NSNull处理NSManagedObject属性值

问题描述

我正在为NSManagedObject的属性设置值，这些值来自正确地从JSON文件序列化的NSDictionary.我的问题是，当某些值为[NSNull null]时，我无法直接分配给该属性:

I'm setting values for properties of my NSManagedObject, these values are coming from a NSDictionary properly serialized from a JSON file. My problem is, that, when some value is [NSNull null], I can't assign directly to the property:

    fight.winnerID = [dict objectForKey:@"winner"];

这将引发NSInvalidArgumentException

"winnerID"; desired type = NSString; given type = NSNull; value = <null>;

我可以轻松检查[NSNull null]的值并分配nil代替:

I could easily check the value for [NSNull null] and assign nil instead:

fight.winnerID = [dict objectForKey:@"winner"] == [NSNull null] ? nil : [dict objectForKey:@"winner"];

但是我认为这不是很优雅，并且要设置很多属性会变得很混乱.

But I think this is not elegant and gets messy with lots of properties to set.

此外，在处理NSNumber属性时，这会变得更加困难:

Also, this gets harder when dealing with NSNumber properties:

fight.round = [NSNumber numberWithUnsignedInteger:[[dict valueForKey:@"round"] unsignedIntegerValue]]

NSInvalidArgumentException现在是:

[NSNull unsignedIntegerValue]: unrecognized selector sent to instance

在这种情况下，我必须先对[dict valueForKey:@"round"]进行处理，然后再对其进行NSUInteger赋值.一线解决方案不见了.

In this case I have to treat [dict valueForKey:@"round"] before making an NSUInteger value of it. And the one line solution is gone.

我尝试制作一个@try @catch块，但是一旦捕获到第一个值，它就会跳过整个@try块，而下一个属性将被忽略.

I tried making a @try @catch block, but as soon as the first value is caught, it jumps the whole @try block and the next properties are ignored.

是否有更好的方法来处理[NSNull null]或使它完全不同但更容易?

Is there a better way to handle [NSNull null] or perhaps make this entirely different but easier?

推荐答案

如果将其包装在宏中，则可能会容易一些:

It might be a little easier if you wrap this in a macro:

#define NULL_TO_NIL(obj) ({ __typeof__ (obj) __obj = (obj); __obj == [NSNull null] ? nil : obj; })

然后您可以编写类似的内容

Then you can write things like

fight.winnerID = NULL_TO_NIL([dict objectForKey:@"winner"]);

或者，您甚至可以尝试将字典填充到托管对象中之前，也可以对其进行预处理，并用nil替换所有NSNull.

Alternatively you can pre-process your dictionary and replace all NSNulls with nil before even trying to stuff it into your managed object.

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