键值编码 - setValue:forKey: 其中值为 NULL
我正在阅读的文档说 setValue:forKey: 会将 NULL 值视为被删除。然而，我有一个如下所示的始终崩溃的示例：
[myObj setValue:[aDictionary valueForKey:@"keyDoesNotExist"] forKey:@"anotherKey"];
如果 < code>aDictionary 不包含键 keyDoesNotExist。有什么办法解决这个问题吗？在这里做什么合适？
myObj 是 NSManagedObject 的一个实例。 “anotherKey”是一对多关系，其中 myObj 可以包含许多“anotherKey”。
I am reading documentation that says setValue:forKey: will treat a NULL value as if it is being removed. Yet, I have an example such as the following that is consistently crashing:
[myObj setValue:[aDictionary valueForKey:@"keyDoesNotExist"] forKey:@"anotherKey"];
This line of code crashes if aDictionary does not contain the key keyDoesNotExist. Is there any way around that? What's the appropriate thing to do here?
myObj is an instance of NSManagedObject. "anotherKey" is a one to many relationship where myObj can contain many of "anotherKey".
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当您使用 nil 值调用
setValue:forKey:时，将调用方法-setNilValueForKey:。默认情况下（在 NSObject 的实现中），这会引发无效参数异常。您可以通过覆盖-setNilValueForKey:并执行您想要的任何操作来使用 nil 值执行其他操作。或者，如果您直接调用 myObj 的 anotherKey setter，则不应出现异常（当然取决于
setAnotherKey:的实现）：您正在阅读的文档可能是针对 NSMutableDictionary 的。如果值为 nil，NSMutableDictionary 的
setValue:forKey:实现会调用removeObjectForKey:。对于 NSObject 子类来说并非如此，我认为 myObj 就是这样。此行为在 NSKeyValueCoding 协议参考文档。
When you call
setValue:forKey:with a nil value, the method-setNilValueForKey:is called. By default (in NSObject's implementation), this raises an invalid argument exception. You can do something else with a nil value by overriding-setNilValueForKey:and doing whatever you'd like.Alternatively, if you call myObj's anotherKey setter directly, you shouldn't get an exception (dependent on the implementation of
setAnotherKey:of course):The documentation you're reading is probably for NSMutableDictionary. NSMutableDictionary's implementation of
setValue:forKey:callsremoveObjectForKey:if the value is nil. That's not true for NSObject subclasses, which I'm presuming is what myObj is.This behavior is discussed in the NSKeyValueCoding Protocol Reference documentation.