 objectForKeyedSubscript：来验证这一点。 。
If you create a class in Swift that descends from NSObject and implements the subscript method on it with a String as the key, this becomes objectForKeyedSubscript:. (For "pure Swift" classes, I suspect this is not the case.) You can verify this by importing your Swift class into Objective-C and verifying that objectForKeyedSubscript: is there.
由于 CKRecord 来自 NSObject ，因此实现了下标覆盖默认实现。此外，似乎 objectForKey：和 valueForKey：最终都被称为 objectForKeyedSubscript：，导致对下标的调用（读为：与相同），导致堆栈溢出。
Since CKRecord descends from NSObject, implementing subscript overrides the default implementation. Further, it seems that objectForKey: and valueForKey: all ultimately called objectForKeyedSubscript:, which results in (read: "is the same as") a call to subscript, which causes the stack overflow.
这可以解释为什么发生堆栈溢出。它仍然不能解释为什么 objectForKeyedSubscript：没有自动桥接的原因，但这也许是因为 setObject：forKeyedSubscript：的定义。类型签名与规范签名的类型签名稍有不同：-（void）setObject：（id< CKRecordValue>）object forKeyedSubscript：（NSString *）key; 。这对Objective-C没有影响，但可能会使桥接代码崩溃。毕竟，Swift是相当新的东西。
That may explain why the stack overflow occurs. It still does not explain why objectForKeyedSubscript: was not automatically bridged, but perhaps it's because the definition of setObject:forKeyedSubscript: has a slightly different type signature from the canonical one: - (void)setObject:(id <CKRecordValue>)object forKeyedSubscript:(NSString *)key;. This makes no difference to Objective-C, but might trip up the "bridging code". Swift is pretty new, after all.
推荐答案
经过一些测试和调试（通过子类）后，我发现对于 CKRecord ， objectForKey：确实调用了 objectForKeyedSubscript：。另外，在隐式标记为 @objc 的Swift类中实现下标（通过从降级） NSObject ）或明确表示下标被实现为 objectForKeyedSubscript：。
After some testing and debugging (via a subclass), I discovered that, for CKRecord, objectForKey: does indeed call objectForKeyedSubscript:. Also, implementing subscript in a Swift class that is marked @objc implicitly (by descending from NSObject) or explicitly means that subscript is implemented as objectForKeyedSubscript:.
这意味着在扩展中的 CKRecord 上实现下标实现，这会导致堆栈溢出。
This means that implementing subscript on CKRecord in an extension hides the default implementation, which causes the stack overflow.
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