Invalid top-level type in JSON write

问题:

I'm trying to create a simple JSON object but I still get error and I know what's wrong in my code:

NSString *vCard = [BRContacts getContacts]; // this is just a string, could be nil
NSDictionary *JSONdic = nil;
if (vCard)
{
    JSONdic = [NSDictionary dictionaryWithObjectsAndKeys:@"1",@"status",vCard,@"data", nil];
}
else
{
    JSONdic = [NSDictionary dictionaryWithObjectsAndKeys:@"0",@"status",@"vCard is empty",@"error", nil];
}
NSError *error = nil;
NSData *JSONData = [NSJSONSerialization dataWithJSONObject:JSONdic options:NSJSONWritingPrettyPrinted error:&error];
return [GCDWebServerDataResponse responseWithJSONObject:JSONdata];

The exception is

Invalid top-level type in JSON write

I checked also JSONdic and it's not nil in every case. Any suggestions?

回答1:

I can't say what is the error, because I tried here and worked.

I tried with NSString *vCard = nil and NSString *vCard = @"SOMESTRING", both cases it worked.

NSString *vCard = @"SOMESTRING"; // this is just a string, could be nil
    NSDictionary *JSONdic = nil;
    if (vCard) {
        JSONdic = @{@"status" : @"1", @"data" : vCard};
    } else {
        JSONdic = @{@"status" : @"0", @"error" : @"vCard is empty"};
    }
    NSError *error = nil;
    NSData *JSONData = [NSData data];

if ([NSJSONSerialization isValidJSONObject:JSONdic]) {
    JSONData = [NSJSONSerialization dataWithJSONObject:JSONdic options:NSJSONWritingPrettyPrinted error:&error];
}

Make sure [BRContacts getContacts] returning a NSString, and I just rewrite to a modern syntax the NSDictionary declaration.

回答2:

Ok I solved. It was a problem related to this line:

return [GCDWebServerDataResponse responseWithJSONObject:JSONdata];

this response of GCDWebServer doesn't want a JSON NSData but a NSDictionary: the error is just because responseWithJSONObject process the input for create a JSON object (and I passed a JSON "pre-processed" object). So my error is not related to my initial code so I updated it just now for future reference, I solved using:

return [GCDWebServerDataResponse responseWithJSONObject:JSONdic]; 

According to the documentation for similar problem be sure to follow this rules:

An object that may be converted to JSON must have the following properties:

• The top level object is an NSArray or NSDictionary.
• All objects are instances of NSString, NSNumber, NSArray, NSDictionary, or NSNull.
• All dictionary keys are instances of NSString.
• Numbers are not NaN or infinity.

回答3:

Swift 4: Worth Considering

JSONSerialization.jsonObject(with: data, options: []) as? [String:AnyObject]

instead of

JSONSerialization.data(withJSONObject: data, options: []) as? [String:AnyObject]