Inserting nil objects into an NSDictionary

问题:

If we have an API that requires only 2 out of an objects 5 properties and iPhone app doesn't require them to instantiate an object, when the object is used in the params NSDicitionary the app will crash. I was told NSDictionary will not let you assign nil values, as when it reaches nil it thinks its finished. Does objective-c have a way to spit out an objects non-nil properties into an NSDictionary?

Example:

[Drunk alloc] init];
drunk.started_drinking = [NSDate date];
drunk.stopped_drinking (we don't set this because he is still a drunk)
drunk.fat = YES;
drunk.dumb = YES;


parameters:@{

             @"auth_token" :token,
             @"name" : drunk.name, @"date_started" : drunk.started_drinking,
             @"date_stopped" : drunk.stopped_drinking, 
             @"prescribing_doctor" : drunk.fat,
             @"pharmacy" : drunk.dumb

            }

This will crash when it gets to the stopped_drinking property. Any suggestions on how to handle this?

回答1:

It's a bit long winded but you could do

static id ObjectOrNull(id object)
{
  return object ?: [NSNull null];
}

parameters:@{
  @"auth_token"         : ObjectOrNull(token),
  @"name"               : ObjectOrNull(drunk.name),
  @"date_started"       : ObjectOrNull(drunk.started_drinking),
  @"date_stopped"       : ObjectOrNull(drunk.stopped_drinking),
  @"prescribing_doctor" : ObjectOrNull(drunk.fat),
  @"pharmacy"           : ObjectOrNull(drunk.dumb),
}

回答2:

You cannot insert nil into collections (dictionaries, arrays, index sets, etc).

You can, however, insert [NSNull null] into them as this is what they made it for

Inserting objects into the dictionary becomes quite easy (if the property is nil, insert an NSNull instead). Then, when pulling things out of the dictionary, a quick if(myReturnedObject == [NSNull null]) will tell you if the returned value is valid, as NSNull is a singleton and thus every NSNull is in fact the same object.

Edit: Paul.s has an excellent example of insertion behavior for your case, complete with ternary operator usage.

Edit Again: Despite the below comment, it is factually confirmed in the Apple docs linked above that NSNull does not crash when added to collections.