When if ever will a property of type NSString be [NSNull null]

Question

I'm parsing some json to return a basic string token or error message.

- (void)callBackWithVerifyHttpResponse:(NSData *)response
{
    SomeResult *result = [self.parser parseVerifyHttpResponseAndReturnResult:response];
    if (result.token) { [self.delegate callBackWithToken:result.token]; }
    if (result.error) { [self.delegate callBackWithError:result.error]; }
}

The tests that prove this

- (void)testVerifyCallbackInvokesErrorCallbackOnDelegateWhenParserReturnsError
{
    SomeResult *result = [[SomeResult alloc] init];
    result.error = @"fail";
    [[self.delegate expect] callBackWithError:@"fail"];
    [[self.delegate reject] callBackWithToken:OCMArg.any];
    [[[self.parser stub] andReturn:result] parseVerifyHttpResponseAndReturnResult:nil];
    [self.sut callBackWithVerifyHttpResponse:nil];
    [self.delegate verify];
}

- (void)testVerifyCallbackInvokesTokenCallbackOnDelegateWhenParserReturnsToken
{
    SomeResult *result = [[SomeResult alloc] init];
    result.token = @"token";
    [[self.delegate expect] callBackWithToken:@"token"];
    [[self.delegate reject] callBackWithError:OCMArg.any];
    [[[self.parser stub] andReturn:result] parseVerifyHttpResponseAndReturnResult:nil];
    [self.sut callBackWithVerifyHttpResponse:nil];
    [self.delegate verify];
}

All was well until I wired this up to an actual endpoint -only to find that unless I modified the callback like the below -it was calling both callbacks (not what I was hoping for)

- (void)callBackWithVerifyHttpResponse:(NSData *)response
{
    SomeResult *result = [self.parser parseVerifyHttpResponseAndReturnResult:response];
    if (result.token != [NSNull null]) { [self.delegate callBackWithToken:result.token]; }
    if (result.error != [NSNull null]) { [self.delegate callBackWithError:result.error]; }
}

So 2 part question

why can't I write a test to prove this? Any time I set the error or token to NULL or NSNull it works fine (yet this code was required for production to work)
why would the production code only fail the conditional if I put the != [NSNull null] (yet I can't seem to get anything but <null> when I NSLog the values while I run it in the simulator?

Keep in mind the token/error properties look like this on the SomeResult object

@interface SomeResult : NSObject

@property (strong, nonatomic) NSString *token;
@property (strong, nonatomic) NSString *error;

@end

score 2 · Accepted Answer · answered Jun 21 '12 at 16:07

Your original code and tests are expecting either token or error to be nil, not [NSNull null]. Presumably when you run it against a production endpoint, your parser is setting the value to [NSNull null].

These tests should pass with your modified code:

- (void)testVerifyCallbackInvokesErrorCallbackOnDelegateWhenParserReturnsError
{
    SomeResult *result = [[SomeResult alloc] init];
    result.error = @"fail";
    result.token = [NSNull null];
    [[self.delegate expect] callBackWithError:@"fail"];
    [[self.delegate reject] callBackWithToken:OCMArg.any];
    [[[self.parser stub] andReturn:result] parseVerifyHttpResponseAndReturnResult:nil];
    [self.sut callBackWithVerifyHttpResponse:nil];
    [self.delegate verify];
}

- (void)testVerifyCallbackInvokesTokenCallbackOnDelegateWhenParserReturnsToken
{
    SomeResult *result = [[SomeResult alloc] init];
    result.token = @"token";
    result.error = [NSNull null];
    [[self.delegate expect] callBackWithToken:@"token"];
    [[self.delegate reject] callBackWithError:OCMArg.any];
    [[[self.parser stub] andReturn:result] parseVerifyHttpResponseAndReturnResult:nil];
    [self.sut callBackWithVerifyHttpResponse:nil];
    [self.delegate verify];
}

When you're dealing with a web service and/or a third party parser that could change its behavior subtly, it's good to code defensively. You could handle both nil and NSNull:

- (void)callBackWithVerifyHttpResponse:(NSData *)response
{
    SomeResult *result = [self.parser parseVerifyHttpResponseAndReturnResult:response];
    if (result.token && result.token != [NSNull null]) { [self.delegate callBackWithToken:result.token]; }
    if (result.error && result.error != [NSNull null]) { [self.delegate callBackWithError:result.error]; }
}

You may also want to make this an if/else--either test for error first, and never set the token if there's an error, or ignore the error if you get a token. If you don't want to ever call both callbacks, then make it impossible in your code paths.

When if ever will a property of type NSString be [NSNull null]

1 Answers1