Object Mapping an empty response with RestKit

问题:

Thanks to the help here and on the RestKit mailing list I've been able to parse my JSON, but now I have a new problem, parsing an empty response. To set the stage, here's what the JSON looks like when my query has results:

    {"blobsList":
        {"blobs":
            [   
                {"createdOn":"2012-03-16T15:13:12.551Z","description":"Fake description ","hint":"And a useless hint","id":400,"name":"Fake CA one","publicId":"FF6","type":0},

                {"createdOn":"2012-03-16T17:33:48.514Z","description":"No hint on this one, but it does have a description.","hint":"Hint","id":402,"name":"Second fake one in CA","publicId":"FF8","type":0}
            ]}}

So I added this to my mapping:

        RKObjectMapping* blobsListMapping = [RKObjectMapping mappingForClass:[GetResponseInRegionResponseList class]];
        [blobsListMapping mapKeyPath:@"blobsList" toAttribute:@"blobsList"];

        [[RKObjectManager sharedManager].mappingProvider setMapping:blobMapping forKeyPath:@"blobsList.blobs"];

        [[RKObjectManager sharedManager].mappingProvider setMapping:blobsListMapping forKeyPath:@"blobsList"];

And are are my Classes:

        @interface GetResponseInRegionResponse : NSObject
        {
            NSString* name;
            NSString* blobId;
            NSString* description;
            NSString* hint;
        }


        @interface GetResponseInRegionResponseList : NSObject
        {
            NSArray  *blobsList;
        }

But now the wrinkle is that my server can also return this for JSON:

        {"blobsList":""} 

Yeah, if the query has no results I get that back. It crashes my app

        restkit.object_mapping:RKObjectMapper.m:255 Performing object mapping sourceObject: {
            blobsList = "";
        }
         and targetObject: (null)

        2012-03-22 11:56:16.233 Ferret[7399:17a07] T restkit.object_mapping:RKObjectMapper.m:269 Examining keyPath 'blobs' for mappable content...
        2012-03-22 11:56:16.233 Ferret[7399:17a07] D restkit.object_mapping:RKObjectMapper.m:279 Found unmappable value at keyPath: blobs
        2012-03-22 11:56:16.233 Ferret[7399:17a07] T restkit.object_mapping:RKObjectMapper.m:269 Examining keyPath 'blobsList.blobs' for mappable content...
        2012-03-22 11:56:16.239 Ferret[7399:17a07] *** Terminating app due to uncaught exception 'NSUnknownKeyException', reason: '[<__NSCFConstantString 0xdb1d0> valueForUndefinedKey:]: this class is not key value coding-compliant for the key blobs.'

So I'm trying to come up with a way to get ResKit to map this empty response. I've tried all of these:

[[RKObjectManager sharedManager].mappingProvider setMapping:blobListMapping forKeyPath:@"blobsList"];
[[RKObjectManager sharedManager].mappingProvider setMapping:NULL forKeyPath:@"blobsList"];
[blobListMapping mapKeyPath:@"" toAttribute:@"blobsList"];
[blobListMapping mapKeyPath:@"blobsList" toAttribute:@"blobsList"];

But they all crash. I'm trying to make heads and tails of the source, the "this class is not key value coding-compliant for the key blobs" is puzzling since there aren't any blobs there, just blobsList. I appreciate any help, thanks!

回答1:

I ran into something similar in my project. I was posting to a web service and that service was returning to me my posted object wrapped in an array. This caused RestKit to crash when parsing because it expected to only get a single object back and not a collection. To fix the problem I implemented the objectLoader:willMapData: delegate method and pulled the response object out of the array and sent that object through the RestKit parsing and all is well.

Maybe you can do something similar. Check to see if you get back a string in blobslist and convert that to an empty array before sending it through the RestKit parsing.

- (void)objectLoader:(RKObjectLoader *)objectLoader willMapData:(id *)mappableData
{
    // Horrible hack to convert posted object that gets returned as an array into a single object
    // that can be mapped.
    //
    if ([objectLoader.targetObject isKindOfClass:[TSLPostPostContract class]])
    {
        if ([*mappableData isKindOfClass:[NSArray class]])
        {
            *mappableData = [*mappableData objectAtIndex:0];
        }
    }
}